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Cues to overeat: Psychological factors influencing overconsumption

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Abstract

Human food intake is driven by necessity, but modern industrialized societies are characterized by food surfeit and an increasingly 'obesogenic' environment. This environment tends to discourage energy expenditure and to facilitate energy intake. The amount eaten in any given eating episode depends less on internal need state and more on environmental contextual factors such as the availability of highly-palatable energy-dense foods. In addition, the process of satiation can easily be disrupted by the introduction within a meal of different foods (variety effect), the presence of others (social context) and competing tasks (distraction). Properties of ingestants such as alcohol promote food intake and characteristics of individuals make them more or less susceptible to situational cues to overeat. In the present review the role of each of these environmental factors in promoting overconsumption are considered and the extent to which these factors might contribute to long-term weight regulation is discussed.
The Summer Meeting of the Nutrition Society, hosted by the Rowett Research Institute and the University of Aberdeen, was held at the
Aberdeen Exhibition and Conference Centre, Aberdeen on 3–6 July 2006
Symposium on ‘Molecular mechanisms and psychology of food intake’
Cues to overeat: psychological factors influencing overconsumption
Marion M. Hetherington
Department of Psychology, Glasgow Caledonian University, School of Life Sciences, George Moore Building,
Cowcaddens Road, Glasgow G4 0BA, UK
Human food intake is driven by necessity, but modern industrialized societies are characterized
by food surfeit and an increasingly ‘obesogenic’ environment. This environment tends to dis-
courage energy expenditure and to facilitate energy intake. The amount eaten in any given
eating episode depends less on internal need state and more on environmental contextual fac-
tors such as the availability of highly-palatable energy-dense foods. In addition, the process of
satiation can easily be disrupted by the introduction within a meal of different foods (variety
effect), the presence of others (social context) and competing tasks (distraction). Properties of
ingestants such as alcohol promote food intake and characteristics of individuals make them
more or less susceptible to situational cues to overeat. In the present review the role of each of
these environmental factors in promoting overconsumption is considered and the extent to
which these factors might contribute to long-term weight regulation is discussed.
Overconsumption: Energy intake: Weight regulation: Environmental factors
Human food intake is motivated by need, but influenced by
a myriad of complex factors including those that reside
within the environment (availability, variety, portion size),
those that characterize the food itself (energy density,
macronutrient composition, sensory features) and the
response of the individual to these factors. Most indus-
trialized countries and many developing nations are now
facing an exponential rise in overweight and obesity that
must largely be a result of changes in the food environ-
ment. The relatively high levels of genetic predisposition
to gain weight (Loos & Bouchard, 2003) in permissive
environments suggest that the characteristics of the obeso-
genic environment and the response of the consumer to
this environment must be understood if the obesity epi-
demic is to be tackled. In order to expose elements of the
environment that may promote overeating, it is important
to identify those controls of ingestion likely to be influ-
enced by the food environment and then to examine how
these features operate to derail homeostatic controls.
Direct and indirect controls of meal size
The brain receives a constant stream of information to
guide food intake based on stored energy as well as current
energy and nutrient sources (Woods, 2005). Thus, the
amount eaten within a meal will depend both on the
metabolism of previous meals and the currently-available
food. However, there is no simple relationship between
energy deficit and energy intake. For example, consumers
in Western cultures tend to eat their smallest meals after
the longest period of energy depletion and their largest
meal at the end of the day, in anticipation of this deficit
(Weingarten, 1986). Meal size does not correspond well
with the immediately preceding inter-meal interval (Le
Magnen, 1969), suggesting that meal size is highly variable
and subject to the influence of many different factors
somewhat uncoupled from specific energy deficits (Wiep-
kema, 1971; Lowe & Levine, 2005). The variability of
meal size indicates that there are a number of situational
factors that can contribute to the amount of food consumed
on any one occasion.
In order to conceptualize the controls of food intake in a
clear and objective way, Smith (1996) has proposed that
there are direct controls that only relate to the process of
food stimuli engaging gastrointestinal mucosal receptors
during eating and indirect controls that encompass all other
cues. Signals arising from the gut provide both orosensory
positive feedback and post-ingestive negative feedback
Corresponding author: Professor Marion M. Hetherington, fax + 1 141 331 3636, email Marion.Hetherington@gcal.ac.uk
Proceedings of the Nutrition Society (2007), 66, 113–123 DOI:10.1017/S0029665107005344
g
The Author 2007
during the eating episode. Orosensory feedback stimulates
eating whilst post-ingestive feedback inhibits eating (Smith,
2000). During the meal positive orosensory stimulation
declines whilst negative feedback increases (Wiepkema,
1971). The meal ends when negative feedback exceeds or
meets the strength of positive feedback (Smith, 1996).
By applying sham-feeding procedures in animal models,
in which positive orosensory stimulation is intact but nega-
tive feedback is absent, the relative impact of orosensory
and gastric feedback in controlling meal size can be
determined. The sham-feeding model has also been used to
investigate the neuromodulation of direct controls of meal
size. For example, it has been shown that central admin-
istration of neuropeptide Y, a powerful orexigenic agent,
affects meal size by increasing the volume of sucrose
solution ingested as well as increasing the frequency and
rate of licks in sham feeding compared with normal feed-
ing (Torregrossa et al. 2006). Thus, neuropeptide Y serves
to amplify orosensory stimulation to eat.
Similarly, the application of pyloric cuffs and/or infu-
sions of nutrients directly into the stomach, bypassing
orosensory inputs, allows the specific effect of negative
gastric feedback to be assessed during a meal. Thus,
under these conditions it has been shown that intake of a
sweetened milk in rats is reduced by a relatively large
volume (12 ml) of either saline (9 g NaCl/l) or milk infu-
sions compared with smaller volumes (3 ml, 6 ml; Eisen
et al. 2001). It is interesting to note that in this example
reduction of intake does not depend on the nutrient content
of the infusion but specifically relates to the larger volume
size, suggesting a mechanical threshold for distension cues
(Eisen et al. 2001). This finding has been confirmed in
human subjects receiving intragastric infusions of milk-
based liquid preloads (Rolls & Roe, 2002). In both lean
and obese women increasing volume (from 200 to 400 ml)
but not increasing energy (from 834 kJ (200 kcal) to
1668 kJ (400 kcal)) was shown to suppress appetite and
food intake. In contrast, intragastric infusions of 425 ml
high-fat or high-carbohydrate soup (Cecil et al. 1999)
reduced appetite but not food intake. Correspondingly,
in human subjects the distension of either the antrum or
fundus, achieved using a gastric balloon, is insufficient
even at high volumes of saline (300–800 ml) to reduce
food intake, despite increasing subjective fullness and
reducing hunger sensations (Oesch et al. 2006). These data
indicate that gastric feedback is sufficient to suppress sub-
jective appetite but does not always produce concomitant
reductions in food intake. Thus, sham feeding and direct
gastric delivery of nutrients allow comparison of the re-
spective roles of positive orosensory stimulation and
negative gastric feedback in determining meal size. Evi-
dently, gastric feedback in the absence of orosensory cues
is relatively weak at reducing food intake in human sub-
jects (Cecil et al. 1999; Oesch et al. 2006).
Indirect controls include all other factors that do not
directly engage with mucosal receptors, e.g. metabolic,
hormonal, cognitive, social and environmental factors.
These controls exert their effects on meal size by affecting
the potency of direct controls (Smith, 1996, 2000). To
assess the relative potency of direct and indirect influences
on food intake, researchers have examined features of the
food (sensory characteristics, volume, energy density,
portion size), features of the consumer (BMI, dietary
restraint, eating habits) and features of the environment
(presence of others, presence of competing tasks). It is
clear that for human subjects indirect controls can override
normal regulatory processes in determining meal size. This
situation has obvious theoretical and applied implications
for research and practice in understanding appetite regu-
lation. Some of these implications are considered in the
present review in the context of cues that promote over-
consumption.
Characteristics of the food: the example of variety
During a meal, and long before the food is fully digested
and absorbed, satiation processes begin to slow and ulti-
mately to terminate eating. There is therefore a delay
between the initiation of eating and the digestion and
absorption of nutrients from the gut. Satiation must depend
on at least some pre-absorptive mechanisms that increase
in strength during the meal and help signal meal termin-
ation. It is not clear, however, to what extent satiation is
entirely pre-absorptive, since changes in glucose and
insulin release occur immediately in response to the sight,
smell and thought of food during the cephalic phase of
ingestion (Powley, 2000). These changes are likely to
facilitate satiation. For example, Langhans et al. (2001)
have shown that infusion of glucose or glucose and insulin
into the hepatic portal vein during a meal attenuates spon-
taneous feeding in the rat. Apparently, there are cephalic-
phase pre-absorptive processes that provide rapid feedback
contributing to satiation. Sensory-specific satiety is a pro-
cess that begins during eating, but before absorption takes
place, and also facilitates meal termination.
Sensory-specific satiety is a phenomenon in which the
pleasantness of an eaten food declines during the meal
whilst preserving or enhancing the pleasantness of other
foods (Rolls, 1986). Thus, the pleasure derived from con-
sumption of a particular food is relatively high in the early
stages of the meal and gradually declines during the course
of eating. Sensory-specific satiety occurs immediately after
eating, in the early pre-absorptive stages of ingestion, and
does not rely on gastric feedback. It has been demonstrated
when foods are sham fed (Rolls & Rolls, 1997; Smeets &
Westerterp-Plantenga, 2006), when the food is merely
smelled for the same duration as a normal meal (Rolls &
Rolls, 1997) and for foods with very low energy densities
(Rolls et al. 1988a,b,c). However, it is not known whether
sensory specific satiety achieved under sham-feeding con-
ditions will have an impact on energy intake, since not all
sham-feeding experiments have offered participants the
opportunity to consume a meal after the sham-feeding pro-
cedure. Nevertheless, Rolls & Rolls (1997) have reported
that in a small sample of ten participants who rinsed (but
did not swallow) coffee or orange juice, both sensory-
specific satiety and reduced ingestion of the rinsed drink
were found. This finding provides preliminary evidence
that sensory-specific satiety in the absence of any gastric
feedback is sufficient to influence intake, at least in the
short term.
114 M. M. Hetherington
Sensory-specific satiety predicts lower intake of the eaten
food compared with consumption of other foods. Thus, if
offered the same food again this food is less likely to be
eaten than if sensorially-different foods are offered (Rolls
et al. 1984). Providing foods that differ only in flavour
promotes intake by between 10 and 15%, but presenting
foods that differ in flavour, texture and macronutrient
content increases energy intake by 40% compared with
offering the same food (Rolls & Hetherington, 1989). Thus,
greater variety along differing dimensions (sensory and
nutritional) promotes greater stimulation of intake. This
effect may be related to habituation processes.
In a series of elegant experiments Epstein and his col-
leagues (for a review, see Raynor & Epstein, 2001) have
examined the effect of variety on salivary habituation.
Salivation in response to the presentation of a food stimu-
lus declines with repeated presentation (Epstein et al.
1992); however, when a different food is introduced during
the session the result is dishabituation to the stimulus
(Temple et al. 2006). Thus, when children are presented
with olfactory cues from a cheeseburger saliva secretion in
response to the stimulus declines over seven trials, but this
trend is reversed in the group provided with a different
food (French fries) in the eighth trial. This change in
salivation is mirrored in motivated responding for either
the cheeseburger or the French fries (Temple et al. 2006).
Thus, variety maintains interest in eating by preserving
both the salivary response to food and motivated respond-
ing to obtain the food, compared with presenting just one
food repeatedly.
Variety may stimulate food intake by a number of
different mechanisms, including dishabituation of salivary
response. Variety maximizes pleasure (positive orosensory
feedback) and sustains appetite, whereas consuming a
single food facilitates satiation. Introducing many different
sensory features within a meal may stimulate eating
through extending positive orosensory feedback.
The relative effects of volume (which promotes negative
gastric feedback) and variety (which may enhance oro-
sensory positive feedback) on meal size have recently been
examined (Norton et al. 2006). Thirty men and women
on four occasions received a low-volume (females 240 ml,
3
.
6 kJ/g; males 300 ml, 3
.
6 kJ/g) or high-volume (females
480 ml, 1
.
8 kJ/g; males 600 ml, 1
.
8 kJ/g) tomato soup
30 min before a sandwich lunch with either a single or
variety of fillings. Reported hunger and fullness indicated
a difference by volume, which suggests that the partici-
pants were sensitive to the immediate effects of volume on
subjective appetite. However, the satiety quotient (change
in subjective ratings of hunger, fullness, desire to eat
divided by weight of soup consumed) calculated just
before lunch indicated a smaller satiety quotient for the
high-volume soup than for the low-volume soup. Thus, the
reported satiety quotient for the high-volume soup is
weaker, indicating some sensitivity to the energy density of
the preloads. Despite the differences in subjective ratings
by volume, there is no effect on the intake at lunch, thus
intakes following the low-volume and high-volume soup
preloads are similar. However, access to a variety of sand-
wich fillings produces a greater intake (by 14%) compared
with providing sandwiches with a single filling regardless
of previous volume ingested. Thus, under these conditions
the orosensory stimulation provided by variety is more im-
portant than the differences in gastric distension produced
by alterations in volume.
It has been suggested that the effect of a volume
manipulation may also depend on energy density exceed-
ing a certain threshold (Gray et al. 2002). The lower satiety
quotient obtained for the high-volume soup in the previous
experiment suggests that this threshold was not reached;
thus, per g soup consumed, the high-volume low-energy
density preload failed to reduce intake at lunch. Under
these conditions, increasing sensory variety (positive feed-
back) has a reliable and marked impact on energy intake in
the short term, regardless of previous ingestion of soup.
Taken together with evidence from intragastric infusion
studies, these data indicate that volume effects will influ-
ence food intake only at relatively high volumes (Rolls &
Roe, 2002) and if paired with a relatively-high energy
density (Gray et al. 2002).
Variety as a distractor
Introducing sensorially-distinct foods, flavours and textures
into a meal may serve to maintain interest in eating. If
motivation to eat is enhanced by the presence of varied
foods, the meal duration will be extended with a con-
sequent increase in meal size. Sensory-specific satiety acts
as a brake on intake and is associated with decreased liking
for an eaten food, limiting selection and intake of that food
whilst directing choice towards foods that differ from the
eaten food in sensory characteristics. This process stimu-
lates intake of a variety of different foods. Providing foods
that vary along sensory and nutritional dimensions is likely
to introduce sufficient novelty to sustain interest in eating
and to promote intake, whereas consuming a single food
enhances satiation processes, including the decline in the
pleasantness of the taste and desire to eat that food.
Consumers who identify the change in pleasantness of
the taste of a food as a primary reason for terminating
intake tend to eat less than those who rely on gastric fill
(Hetherington, 1996). This finding suggests that if the
attention of the consumer is drawn to the change in pleas-
antness of the eaten food during a meal, this process will
facilitate the development of satiation and will limit intake.
Attention to what is eaten and memory for recent eating
contributes to the regulation of intake (Higgs, 2002, 2005).
Multiple attentional targets (whether food- or non-
food-related) will delay satiation.
This prediction has been tested in two studies in which
participants were asked to consume a snack of either pop-
corn or chocolate. In the first within-subjects experiment
participants attended the laboratory on four occasions for a
snack. On one occasion they simply ate their preferred
flavour (salty or sweet) of popcorn ad libitum without
interruption. In this control condition participants rated the
pleasantness of their preferred flavour of popcorn, imme-
diately before and after intake. During the other three
experimental sessions participants consumed their pre-
ferred flavour of popcorn, rated its pleasantness before and
after intake and, in addition, made ratings of pleasantness
Molecular mechanisms and psychology of food intake 115
and desire to eat the eaten food (same food) or a different
uneaten food (either congruent or incongruent in flavour to
the popcorn) at intervals of 1 min during the snack. Instruc-
tions to stop and rate the foods were delivered by audio-
tape. Interrupting participants to taste and rate a different
food during eating increases consumption of their snack by
13% in the congruent condition (where the other food
shared similar sensory characteristics) and by 18% in the
incongruent condition (where the other food differed in
sensory characteristics) compared with intake when atten-
tion is specifically drawn to the taste of the eaten food.
This same taste condition maximizes focus on the sensory
properties of the popcorn, with the congruent and incon-
gruent taste conditions appearing to disrupt this focus.
When ratings of pleasantness of the eaten food are com-
pared across conditions, pleasantness declines rapidly in
the same condition but remains relatively high in the con-
gruent and incongruent conditions. Thus, increased food
intake during congruent and incongruent conditions may
be related to the delay in satiation observed in these con-
ditions.
In the second between-subjects experiment participants
attended the laboratory on only one occasion. Men and
women were allocated to either a food-focus or a food-
distraction condition. In the food-focus condition the parti-
cipants were given chocolate to consume as a snack and
were interrupted at 1 min intervals during eating to rate
pleasantness and desire to eat that food. In the food-
distraction condition the participants were asked to make
the same ratings of a different food (cheese crackers) pro-
vided alongside the chocolate. Again, by simply introduc-
ing a salty food during the consumption of a sweet snack
the pleasantness of the snack remains relatively high com-
pared with attending to this food alone during the snack.
Introducing variety during an eating episode appears to
delay the normal decline in pleasantness associated with
sensory-specific satiety (Hetherington et al. 2006b). This
finding could explain, in part, why variety increases energy
intake.
Variety is likely to stimulate overconsumption through a
variety of mechanisms including the process of salivary
dishabituation (Epstein et al. 1992) and extending positive
orosensory feedback during the meal (Hetherington et al.
2006b). Short-term experiments suggest only that over-
consumption in response to variety occurs within a meal,
but it is not clear whether variety is relevant to long-term
energy balance. In a study of variety effects in the longer
term Stubbs et al. (2001) have studied six lean and six
overweight men during a 9 d period with a limited (five),
medium (ten) or a high variety (fifteen) of foods offered. In
this experiment participants were monitored within a resi-
dential environment and the composition of foods was
controlled, having identical macronutrient composition and
energy density. For all twelve men access to the high
variety of foods was found to increase energy intake. Also,
the lean men were found to gain weight with the high
variety of foods compared with the medium and limited
varieties, and the overweight men tended to lose weight
during the study, losing least weight with the high variety
of foods. Thus, providing ad libitum access to foods that
differ in sensory attributes enhances energy intake and in
some cases produces excess intake and positive energy
balance. This result is an important demonstration of how
short-term experimental studies can identify a key deter-
minant of energy intake in the laboratory that then trans-
lates well to the longer-term context of body-weight
regulation. This study also shows very distinctly that not
all individuals are equally susceptible to environmental
manipulations.
Increasing pleasure during eating
Variety may enhance eating by its effects on habituation and
the extension of positive orosensory feedback; however, it is
unlikely that variety itself enhances intake. The previously
described studies have all involved a variety of highly-
palatable foods within a meal or over several days. The
implication is that less-liked foods offered over many days
or months may not maximize pleasure and therefore intake.
In a series of studies conducted over many years by
researchers at Natick, MA, USA on US army rations (Hirsch
et al. 2005) reduced energy intake has been found repeat-
edly when soldiers receive a variety of ready-to-eat meals in
the field compared with the same foods offered in cafeteria
settings. A comparison of acceptance ratings between field
and control conditions shows higher ratings in the field
group but a consistently lower energy intake. However, in
the field soldiers discard items they do not like, or trade
them for foods they like, which serves to maintain accep-
tance at a relatively high level despite lowered energy
intakes. Reduced energy intake probably reflects long-term
monotony and relatively unfavourable situational factors
associated with eating in the field. This ‘real world’ example
demonstrates the limits of variety in enhancing food intake,
the importance of providing frequent changes in menus to
promote novelty and the strategies consumers use to main-
tain the pleasure they derive from food such as trading
items.
Another obvious means to extend positive orosensory
feedback is the manipulation of palatability. Maintaining
the energy and nutrient content of foods whilst manip-
ulating palatability is the only method of determining the
relative importance of pleasure over other variables in
guiding energy intake and whether changing palatability
alone can affect weight regulation. In a series of experi-
ments on rats using an ‘electronic oesophagus’, in which
animals can determine the rate of infusion of nutrients
directly into the stomach when ingesting a flavoured solu-
tion, Sclafani (2004) has demonstrated that improving the
palatability of the flavoured solution increases both energy
intake and weight gain.
In human subjects palatability of individual foods tends
to be associated with high levels of energy density, inde-
pendent of fat levels, and high taste pleasantness is asso-
ciated with greater levels of energy intake (McCrory et al.
2006). However, since palatability co-varies with energy
density, it is difficult to disentangle the relative impact of
each factor on weight gain. However, if energy content is
kept the same and foods are made more palatable, as in the
rat model, then the relative effects of energy and palat-
ability can be determined.
116 M. M. Hetherington
Yeomans et al. (2001) have investigated this approach
by developing a palatable version of a test meal using the
simple addition of oregano. This alteration in flavour is
sufficient to promote greater food intake. It has been sug-
gested that increasing the palatability of foods may reduce
their satiating effects (Yeomans et al. 2001; Robinson
et al. 2005). Thus, the efficacy of a high-energy fat or
carbohydrate preload in reducing subsequent food intake
has been shown to be influenced by whether the test meal
is bland or palatable. Each of the high-energy preloads
reduces intake of the bland meal but not the palatable meal
(Yeomans et al. 2001). Providing infusions of 300 ml high-
energy high-fat soup directly into the gastrointestinal tract
fails to inhibit intake of a palatable test meal relative to a
bland test meal (Robinson et al. 2005), although these
differences are not replicated with the high-carbohydrate
soup preload. This outcome suggests that palatability may
stimulate eating despite post-ingestive signals associated
with satiation, particularly when these signals are relatively
weak, as they are for fat.
Flavours paired with high-energy infusions are less pre-
ferred than flavours paired with more-dilute nutrient infu-
sions (Sclafani & Ackroff, 2004). Thus, there is no simple
linear relationship between palatability and increased
energy density in stimulating both food intake and weight
gain. Nevertheless, in animals flavours paired with nutri-
ents may come to be preferred and this preference can
occur even in animals that have been previously fed to
satiety on lab chow. Thus, manipulations of palatability
may, under certain conditions, disrupt normal satiation
processes (Yeomans et al. 2001) and override homeostatic
mechanisms of energy balance (Sclafani, 2004). It remains
to be seen whether the same can be applied to human
subjects over the long term, since the complexity of
learning the consequences of eating paired with the diver-
sity of flavours available in the human diet precludes a
straightforward comparison. However, the possibility that
palatability may impair satiation, and that this process
could contribute to overeating and weight gain is worthy of
further consideration, particularly given the consumer’s
efforts to maximize the pleasantness of the eating experi-
ence and the palatability of the foods they choose.
Pleasure and alcohol
Alcoholic beverages are often consumed before and with
food in order to heighten appetite and to enhance the
pleasure derived from the meal. Alcohol has the capacity
to influence energy intake and how substrates are utilized
(Suter, 2005), thus alcohol is an interesting and important
component of the diet for body-weight regulation. Energy
derived from alcohol is additive to total energy intake, i.e.
there is no apparent compensation for alcohol energy
within a meal (Poppitt et al. 1996, 1998). Alcohol also
stimulates short-term intake relative to consuming no
alcohol (Yeomans et al. 1999, 2003; Hetherington et al.
2001). Thus, when alcohol is consumed within 1 h of eat-
ing there is no adjustment made in intake (additive effect)
and it can increase the amount consumed relative to no
alcohol (stimulatory effect). It is not clear whether these
effects are compensated by reduced intake and/or increased
physical activity later in the day or on subsequent days.
However, in the short term at least, there is a highly-
reliable effect of alcohol on promoting energy intake
(Yeomans, 2004).
Interestingly, when consumers have a variety of foods to
select from, alcohol intake favours selection of high-fat
palatable savoury foods (Caton et al. 2004). It is not
known whether alcoholic drinks produce these effects as a
function of enhancing pleasure or through learning and
expectancy effects. For example, consumers may have
long conditioning histories of pairing alcohol with specific
types of foods; thus, choice is directed by previous learn-
ing or it could be that ingestion of alcohol promotes high-
fat high-energy foods as a function of metabolic processes.
To investigate the role of palatability in the ‘aperitif’
effect of alcoholic beverages an experiment has been con-
ducted on lean males (Caton et al. 2006). The objective
was to test the effects of alcohol intake on the development
of sensory-specific satiety, perceived palatability of foods
and food intake. Thus, a comparison was made of the
amount eaten from two versions (bland or flavoured) of the
same food items (crackers and cheese) at lunch following
either a preload with or without alcohol. The no-alcohol
preloads consisted of 405 ml chilled lightly-carbonated
white grape juice (867
.
3 kJ), with pure ethyl alcohol (96%,
v/v) wiped around the rim to reduce expectancy effects.
The alcohol preloads consisted of 375 ml carbonated
white grape juice with the addition of 24 g (30 ml) ethyl
alcohol (7
.
5% (v/v); 1508
.
6 kJ). Pleasantness ratings of the
lunch foods were taken alongside a selection of sweet and
savoury bland and flavoured foods before and after the
drinks and again after the lunch.
It was found that bland foods are rated as less pleasant
than flavoured foods; however, alcohol increases intake of
both bland and flavoured versions of the lunch compared
with intake following no alcohol. Alcohol has no selective
effects on perceived palatability and does not affect the
development of sensory-specific satiety. Thus, alcohol stim-
ulates food intake despite differences in palatability. What
are the implications of this robust stimulatory effect of
alcohol on energy intake in the long term and on weight
regulation?
Most studies of short-term food intake have not fol-
lowed participants beyond a single meal or have relied
on diet records for subsequent meal intake (Mattes, 1996;
Westerterp-Plantenga & Verwegen, 1999). In contrast,
Foltin et al. (1993) have measured accurately all foods and
drinks consumed in a residential setting after dextrose- or
ethanol-containing drinks. In this study energy from both
beverages was found to be only partially compensated
within a 24 h period, although energy compensation on
the days after beverage intake was not calculated. It has
been estimated that there is a 2 d lag in compensation (de
Castro, 1998) and, therefore, any adjustments in intake
should be followed for 2 d following alcohol. A recent
study of lean men (Caton et al. 2007) has compared the
intake of a two-course meal following no alcohol, 375 ml
glass of red wine before lunch (aperitif) or 125 ml wine
served with the first course and 200 ml wine served with
the second course (co-ingestion). Energy intake for the
Molecular mechanisms and psychology of food intake 117
remainder of that day and the following 3 d was then
measured using diet records. Total energy intake was
found to decrease the day after the experiment in all con-
ditions and again on day 4, regardless of alcohol intake.
Thus, under these conditions at least, the men adjusted
for overconsumption in the laboratory on days 2 and 4. It
is possible that for these lean men sufficient adjustment
for overeating has a protective effect on weight status.
However, these results may differ in older or overweight
participants.
There is considerable interest in the relationship between
alcohol intake, adiposity and obesity (Wannamethee et al.
2005). Large-scale epidemiological surveys (Arif &
Rohrer, 2005; Wannamethee et al. 2005) indicate that
moderate social drinking is protective against overweight
and obesity but that excessive or binge drinking (more than
four drinks daily) is associated with a higher BMI (Arif &
Rohrer, 2005) and with central adiposity (Wannamethee
et al. 2005). Similarly, higher alcohol intakes are related to
poorer diet quality (Breslow et al. 2006). However, even
this apparently linear relationship masks the observation
that the poorest diet quality is associated with the highest
intakes of alcohol at the lowest frequencies. Thus, it is not
simply the amount of alcohol consumed but the frequency
and pattern of drinking as well as the amount consumed
that determines its impact on dietary intake and potential to
weight gain.
The variable response to alcohol ingestion and body-
weight regulation is supported by laboratory investigations
(Clevidence et al. 1995) that indicate that weight gain
following alcohol is linked to individual differences. The
particular case of alcohol highlights the importance of the
contribution of this macronutrient to the diet, its effect on
stimulating intake and on individual differences in the
ability of consumers to adjust energy intake in response to
both energy from alcohol and additional food energy in
regulating body weight.
Characteristics of the food environment
Human food intake is influenced by social setting; thus,
energy intake in the presence of friends or family can
increase by between 40 and 70% compared with eating
alone (Shide & Rolls, 1991; de Castro, 1990, 1991, 1994).
Eating with friends and family encourages greater food
intake both by extending the duration of the meal and by
providing a more pleasant context in which to consume
meals and snacks (Herman et al. 2003). Thus, for example,
participants rated a test meal as more palatable when this
meal was eaten with a group of others than when eating the
same meal alone (Bellisle & Dalix, 2001). Meals eaten
alone tend to be somewhat functional, tedious and short. In
contrast, eating a meal with others is perceived as an
important and enjoyable part of the cultural experience.
Indeed, most important social and cultural events tend to
revolve around social eating (Rozin, 2005). When con-
sumers eat in groups of familiar others they expect the
focus to be on food sharing and the time frame is likely to
be much longer than that set aside for eating alone
(Herman et al. 2003). Similarly, the sorts of foods provided
for a social function will differ markedly from those eaten
during a solo meal (de Castro, 1994). The social, psycho-
logical and environmental factors contributing to social-
facilitation effects are highly variable (Hetherington et al.
2006a). However, it could be that eating with others sim-
ply stimulates eating as a function of duration, with longer
sessions increasing exposure to food cues and providing
greater opportunities to eat (de Castro, 1990). It is not
possible in most everyday social contexts to identify the
relative importance of meal duration or the type of food
offered in stimulating consumption.
Thus, social-facilitation effects have been studied in the
laboratory so that foods offered can be controlled as well
as the duration of the meal. Importantly, key features of the
social context can also be manipulated, including the
number of diners and their familiarity to the participant. A
recent experiment (Hetherington et al. 2006a) has con-
sidered the possibility that social eating, like eating in the
presence of any distraction, increases food intake partly as
a function of drawing the consumer’s attention away from
the food eaten, thereby extending meal duration and
amount consumed. Therefore, a comparison was made of
intake when eating alone and intake when consumers
watched television or had their meal in the presence of
familiar or unfamiliar others. Since the focus of interest
was in how consumers allocated their attention during the
meal (eating, drinking, talking) all meals were videotaped
and then coded. A buffet-style meal was offered for lunch
to approximate the typical foods eaten during social occa-
sions. The meal consisted of a variety of different foods,
including energy-dense palatable foods (bread, cheese,
green salad, coleslaw, crisps, cakes). It was predicted that
social-facilitation effects would favour selection of the
most energy-dense and palatable foods. Thus, the four
meal sessions consisted of eating alone with no distrac-
tions, eating the same meal with the television switched on
or eating with two same-gender friends or two same-
gender strangers. Inviting same-gender diners is important
since women eat differently in front of men, tending to eat
less in order to appear feminine (Mori et al. 1987).
Under these conditions three key findings emerge:
energy intake is higher when eating alone with the tele-
vision switched on or when eating with friends; meal
duration is longer when eating with others (friends or
strangers) compared with eating alone; videotapes indicate
that attention is largely allocated to the meal when eating
alone, but to non-food cues in the other conditions. The
finding that when eating alone in the presence of the tele-
vision (14 %) or when eating with friends (18%) energy
intake is higher than when eating alone supports the find-
ings of the previous studies. However, the magnitude of
the social-facilitation effect is much less than would be
predicted from free-living studies (Bellisle et al. 1999;
de Castro, 1990, 1991, 1994). This disparity is probably a
result of the constraints of the laboratory (limited variety,
limited time frame) compared with spontaneous meals in
the ‘real world’. Nevertheless, the study shows a selective
enhancement of sweet high-fat foods (by >50%) when
eating with friends compared with eating alone, indicating
that social facilitation favours particular foods (easy-
to-eat highly-palatable high-energy snacks) typical of
118 M. M. Hetherington
social eating occasions. Such an observation has been
reported by Clendenen et al. (1994) who have found that
eating with friends specifically increases intake of cookies
compared with intake when eating with strangers.
It is interesting to speculate on why social contexts may
favour overconsumption of high-energy high-fat foods
rather than, for example, low-energy-density foods such as
salad. It could be ease of eating or the palatability of these
foods; it could be that consumers match their eating to
what others eat (Herman et al . 2005) or that when dis-
tracted it is more difficult to monitor and limit intake of
foods that are typically restricted. Of course, all these
factors are likely to play a role in the social facilitation
effect.
Key to social facilitation is familiarity of the other
diners (Shide & Rolls, 1991; de Castro, 1994), since eating
with strangers fails to reliably increase intake. Eating with
strangers may necessitate greater vigilance than eating with
familiar others, since the consumer may self-monitor (im-
pression management) and compare their own intake
against what others eat (social norms). When unfamiliar
women are paired for lunch to eat pizza, the amount eaten
is highly matched between the pair (Herman et al. 2005),
suggesting both high levels of self-monitoring and atten-
tion to what the other is eating.
The finding in the experiment (Hetherington et al.
2006a) that television viewing increases energy intake by
about 14 % of a similar magnitude of effect to that reported
by Bellisle et al. (2004). Watching television and eating
with friends share the common property of introducing
competing pleasurable tasks during eating that distract
attention away from processing food cues. Scrutiny of the
videotapes of each session indicates that the amount of
time spent looking away from the meal is highest when
watching television, thus the programme engaged the par-
ticipant’s attention. On the other hand, during social eating
much of the time is spent looking away from the meal,
with 40% of the time spent talking. Thus, when less
attention is paid to the meal the ability to self-monitor is
reduced and may interfere with the development of normal
satiation. However, the presence of distraction alone is
not sufficient to increase food intake, since eating with
strangers had the same effect on time spent looking away
from the meal, and time spent talking had no impact on
energy intake, in this condition.
Long-term energy balance: implicating
situational factors in weight gain
Given the earlier observations, is there any evidence that
watching television contributes to weight gain as well as
increasing time spent being sedentary? Television viewing
is associated with snacking, sedentary behaviour and
obesity (Hancox et al. 2004; Viner & Cole, 2005). One
study of adolescents (Van den Bulck & Van Mierlo, 2004)
has estimated that, since many of them eat while watching
television, 1 h of television viewing is associated with
approximately 653 kJ additional energy intake, and another
study (Viner & Cole, 2005) has shown that television
viewing at the weekends at 5 years of age predicts obesity
at 30 years of age. Thus, television viewing reduces phy-
sical activity and provides an opportunity for unmonitored
additional energy intake that may track well into later life.
What about eating in a social context; does it contribute
to weight gain? One study of the effects of transition from
living alone to living with a partner (Kemmer et al. 1998;
Marshall & Anderson, 2002; Anderson et al. 2004) has
examined differences in food choice, eating habits, energy
intake and body weight. In this study twenty-two couples
were followed during the 3 months before and up to 1 year
after moving in together. It was found that social eating
occasions increase after marriage or cohabitation as cou-
ples eat together more often and the number of occasions
spent eating with the extended family and friends in-
creases. Couples are more likely to include alcohol with
meals and encourage each other to break dietary restric-
tions (Anderson et al. 2004). There is a shift in the kinds of
foods eaten as well as a net increase in total energy intake.
Overall, it was found that both men and women gain
weight within the first year of moving in together (women
1
.
5 kg; men 1
.
7 kg). The short timescale of the study may
limit extrapolation to a wider context; however, there is
some evidence that marital status influences BMI (Ball &
Crawford, 2005).
A ‘real world’ example of transition that incorporates
many different changes in eating habits involves leaving
home to go to college. In this transition college students
are exposed to a wider variety of foods, they eat together in
social groups, regularly drink alcohol and are perhaps more
likely to eat while distracted than when eating at home.
Several studies (Anderson et al. 2003; Levitsky et al. 2004;
Lowe et al. 2006) have shown a reliable increase in body
weight, known in the USA as the ‘freshman 15’. This
phenomenon is widely acknowledged anecdotally but few
systematic studies have been conducted. One such study
(Anderson et al. 2003) has monitored body weight in 135
first-year men and women over the first 14-week semester
and a smaller subsample until the end of the first year. An
overall gain in weight of 1
.
3 (range - 3
.
6– + 5
.
2) kg was
found in the first semester, and in the subset of freshmen
monitored until the end of the first year the net weight gain
was 1
.
7 (range - 6
.
2– + 11
.
4) kg. These findings are of a
similar magnitude to that found in the couples moving in
together (Anderson et al. 2004), suggesting some shared
experience in the transition process.
Another study (Levitsky et al. 2004) that has provided
further support for this weight change has reported an
average gain of 1
.
9 (range - 5
.
9– + 8
.
6) kg in the first
semester. Using a self-report questionnaire to investigate
predictors of weight gain it was found that the variance in
the regression models is largely accounted for by eating at
‘all-you-can-eat’ dining halls, increased snacking and
consumption of high-fat snacks. Clearly, situational factors
play a profound role in determining how much and what
types of food are selected by consumers, and at times of
transition (going to college, living as a couple) these
changes in eating are sufficient to promote weight gain. It
is obvious, however, that not everyone who experiences
this sort of transition gains weight. In each of these studies
it was found that the majority of participants gain a modest
amount of weight; however, some lose weight and others
Molecular mechanisms and psychology of food intake 119
stay the same. Obvious questions that arise are: what
predicts susceptibility to these environmental factors pro-
moting overconsumption and weight change and who is
most likely to gain weight under these circumstances?
To address these questions it is necessary to consider
individual differences; however, it is clear that few studies
examining situational cues to overeat have identified
specific individual characteristics that identify those sus-
ceptible to weight gain.
Characteristics of the consumer
If consumers are aware of the tendency for small changes
in their dietary intake to produce changes in body weight
they may attempt to limit food intake. Restrained eating
is the construct used to describe this type of behaviour,
i.e. imposing cognitive limits on food intake for the pur-
pose of maintaining or losing weight (for a review, see
Herman & Polivy, 2005). In the case of the weight gain
that occurs in the first year of college, it might reasonably
be predicted that higher levels of restrained eating would
result in less weight gain than in individuals with low
levels of restraint. However, Lowe et al. (2006) have
examined weight gain in sixty-nine female first-year stu-
dents and have related this gain to various measures of
dieting and restrained eating. It was found that cognitive
restraint scores from validated questionnaires fail to predict
weight change; however, women with a history of dieting
and with a high level of weight suppression (difference
between current and highest-ever weight) are most likely
to gain weight during their first year. The average weight
gain across the year was found to be 2
.
1 kg, with this gain
being greater (+ 5 kg) in those currently attempting to diet
compared with former dieters (+ 2
.
5 kg) and those who had
never been on a weight-loss diet (+ 1
.
6 kg). Similarly, those
students with the highest levels of weight suppression were
found to gain more weight over the academic year than
those with low weight suppression. Evidently, individuals
with a tendency to gain weight, as indicated by high levels
of weight suppression, are those most likely to be on a diet
and most vulnerable to the situational factors that promote
energy intake during their first year. However, it is curious
that those claiming to be restricting food intake to lose
weight are also those most likely to gain weight. This
seeming paradox has been illustrated in a series of studies
by Stice et al. (2004) in which those subjects who score
high on restrained eating measures fail to restrict their food
intake under a number of different circumstances: in a
laboratory-based snack; a laboratory-based healthy meal; a
fast-food restaurant; a university dormitory dining hall. In
all these different settings it is reasonable to suppose that
there would be an inverse relationship between dietary
restraint and energy intake; however, this relationship is
not observed. Highly-restrained individuals may be restrain-
ed for the very reason that they are highly susceptible to
food cues and tend to overeat. There are numerous studies
(Polivy & Herman, 1985; Stice et al. 1999) that indicate a
strong relationship between dietary restriction and the
tendency to binge eat. It is not always clear whether
restraint is imposed to counter episodes of overeating or if
efforts to restrict food intake increase the likelihood of
overeating.
Whatever the direction of causality, there seem to be
some circumstances in which restraint is accentuated and
the tendency to restrict is enhanced and other circum-
stances in which restraint is abandoned. For example,
Bellisle & Dalix (2001) have demonstrated that when
restrained eaters are asked to focus on the food they are
eating a lower intake is produced than when eating while
listening to an audiotape. There is a correlation between
restraint score and the difference in intake between base-
line and distraction. Focusing on food appears to accentu-
ate restraint by producing similar intakes to the baseline
condition, but distraction increases intake especially in
those individuals with higher restraint scores.
When restrained eaters are faced with cues that remind
them of their intention to restrict food intake restraint is
heightened, but if attention is narrowed to other cues then
this situation promotes disinhibited eating (Mann & Ward,
2004). Dietary restriction is an effortful activity that ‘costs’
the restrained eater (Green & Rogers, 1999). Thus, if
restrained eaters are asked to perform a task with a high
cognitive load during eating, this additional demand on
attention may impair monitoring of food consumption and
produces an increase in intake relative to a low-cognitive-
load activity (Ward & Mann, 2000).
There is, therefore, no simple relationship between
an individual’s tendency to restrict food intake and their
susceptibility to gain weight in situations that promote
overconsumption. It is not clear what specific individual
characteristics protect against the obesogenic environment,
although it is the case that some individuals are extremely
good at resisting even the most systematic attempts at
overfeeding. For example, a study of overfeeding (Jebb
et al. 2006) has been conducted in which six lean healthy
men took all meals in a metabolic ‘hotel’ for a period of
17 weeks. Following a 21 d baseline there were three
overfeeding phases interspersed by 7 d of free feeding. The
men were required to eat + 20, + 40 and + 60 % energy
requirements, which was achieved by increasing the energy
density of the diet through increasing fat content system-
atically. As expected, the result was weight gain and, on
average, a failure to compensate for overfeeding during
the periods of free feeding. However, there was an attempt
by at least two men to compensate during the 7 d ad
libitum periods, although 7 d may be insufficient for this
ability to be fully expressed, and perhaps a longer period
of free feeding would have permitted compensation to
occur.
Jebb et al. (2006) have demonstrated that individual
differences will determine how much weight is gained
during systematic overfeeding. Evidently, there are some
individuals who are resistant to situational cues to overeat,
even when overfeeding is forced. However, it is not
obvious what these individual differences are and how they
arise. Are susceptible individuals more motivated by food
reward or more sensitive to the presence of food cues?
Recent imaging studies (Beaver et al. 2006) suggest that
there is a correlation between sensitivity to reward traits
and brain activation in response to palatable food images.
There are, therefore, individual differences in the response
120 M. M. Hetherington
to situational cues involving food, and these differences
may play a role in susceptibility to overeating and weight
gain in the long term.
Conclusions
It is relatively easy to promote overeating in response to
diverse situational cues. Most individuals have a ‘thrifty’
genotype that confers a slight or strong disposition to lay
down fat stores. However, there are important individual
differences in response to these cues. Although the present
review has emphasized those circumstances that promote
overeating (variety, alcohol, social context, distraction),
these effects are mostly observed in the short term. Where
longer-term studies have been conducted, individual dif-
ferences become more salient, with some individuals
highly susceptible to situational cues that promote over-
eating and weight gain, and others who tend to resist these
cues. It is not clear whether this susceptibility is learned or
endowed.
The specific effect of different gene variants on behav-
ioural and physical phenotype has recently been examined
(Cecil et al. 2006). This research has identified children
who carry gene variants that confer some protection from
obesity (Cecil et al. 2005a), but has also found that situ-
ational factors such as socio-economic status profoundly
influence the risk of developing overweight and obesity
(Cecil et al. 2005b).
Given that excess body weight tracks into adulthood and
that circumstances converge in adulthood to promote
weight gain, it is important to characterize the behavioural
phenotype of children who resist overeating as well as
identifying the source of behaviours that facilitate over-
eating, such as eating in the absence of hunger (Birch et al.
2003; Faith et al. 2006). Much of human eating behaviour
occurs outside of metabolic requirements; however, there
is a particular need to identify circumstances that are ‘high
risk’ and that promote overeating, as well as characteristics
of ‘high-risk’ individuals who are predisposed either
through genes or learning to overconsume. Thus, through
identifying the psychobiological profile of those individ-
uals who fail to respond to situational cues to overeat and
who might be resistant to obesity it will be possible to
provide clues about how to challenge the obesogenic en-
vironment and develop a behavioural model on which to
base interventions to in order to manage cues to overeat.
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Molecular mechanisms and psychology of food intake 123
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... Edmund Rolls quickly went on to show with colleagues that sensory-specific satiety was present in humans, and ran generations of Oxford undergraduates on sensory-specific satiety paradigms, showing that they showed sensory-specific satiety for food, and that variety of taste and flavour in a meal was a major factor in influencing how much food is eaten in a meal Rolls, 1977, 1997;Rolls et al., 1981aRolls et al., ,b, 1982Rolls et al., , 1983aRolls et al., ,b, 1984Hetherington, 2007). Further, it was shown in an Ethiopian refugee camp that there is a long-term form of sensory-specific satiety, which needs to be allowed for when designing foods to be offered on a long time scale (Rolls and De Waal, 1985). ...
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Over the last decades, eating episodes in addition to the three daily main meals have been observed worldwide; the prevalence of these “snacking” episodes raises health questions that mindful eating may contribute to answering. The goal of the symposium entitled “Mindful eating applied to snacking: a promising behavioral approach supported by research” was to introduce, for the first time in a scientific congress, the emerging science related to mindful eating and to evaluate its application to snacking occasions. It was held at the 21st International Congress of Nutrition (IUNS), which took place in Buenos Aires from October 15-20, 2017. Three primary topics were presented: 1) the definition of snacking and its role in dietary quality in adults; 2) the value of eating mindfully as an emerging concept, in relation to snacking occasions; 3) a detailed approach to mindful eating from theoretical principles to applications. Promoting mindful eating is a relatively new ‘third-wave’ cognitive-behavioral approach that enhances individuals’ awareness of, and attention to, physiological hunger and satiety, eating enjoyment, portion size and nutritional health when eating or when making food choices. Encouraging results have been obtained in obese individuals. Applied to snacking, mindful eating may help individuals’ better control food intake, and help orient their choices without compromising pleasure while eating. This symposium was organized by Mondelez International R&D.
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The amount eaten by humans in spontaneously ingested meals is positively correlated with the number of other people present. In order to investigate whether this social facilitation of eating was due to an increase in arousal, emotionality, hunger, or social interactions, analyses were performed on the data obtained from 82 adult humans. They were paid to maintain 7-day diaries of everything they ingested, when and where they ingested it, the number of other people present, and their subjective states of hunger, elation, and anxiety. The presence of other people was found to be associated with the duration of meals and not the rate of intake, whereas self-rated hunger was found to be associated with the rate of intake and not the duration of meals. Self-rated anxiety was not found to be associated with the number of people present, whereas self-rated elation was positively correlated with the presence of others. Multiple regression analyses suggested that the presence of other people facilitates intake and increases elation independently. It also suggested that social facilitation operates by independently increasing the size and the duration of meals and that it operates independently of the subjective state of the individual. These results contradict the predictions of increased arousal, increased hunger, and increased emotionality models but support attentional, disinhibitory, and time extension models of social facilitation.
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We have studied the effects of consumption of foods sweetened with either sucrose or aspartame on appetite ratings and food intake. Normal weight, non-dieting subjects ate the same amount of high- and low-calorie versions of pudding or jello and despite the resulting difference in caloric intake, showed only a non-significant trend towards compensation in a lunch one or two hours later. There were no significant differences between rated hunger, fullness, desire to eat, the amount subjects wanted to eat, or sensory-specific satiety following the high- and low-calorie foods. Knowing the caloric values of the foods did not influence intake or appetite ratings in that both informed and uninformed subjects responded similarly. Thus in the short term subjects tended to eat a constant amount of a particular food and this volume had a greater effect on appetite ratings and subsequent intake than the calories consumed.
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The effects of consuming equicaloric preloads with different macronutrient contents on the development of satiety were investigated. Ten normal-weight, nondieting women fasted overnight and then rated hunger, fullness, and food preferences, and the pleasantness of the taste of food samples before and after five different preloads. The greatest changes in the pleasantness of the taste of the food occurred for the eaten food relative to the uneaten food. Macronutrient content of the preloads did not significantly influence the magnitude of these changes or the pleasantness of foods with similar nutrient contents. Therefore, no evidence of nutrient-specificity was obtained. Nutrient composition had a differential effect on hunger, fullness and food intake. Ratings of hunger decreased and fullness increased following the high starch and high protein preloads to a greater extent than after the high fat, high sucrose and mixed content preloads. When a self-selection meal was offered 2 hours after the preloads energy intake and preference for high carbohydrate and high fat food items were significantly decreased by the high protein and high starch preloads. However, no specific reduction in macronutrient intake was observed. In conclusion, differential effects of the preloads were observed in subjective ratings of hunger, fullness, preferences and subsequent food intake, but there was no indication that satiety was macronutrient-specific.