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Dairy proteins and the control of satiety and obesity

Authors:

Abstract

Over the past decade there has been growing scientific evidence and public acceptance of the role that dietary protein plays in regulation of satiety, feed intake and obesity-related disorders. Dietary protein appears to suppress food intake and delay the return of hunger more than fats or carbohydrates in a manner not due to energy content alone. Also, high-protein diets support the maintenance of muscle mass when subjects reduce their energy intake, ensuring primarily adipose tissue loss. Some protein sources, particularly dairy, contain specific peptides or proteins that may elicit direct effects on satiety. The major proteins present in milk include beta-lactalbumin, alpha-lactoglobulin, immunoglobulins, bovine serum albumin, and the various caseins. In addition, processed whey contains glycomacropeptide, which stimulates pancreatic and gastrointestinal secretion of hormones involved in satiety to a greater extent than whey alone. In the context of the literature, we show that a glycomacropeptide-rich whey protein isolate decreases feed intake and weight gain to a greater extent than a soy protein isolate in obese pigs. Also, insulin sensitivity is improved in pigs consuming high-protein diets, with these effects being independent of protein source. While, high-protein diets may decrease calcium balance and bone strength, it appears that these effects are attenuated by dairy proteins and dairy sources of calcium. These findings suggest that high-protein diets, and in particular those that contain whey proteins, may reduce hunger and food intake, thereby reducing fat deposition and improving insulin sensitivity.
Introduction
The role of dairy foods in human nutrition and their impact on
consumer health is one that has been hotly debated by both the
food and health industries. Dairy products have been frequently
associated with a ‘negative’health image as being high in fat and
being associated with increased obesity and a greater risk of
cardiovascular disease. However, this discussion often fails to
mention the fact that dairy products are among the most versatile
protein sources available and, in addition to a unique amino acid
composition, have several bioactive properties that make them
important nutrients for human health and development. This
paper focuses on some of the functional properties of dietary
protein, specifically dairy protein, with respect to satiety and
weight control. In addition to reviewing the literature, we will
provide evidence from our own studies with obese minipigs that
shows that a glycomacropeptide (GMP)-rich whey protein
isolate (WPI) decreases feed intake and weight gain to a greater
extent than a soy protein isolate (SPI).
Dietary protein
Satiety and hunger
There has been a growing interest in the role of dietary protein
in weight and appetite control. Indeed, several very popular
diets that have a high dietary protein content as their basis have
been published and advocated. Among these, the Atkins (Atkins
1992), Protein Power (Eades and Eades 1996) and CSIRO
Wellbeing (Noakes and Clifton 2005) diets have gained the most
traction at various times over the last decade. In a recent review
of the role of dairy proteins in satiety and weight control, four
lines of evidence were presented to support a role for dietary
proteins in the regulation of food intake and weight maintenance
(Anderson and Moore 2004). First, proteins suppress food
intake more than fats or carbohydrates and the extent of this
reduction is greater than can be accounted for by their energy
content alone. Second, proteins make a stronger contribution to
satiety and delay the return of hunger compared with fat and
carbohydrates. Third, high-protein diets support the
maintenance of lean body mass under circumstances of energy
restriction, thereby promoting weight loss primarily as adipose
tissue. Finally, protein digestion leads to the stimulation of many
physiological and metabolic responses involved in the
regulation of feed intake. In addition, some protein sources
contain specific peptides that may elicit direct effects on satiety
and metabolism. This fact can be added to the arguments
mentioned above.
Several studies have investigated the effect of high-protein
diets on hunger, satiety and energy intake (see reviews by
Eisenstein et al. 2002; Anderson and Moore 2004; Layman and
Baum 2004). Although responses may vary because of the
methodologies used, the general consensus from these reviews
and studies is that high-protein diets or meals have the potential
to suppress hunger to a greater degree than diets or meals high
in carbohydrates or fat. Furthermore, the enhanced sensations of
satiety may lead to lower energy intakes and greater weight loss
or better weight control. Of course, in reviewing these studies it
is important to take into account that some studies looked at
acute or short-term responses to a single preload (often a drink)
Australian Journal of Experimental Agriculture, 2007, 47, 1051–1058
0816-1089/07/09105110.1071/EA06263© CSIRO 2007
Frank R. Dunshea
A,B,C
, Ewa Ostrowska
A
, Josie M. Ferrari
A
and Harsharn S. Gill
A
A
National Centre of Excellence in Functional Foods, Department of Primary Industries,
Werribee, Vic. 3030, Australia.
B
Faculty of Land and Food Resources, The University of Melbourne, Parkville, Vic. 3010, Australia.
C
Corresponding author. Email: fdunshea@unimelb.edu.au
Abstract. Over the past decade there has been growing scientific evidence and public acceptance of the role that dietary
protein plays in regulation of satiety, feed intake and obesity-related disorders. Dietary protein appears to suppress food
intake and delay the return of hunger more than fats or carbohydrates in a manner not due to energy content alone. Also,
high-protein diets support the maintenance of muscle mass when subjects reduce their energy intake, ensuring primarily
adipose tissue loss. Some protein sources, particularly dairy, contain specific peptides or proteins that may elicit direct
effects on satiety. The major proteins present in milk include β-lactalbumin, α-lactoglobulin, immunoglobulins, bovine
serum albumin, and the various caseins. In addition, processed whey contains glycomacropeptide, which stimulates
pancreatic and gastrointestinal secretion of hormones involved in satiety to a greater extent than whey alone. In the context
of the literature, we show that a glycomacropeptide-rich whey protein isolate decreases feed intake and weight gain to a
greater extent than a soy protein isolate in obese pigs. Also, insulin sensitivity is improved in pigs consuming high-protein
diets, with these effects being independent of protein source. While, high-protein diets may decrease calcium balance and
bone strength, it appears that these effects are attenuated by dairy proteins and dairy sources of calcium. These findings
suggest that high-protein diets, and in particular those that contain whey proteins, may reduce hunger and food intake,
thereby reducing fat deposition and improving insulin sensitivity.
Dairy proteins and the regulation of satiety and obesity
www.publish.csiro.au/journals/ajea
CSIRO PUBLISHING
Dairy Science Research Front
F. R. Dunshea et al.1052 Australian Journal of Experimental Agriculture
or meal while others looked at a more chronic response to the
consumption of a diet. In one such short-term study, Stubbs
et al. (1997) reported that men who consumed a high-protein
breakfast had lower subjective hunger, and higher fullness
scores compared with men who had a high-fat or high-
carbohydrate diet. However, there was no difference in
subsequent lunch or dinner intakes despite the lower hunger
scores. In contrast, Latner and Schwartz (1999) found that
women who ate a high-protein lunch subsequently consumed
31% less energy and exhibited less pre-dinner hunger sensations
compared with women consuming a high-carbohydrate meal.
Similarly, Poppitt et al. (1998) reported that women consuming
a high-protein meal exhibited greater satiety and had lower
energy intakes than women consuming a high-carbohydrate
meal. High-protein and low-fat meals provided to preschool
children resulted in greater short-term satiety and lower food
intake in the subsequent meal than a high-carbohydrate, low-fat
diet. Porrini et al. (1997) found that subjects consuming a
high-protein preload displayed a greater degree of intrameal and
postmeal satiety than those consuming a high-fat preload.
Adults fed a protein-rich lunch consumed less energy in a
subsequent meal than subjects who consumed a low-protein
lunch (Booth et al. 1970).
In addition to the acute effects of high-protein meals on
increasing satiety and reducing energy intake, several more
chronic studies have reported that subjects were more likely to
adhere to a high-protein diet rather than a high-carbohydrate
diet as part of a weight loss program. For example, Skov et al.
(1999) found that men and women consuming ad libitum a
high-protein (meat and dairy), low-fat diet lost 8.9 kg in
6 months compared with a loss of 5.1 kg by those consuming a
high-carbohydrate diet. In addition, more subjects on the
high-protein diet adhered to the diet and achieved the clinically
relevant weight loss (35%) compared with those consuming the
high-carbohydrate diet (9%). A study investigating the impact of
a high-protein meat diet compared with a high-carbohydrate
vegetarian diet on satiety and weight loss found that subjects
consuming the meat diet at lunch ate 12% less at dinner than
those consuming the vegetarian diet, resulting in greater weight
loss (Barkeling et al. 1990).
Increased thermogenesis and postprandial metabolism
It has also been hypothesised that consumption of high-protein
meals increases postprandial energy expenditure over that
observed when meals rich in carbohydrate are consumed
(Eisenstein et al. 2002; Halton and Hu 2004). Typically, the
thermic effect of feeding represents 10–15% of total energy
expenditure (Sims and Danforth 1987). Robinson et al. (1990)
reported that men consuming a high-protein diet had a higher
thermic response and whole-body nitrogen turnover than men
consuming a high-carbohydrate diet. In addition, the metabolic
cost of the protein synthesis was 68 and 36% in subjects
consuming the high-protein and high-carbohydrate diets,
respectively. Similarly, Karst et al. (1984) reported that the
energy expenditure following a high-protein meal was three
times greater than energy expended after a high-carbohydrate
meal. Nair et al. (1983) reported that the thermic response was
greater and also more prolonged in subjects fed a high-protein
diet compared with a high-carbohydrate diet. Halton and Hu
(2004) conducted a meta-analysis of the available data and
concluded that there is convincing evidence that a higher protein
intake increases thermogenesis compared with diets of lower
protein content. Schoeller and Buchholz (2005) reviewed the
literature on diet composition and energy expenditure and
concluded that although the carbohydrate content of the diet
per se had no effect on energy expenditure, where carbohydrate
was replaced by protein, there was an increase in energy
expenditure. These studies clearly indicate that the consumption
of high-protein diets by normal or obese subjects is associated
with greater postprandial energy expenditure.
However, the contribution of this increased thermogenesis to
weight loss is unknown, as most studies were acute and
conducted over a maximum of only 24 h. In one of the few
chronic studies, Noakes et al. (2005) fed energy restricted
(5600 kJ/day) diets that were either high in protein or high in
carbohydrate to overweight women and found that although
weight loss was the same in both groups, subjects with high
serum triacylglycerol (>1.5 mmol/L) lost more fat mass and
experienced a greater decrease in circulating triglycerides with
the high-protein diet than with the high-carbohydrate diet.
Given that these subjects were consuming the same amount of
energy per day, these data suggest that there is a higher energy
expenditure in the subjects consuming the high-protein diet.
Mikkelsen et al. (2000) studied energy expenditure over 4 days
in subjects consuming energy restricted diets that were either
high in carbohydrate or high in soy or pork protein. Mean
energy balance was negative in all dietary groups but was more
negative in the subjects consuming the high-protein diets. Men
consuming the high-protein pork diet had the lowest energy
balance over the entire 4-day period and during the chamber stay
on day 4. The difference in energy balance could largely be
ascribed to the higher diet-induced thermogenesis in these
subjects (Mikkelsen et al. 2000).
Another possible means by which dietary protein could
stimulate postprandial metabolism and satiety is through portal
sensing of intestinal gluconeogenesis from digested and
absorbed amino acids (Mithieux et al. 2005). In an elegant
series of studies in rats, these authors were able to demonstrate
that consumption of dietary protein increased gluconeogenesis
in the small intestine with subsequent release of glucose into
portal blood. The resultant decrease in food intake was similar
to that observed in rats in which a quantitatively similar amount
of glucose was infused directly into the portal vein. In turn, this
reduction in food intake could be completely reversed through
denervation of the portal vein. There are glucose-sensitive cells
that transmit a signal to the brain via afferences of the vagus
nerve that are present in the wall of the portal vein (Thorens and
Larsen 2004). Therefore, Mithieux et al. (2005) suggest that
intestinal gluconeogenesis from amino acids may be one means
by which dietary protein decreases appetite and food intake and
improves glycaemic control (see below).
Increased glycaemic control
Diets designed to lower the insulin response to ingested
carbohydrate [i.e. low glycaemic index (GI) foods] may improve
access to stored metabolic fuels, decrease hunger, and promote
weight loss (Ludwig 2000). The GI is a property of
carbohydrate-containing foods that predicts the circulating
Australian Journal of Experimental Agriculture 1053
blood glucose response. As protein has a minimal short-term
effect on blood glucose compared with carbohydrates, meals
high in protein have been used to reduce the GI of meals.
Farnsworth et al. (2003) reported that subjects consuming a
high-protein diet had a lower glycaemic response than those
consuming a standard protein, high-carbohydrate diet.
Similarly, Layman et al. (2003) reported that women who
consumed a high-protein diet had a lower insulin response to
meals, greater satiety (subjective score), lost more bodyweight
and had a greater fat:lean loss compared with women
consuming a high-carbohydrate diet. Nilsson et al. (2004)
examined the glycaemic response to several dietary proteins and
found a lower GI compared with whole wheat bread, although
there was variability among protein sources. Interestingly, whey
and milk had low and similar glucose responses but whey had a
markedly higher insulin response.
Dairy proteins
Satiety, weight loss and metabolism
There is some evidence that different types of proteins may
have differing effects on satiety. For example, anecdotal
observations indicate that different types of meat protein effect
satiety and weight loss to varying degrees, with red meat (beef,
lamb and pork) being more filling and resulting in greater
weight loss than white meat (chicken and fish). However,
Melanson et al. (2003) reported similar weight and fat loss in
subjects fed beef or chicken meals. Similarly, Uhe et al. (1992)
reported no difference in satiety between beef and chicken, but
found that satiety was greater following the consumption of a
meal containing fish compared with a meal containing beef or
chicken. The case for differential effects of specific dairy
proteins on satiety appears to be stronger, although is still not
conclusive.
Food intake was suppressed to a greater extent in rats
gavaged with whey protein as compared with egg albumen or
soy protein (Morgan 1998). However, others have found that the
source of protein had no effect on food intake suppression. Lang
et al. (1998) compared six different protein sources including
casein, gelatin, egg albumen, gluten, soy protein and pea protein
and found no difference in energy intake at the next buffet meal
8 h later. In a subsequent study, these same workers found that
appetite was reduced to a greater extent after consumption of a
high-protein meal containing gelatin than after a meal
containing casein (Lang et al. 1999). In a more recent study with
individuals on a weight-reducing caloric intake, it was found
that there was no difference in chronic feed intake or weight and
fat loss in individuals consuming either a high-dairy protein,
high-calcium diet (low-fat cheese and yoghurt as major protein
sources) or a mixed protein, low-calcium diet (lean ham, eggs as
major protein sources) (Bowen et al. 2005a). Although insulin
resistance was improved over the duration of the weight loss
program, there were no differences between the diets in any
metabolic parameters normally associated with insulin
resistance. Similarly, Bowen et al. (2005b) found that although
consumption of a high-protein diet reduced energy intake and
plasma gherelin (a stimulator of hunger) and increased plasma
glucagon-like peptide-1 (GLP-1) and cholecystokinin (CCK)
(inhibitors of hunger) compared with a glucose meal, there was
no difference between protein sources (WPI, SPI or gluten).
However, within dairy proteins there may also be differences
between whey and casein and other products of processing.
Milk contains a mixture of proteins, each having unique
attributes for nutritional, biological, and human food ingredient
applications (Smithers et al. 1996). The major proteins present
in milk include β-lactalbumin, α-lactoglobulin,
immunoglobulin, bovine serum albumin, and the caseins:
κ-casein, β-casein, and the α-caseins (Etzel 2004). Minor but
commercially important proteins are lactoferrin and
lactoperoxidase. In addition, rennet whey contains GMP, which
is cleaved from κ-casein by chymosin to initiate precipitation of
the caseins forming curd. Protein from bovine whole milk
consists of ~20% whey protein. When casein is removed from
whole milk, liquid whey remains, having a protein
concentration of ~65% on a dry matter basis. Several different
proprietary processes exist to further treat or purify whey
protein resulting in various WPIs, some of which may be rich in
specific bioactive peptides such as GMP. Recently, McIntosh
et al. (2005) demonstrated that supplementing diets containing
WPI with GMP, reduced weight gain, abdominal fat, plasma
insulin and lipid status in growing rats compared with diets
containing WPI alone as a protein source. Hall et al. (2003)
found that energy intake from a buffet meal was significantly
less 90 min after a 1700 kJ liquid preload containing 48 g whey,
compared with an equivalent casein preload. Also, subjects who
received the whey preload were less hungry and felt more full
both before and after the buffet meal. While these authors
provided the amino acid profiles of the whey and casein there
was no information on the GMP content of the whey.
Minipig model
In order to test whether a WPI rich in GMP has an effect on
food intake, bodyweight and other indices of obesity, we
conducted a study in obese minipigs as a model for the obese
human (Ferrari et al. 2005). The minipig is an excellent model
for obesity as it contains 50% body fat and displays insulin
resistance with respect to both glucose and amino acid uptake
(Dunshea et al. 2005a, 2005b). Sixteen obese female minipigs
(133 kg, 50% body fat) were randomly allocated to a 2 × 2
factorial design with the respective factors being source of
protein [WPI (NaturaPro MG2460, MG Nutritionals,
Brunswick, Victoria, Australia) v. SPI (Profarm 974, ADM, Palm
Beach, Queensland, Australia) or level of dietary protein – 11%
low-protein v. 22% high-protein crude protein]. The WPI
contained 46, 30, and 8% β-lactalbumin, GMP and
α-lactoglobulin, respectively. After consuming their respective
diets for 10 weeks, the surgically prepared pigs were infused
intravenously with insulin at 0.6 and 6.0 mU/(kg.min) and blood
glucose and amino acids clamped at preinfusion values by
simultaneous infusions of dextrose (50% w/v) and a parenteral
amino acid mix (10% w/v), respectively, using the
hyperinsulinemic/euglycemic/eulysinemic clamp (Dunshea
et al. 1992a, 1992b, 1995, 2005c; Wray-Cahen et al. 1997).
Composition of the ham region was determined by dual energy
X-ray absorptiometry (DXA) at 0, 4 and 8 weeks (Suster et al
.
2003).
Food intake was decreased by feeding the high-protein diet
(2070 v. 2352 g/day for high- and low-protein, respectively,
Dairy proteins and the regulation of satiety and obesity
F. R. Dunshea et al.1054 Australian Journal of Experimental Agriculture
P < 0.001; Fig. 1), although average food intake varied
(P < 0.001) from week-to-week with the response decreasing
with time. There was no main effect of type of dietary protein on
food intake (2179 v. 2243 g/day for pigs consuming whey and
soy diets, respectively, P = 0.42) but there was an interaction
(P = 0.027) between type and level of protein. Thus, feed intake
was decreased by feeding a high-protein diet when the major
protein source was WPI (1951 v. 2408 g/day) to a greater extent
than when the protein source was SPI (2189 v. 2296 g/day).
Most pigs gained weight over the duration of the study although
the weight gain was lower in pigs consuming the high-protein
diet than in pigs consuming the low-protein diet (231 v.
382 g/day, P = 0.045). Although there was no main effect of type
of protein on weight gain, there was an interaction such that the
high level of dietary protein decreased weight gain to a greater
extent in the pigs consuming WPI in comparison with those
consuming SPI.
The DXA analyses confirmed that at the commencement of
the study the minpigs were indeed obese and contained 49.9 ±
1.25% (mean ± s.e.) fat and 38.0 ± 1.25% lean tissue. During the
study there was no effect of type of dietary protein on ham fat
content (13.2 v. 13.0 kg for SPI and WPI respectively, P = 0.74),
while the fat content tended to be lower in minipigs consuming
the high-protein diets (13.5 v. 12.7 kg, P = 0.085). There was no
effect of type of dietary protein (17.7 v. 18.0 kg for SPI and WPI
respectively, P = 0.54) or protein level (17.7 v. 18.0 kg, P = 0.28)
on the amount of lean tissue in the ham. During the study there
was no effect of type of dietary protein on the ratio of fat:lean
tissue in the ham (0.75 v. 0.72 for soy and whey protein,
respectively, P = 0.55), whereas the ratio of fat:lean tissue was
lower in minipigs consuming the high-protein diets (0.76 v.
0.70, P = 0.026).
Protein source had no effect on the amount of dextrose
infused to maintain euglycemia (108 v. 115 mL/h P = 0.59) but
the amount infused was lower in the minipigs fed the low-
protein diet (101 v. 125 mL/h, P = 0.048, Fig. 2). Protein source
had no effect on the amino acid infusion rate required to
maintain plasma lysine concentrations (50 v. 50 mL/h, P = 0.98)
but the amount infused was lower in pigs fed the low-protein
diet (45 v. 55 mL/h, P = 0.030). Taken together, these data show
that a high-protein diet reduced feed intake, weight gain and fat
deposition and reduced insulin resistance in obese minipigs. The
high-protein diet containing WPI that was enriched in GMP had
the greatest effect on feed intake and weight gain. The obese
minipigs appeared to be more insulin resistant than younger
pigs (Dunshea et al. 2005a), but nevertheless the higher protein
diets decreased insulin resistance with respect to both glucose
and amino acid utilisation. With respect to metabolic parameters
of insulin resistance, the source of dietary protein did not appear
to be important. The observations on insulin resistance are
interesting given that several researchers have found that milk,
and in particular whey, elicit low postprandial glucose responses
but relatively high insulin responses (Östman et al. 2001;
Nilsson et al. 2004). This insulin response may be due to some
intrinsic factor in dairy proteins although pea amino acid
hydrolysates also stimulate insulin secretion (Calbet and
MacLean 2002), suggesting that it is the rate of appearance of
amino acids that stimulate insulin secretion. In turn, the
stimulation of increased insulin secretion after consuming high-
protein diets may be in part a response to increased intestinal
gluconeogenesis (Mithieux et al. 2005). Also, the heightened
insulin secretion in subjects consuming high-protein diets may
be in part causative of the reduction in hunger and may be
related to heightened whole body insulin sensitivity.
Effects of GMP on satiety and feed intake may be mediated
via the gastrointestinal hormone CCK, which is a potent
satiating signal. In humans, GMP is a potent stimulator of CCK
secretion (Corring et al. 1997), while in rats it stimulates
0
500
1000
1500
2000
2500
Soy protein
isolate
Whey protein
isolate
Food intake (g/day)
Fig. 1. Effect of source and amount of dietary protein on feed intake in
obese minipigs. Sources of dietary protein were soy protein isolate (SPI) and
whey protein isolate (WPI) enriched with glycomacropeptide fed at either
11% (low protein, shaded bars) or 22% (high protein, black bars) of total
dietary protein. Significances of effect of type of protein, level of protein
and interaction were P = 0.42, P < 0.001 and P = 0.027, respectively (Ferrari
et al. 2005).
0
100
200
300
0.6 6.0
Dose of insulin [mU/(kg.min)]
Glucose infusion rate (mL/h)
Fig. 2. Effect of amount of dietary protein on the amount of exogenous
glucose (50% dextrose) required to maintain glycaemia during a
hyperinsulinemic/euglyacemic/eulysinic clamp conducted at two doses of
insulin [0.6 and 6.0 mU/(kg.min)]. Sources of dietary protein were soy
protein isolate (SPI) and whey protein isolate (WPI) enriched with
glycomacropeptide fed at either 11% (low protein, shaded bars) or 22%
(high protein, black bars) of total dietary protein. Data have been pooled
across dietary protein source as there was no effect of protein type
(P = 0.59). Significances of effect of level of protein and dextrose were
P = 0.048 and P < 0.001, respectively (Ferrari et al. 2005).
Australian Journal of Experimental Agriculture 1055
pancreatic secretions (which contain CCK) with a much greater
potency than whey protein alone (Pedersen et al. 2000). Plasma
CCK, GLP-I and glucose-dependent insulinotropic polypeptide
were higher in subjects who consumed a liquid whey
supplement as compared with subjects who consumed an
equivalent amount of a casein supplement (Hall et al. 2003).
Alternatively, Bowen et al. (2005b) found that although plasma
CCK and GLP-1 were higher and gherelin lower in humans after
consuming a protein meal compared with a glucose meal, there
was no difference between dairy or other proteins. Dairy
proteins also are much higher in branched chain amino acids
(BCAA), particularly leucine, than vegetable protein sources
and some have proposed that high levels of BCAA can inhibit
feed intake (Layman 2003). In this context, plasma amino acid
concentrations were 28% higher in subjects who consumed a
liquid whey supplement as compared with subjects who
consumed an equivalent amount of a casein supplement (Hall
et al. 2003). In addition, high BCAA concentrations may
conserve muscle tissue and ensure that weight loss is primarily
as fat during energy restriction.
Calcium balance
The role of high-protein diets, especially the Atkins and the
Protein Power diets, in weight loss have however been widely
criticised for possible adverse effects on calcium balance,
cardiovascular disease and renal and liver function. However,
the data on the effects of high-protein diets on calcium
metabolism are equivocal, with negative calcium balance and
bone loss implicated in some studies (Abelow et al. 1992) but
not others (Lacey et al. 1991; Chiu et al. 1997; Munger et al.
1999). These differences may be largely due to differences in
calcium levels and amino acid composition between the
experimental treatments. Furthermore, separate treatment of
calcium as a variable can be an oversimplification because
calcium intake may be associated with high intakes of protein
and other nutrients (Holbrook and Barrett-Conner 1991).
One epidemiological study associated long-term
consumption of diets high in animal protein with increased hip
fracture rates in an elderly population (Abelow et al. 1992).
These authors attributed this to increased glomerular filtration
rate and decreased fractional renal reabsorption, which in turn
may be mediated by changes in acid load (Barzel 1995) or
increased circulating insulin concentrations (Kerstetter and
Allen 1990). However, it is unclear whether this is true for both
animal and plant protein sources. For example, it may be that
dairy proteins may prevent osteoporosis by providing key
nutrients important to bone development and maintenance, by
enhancing calcium absorption or retention, by building peak
bone mass or by suppressing bone turnover, and, therefore, bone
loss (Weaver and Liebman 2002). Bowen et al. (2004) looked at
indices of bone turnover and calcium metabolism in human
subjects consuming high-protein, energy restricted diets
formulated around either dairy or mixed proteins. Calcium
excretion decreased during both interventions perhaps due to
the reduction in energy intake. By week 16, the subjects
consuming the mixed protein diet had a 40% larger increase in
deoxypyridinoline (a bone turnover marker) compared with
those consuming the dairy protein diet. Osteocalcin (a marker of
bone formation) increased in subjects consuming the mixed
protein diet only. Overall, the dairy protein conferred a modest
advantage over the mixed protein diet by reducing the
accelerated bone turnover associated with weight loss.
Using the growing pigs as a model for rapid bone growth
(Ikeda et al. 2003) we recently investigated the effects of
dietary source and level of protein on bone growth and bone
density (Cox et al. 2005). Pigs were fed one of six dietary
treatments formulated to be adequate for all nutrients and to
provide either 100, 140 or 180% of dietary protein
requirements using the same WPI and SPI used in the study
outlined previously. Body composition and bone mineral
content was measured using DXA. Male pigs had higher rates
of bone mineral deposition than females (22.1 v. 20.5 g/day,
P < 0.001) while pigs consuming WPI had higher rates of bone
mineral deposition than those consuming SPI diets (23.0 v.
20.5 g/day, P < 0.001). Similarly, the change in bone density
was greater in pigs consuming WPI than those consuming SPI
(4.69 v. 3.36 mg/cm
2
.day, P < 0.001). There was a linear
decrease in both bone mineral deposition rate and change in
bone density with increasing dietary protein content. These
data suggest that increasing dietary protein reduces bone
mineral deposition and the rate of change in bone density
during rapid growth but that this can be alleviated by WPI.
The mechanism by which dietary protein intake may
strengthen bone is still unclear, but an effect on the structure and
function of bone-related proteins is plausible. Differences in
protein quality and availability between dairy and vegetable
sources, related to amino acid distribution or associated dietary
constituents with effects on digestibility, absorption, and
metabolism of amino acids, may underlie the different
associations between dairy and vegetable protein intake and
bone development. Takada et al. (1996) found that milk whey
protein stimulated the proliferation and differentiation of
osteoblastic cells in vitro and milk whey protein, especially its
basic fraction (milk basic protein), was shown to suppress bone
resorption by its direct effects on osteoclasts in healthy adult
men (Toba et al. 2000). Toba et al. (2000) suggested that milk
cystatin in the milk basic protein fraction is one of the possible
components that prevents bone resorption by inhibiting
cathepsin, which is a protease secreted by osteoclasts. This was
confirmed in a more recent study by Brage et al. (2004), who
showed that cysteine proteinase inhibitors decrease formation of
osteoclasts by interfering at a late stage of preosteoclast
differentiation.
Effect of dairy calcium on fat and weight loss
No discussion about dairy protein and weight loss would be
complete without some reference to the recent interest in
calcium, specifically of dairy origin, on fat mobilisation.
Several publications have appeared over the past 5 years that
suggest that increasing dietary calcium intake without changing
energy intake will result in a net mobilisation of body fat (see
review by Zemel and Miller 2004). It has been hypothesised that
high calcium intakes lower intracellular calcium via lowered
calcitriol, resulting in reduced lipogenesis and increased
lipolysis and lipid oxidation (Zemel and Miller 2004; Zemel
2005). Moreover, it appears that dairy forms of calcium are
more efficacious than calcium alone. The augmented effects of
dairy-bound calcium have been isolated to the whey fraction
Dairy proteins and the regulation of satiety and obesity
F. R. Dunshea et al.1056 Australian Journal of Experimental Agriculture
and are possibly due to an interaction with other bioactive
compounds such as angiotension converting enzyme inhibitors
or GMP. While much of the data from Zemel’s laboratory on the
role of dairy calcium in fat loss is compelling, it should be noted
that not all studies support these findings. For example, there
was no difference in feed intake or weight and fat loss in
individuals consuming restricted amounts of either a high-dairy
protein, high-calcium diet or a mixed protein, low-calcium diet
(Bowen et al. 2005a).
Conclusions
It appears that high-protein diets increase satiety signals and the
feeling of fullness to a greater extent than other macronutrients.
As a consequence, high-protein diets have a higher acceptance
and conformance, especially when energy intake is restricted in
an effort to lose weight. The high amino acid content of a
high-protein diet protects muscle mass and ensures that most
weight is lost as fat. Some protein sources, particularly dairy,
contain specific peptides or proteins such as GMP that may
elicit direct effect on satiety. GMP stimulates pancreatic and
gastrointestinal secretion of hormones involved in satiety to a
greater extent than whey alone and so may contribute to the
satiating effects of whey and other dairy proteins. In addition,
the calcium found in dairy protein products such as whey can
also decrease fat deposition. In conclusion, high-protein diets,
and in particular those that contain whey proteins, may reduce
hunger and food intake, thereby reducing fat deposition and
improving insulin sensitivity.
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http://www.publish.csiro.au/journals/ajea
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