Type 2 Diabetes and obesity: A review

Abstract

Abstract:
The article reviews the relationship between type 2 diabetes and obesity. This also includes types of obesity and its genetic predisposition. The modern generalization of sedentary life and caloric abundance has created new physiological conditions capable of changing the level of expression of a number of genes involved in fuel metabolism and body weight regulation. It is likely that the genetic variants or alleles of these genes have in the past participated in the adaptation of human physiology to its evolutionary constraints. In this article, we underscore the importance of obesity in relation to disorders of diverse etiologies characterized by disturbances of free fatty acids, visceral adiposity and insulin resistance. Further, we have investigated the role of selecting the traits to be subjected to quantitative genetic analysis in the occurrence of obesity.
Key words:Diabetes, Obesity, Review

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Review Article:
Type 2 Diabetes and obesity: A review
* A. Hussain1, M.Z.I. Hydrie 1,2, B. Claussen1, S. Asghar1
Abstract:
The article reviews the relationship between type 2 diabetes and obesity. This also includes types of
obesity and its genetic predisposition. The modern generalization of sedentary life and caloric
abundance has created new physiological conditions capable of changing the level of expression of
a number of genes involved in fuel metabolism and body weight regulation. It is likely that the genetic
variants or alleles of these genes have in the past participated in the adaptation of human physiology
to its evolutionary constraints. In this article, we underscore the importance of obesity in relation to
disorders of diverse etiologies characterized by disturbances of free fatty acids, visceral adiposity and
insulin resistance. Further, we have investigated the role of selecting the traits to be subjected to
quantitative genetic analysis in the occurrence of obesity
Key words: Diabetes, Obesity, Review
Introduction:
Diabetes mellitus (DM) and obesity have a
complex relationship, with type 2 diabetes
strongly associated with obesity [1]. Obesity
stands out as a risk factor for Type 2 DM, but we
see some lean type 2 diabetes subjects probably
having Latent Autoimmune Diabetes in Adults
(LADA). Thus obesity may be a precursor for Type
2 DM, following insulin resistance [2-3]. Most
researchers consider that this relationship is
different in different types of obesity and Type 2
DM [4]. Further studies are needed to fully
understand this.
Causes of obesity are probably different for many
types. Genetic disposition is clearly one [5-7].
Different demographic groups according to
lifestyle and genetics must be studied in a
comprehensive way in order to understand more
of these patterns.
1 Institute of General Practice and Community
Medicine, Department of International Health,
University of Oslo, Norway
2 Research Department, Baqai Institute of
Diabetology and Endocrinology, Baqai Medical
University, Karachi – Pakistan
*Corresponding Author:
Prof. Akhtar Hussain
Institute of General Practice and Community
Medicine, Department of International Health,
University of Oslo, Norway
Email: akhtar.hussain@medisin.uio.no
For treatment and prevention of Type 2 DM,
reduction of obesity is a key goal all over the
world [6, 8, 9]. The main treatment of both
conditions is reduced caloric intake and
increased physical activity. Newly diagnosed
cases of Type 2 DM and obesity are always
treated in this way.
Free fatty acids and triglycerides in the
development of Type 2 DM
Free fatty acids (FFA) are a major source of
energy for liver, kidney and skeletal muscle and a
key substrate for triglyceride production by the
liver. In periods of prolonged fasting, FFA provide
an alternative energy source to glucose,
preserving glucose for cerebral requirements and
also preserving body proteins, which can serve as
substrates for gluconeogenesis. FFA are stored in
the body in the form of triglycerides, the vast
majority of which are located in white adipose
tissue, and are released from triglycerides by the
process of lipolysis. After transportation into the
tissue, FFA are mainly oxidized in muscle cells to
release energy, or converted into lipoproteins by
the liver. The enzyme that controls the rate-
limiting step for mobilization of triglycerides in
adipose tissue is hormonally regulated. Insulin is
one of the main hormones involved in this
regulatory process [10], and the most potent
antilipolytic hormone [11].
In insulin resistance, the insensitivity of adipocytes
to insulin, results in elevated FFA [12-13] which is a
characteristic feature of Type 2 DM [5, 6] and is
strongly implicated in the development of insulin

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resistance and beta-cell dysfunction. Thus, it is
becoming increasingly apparent that reducing
FFA levels is an important goal in the
management of patients with Type 2 DM.
Prospective epidemiological studies have shown
that an elevated FFA level is a risk marker for
long-term development of glucose intolerance
and progression to Type 2 DM [7], in addition to
being associated with several other independent
risk factors for cardiovascular diseases [1-3,8,14].
It is becoming increasingly clear that
management of dyslipidaemia is of equal
importance to control of hyperglycaemia and
hypertension in the care of patients with Type 2
DM. The majority of these patients are obese and
have elevated plasma FFA levels [5]. It has been
suggested that FFA may be an important link
between obesity, insulin resistance and Type 2
DM [4]. Insulin resistance might indeed originate
in the adipose tissue [15] by mechanisms as
suggested in Box 1 [16].
Box 1. Hypothesis: How Obesity Causes Insulin
Resistance
The Adipokine Hypothesis
Obesity leads to an alteration in the profile of
hormones secreted by adipose tissue
(adipokines). In the obese state, adipose tissue
secretes proportionally more adipokines that
cause insulin resistance and fewer that promote
insulin sensitivity.
The Inflammation Hypothesis
Obesity is associated with an increase in
adipocyte secretion of chemokines, which
promote macrophage infiltration [17]. In addition
to increased macrophage infiltration, obesity is
also associated with increased macrophage
activation. Activated macrophages produce
cytokines that can negatively impact insulin
sensitivity [17].
Interplay between central obesity and free fatty
acids
Central obesity, i.e. fat accumulation in the
subcutaneous abdominal and visceral depots, is
most strongly associated with the risk of
metabolic and cardiovascular complications [18-
20]. Upper-body obesity results in an increased
mass effect in the visceral region coupled with
increased mobilization of FFA from the individual
fat cells in the visceral depot into the portal vein
[21]. The combination of these factors produces
markedly elevated 'portal' FFA levels in obese
subjects, resulting in hyperglycaemia,
hyperinsulinaemia, and hepatic insulin resistance
(Figure 1). In addition, although the effect is less
marked, the increase in upper-body
subcutaneous fat in obese subjects generate an
excess of FFA in the peripheral circulation, which
is likely to inhibit insulin-stimulated glucose uptake
in muscle and, maybe, impaired insulin secretion
by the pancreas (10).
Figure 1: FFA turnover in visceral and
subcutaneous adipose tissues From: Arner P.
Diabetes obesity and Metabolism 3 (s1) 11-19,
2001.
Visceral obesity and Adipocytokines
Several studies have investigated the biological
characteristics of both visceral and
subcutaneous adipose tissue by analysis of the
gene-expression profile to establish a molecular
basis of visceral fat related disease.
Approximately 20% of all genes in subcutaneous
adipose tissue and about 30% in the visceral
adipose tissue have been identified. These
bioactive substances are classified as
‘adipocytokines’, and are subdivided into
adipocytokines which are adipose tissue-specific
bioactive substances (i.e. leptin and
adiponectin) and adipocytokines abundantly
secreted from adipose tissue, but which are
nonspecific for adipose tissue (i.e. PAI-1, tumor
necrosis factor-alpha, interleukins, etc.) [22].
Adipocytokines are involved both in regulation of
glucose and lipid metabolism, control of
oxidative stress and in maintenance of the
vascular wall integrity. For example, TNF- a, IL-6
and leptin are able to induce insulin resistance,
whereas adiponectin improves insulin sensitivity.

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Several studies suggest that leptin may be
considered an important link between central
obesity, hypertension and Metabolic Syndrome.
Results from an Italian prospective study
emphasize that higher circulating plasma leptin
levels are a significant predictor of the risk of
Metabolic Syndrome and, in particular, of higher
blood pressure and IFG components [23].
Insulin resistance
Visceral obesity may play an important role in
developing Insulin resistance, through certain
inflammatory cytokines, such as IL-6, TNF-alpha,
TGF b1 and monocyte chemotactic protein-1,
produced by the resident fat macrophages (Box
1). These inflammatory cytokines are involved in
the increased cardiovascular risk of the obese
patient, independent of their role in insulin
resistance (Figure 2).
Figure 2: Role of visceral obesity in the
pathogenesis of metabolic syndrome. * From
Kadowachi et al (24)
Visceral obesity is often associated with the
endothelial up-regulation of the adhesion
molecules, hypercoagulability. The inflammatory
background is also probably promoted by a
concomitant deficit of nitric oxide, which also
appears to decrease the vasodilator properties
of perivascular adipose tissue (PVAT) leading to
hypoxia, inflammation, and oxidative stress. (25-
28). Nevertheless, genetic factors play a role in
the genesis of insulin resistance. Visceral obesity
and hyperinsulinemia contribute also to the
arterial hypertension. Accordingly, a vicious circle
possibly develops between sympathetic
activation, hyperinsulinemia and weight gain in
patients with visceral obesity. Clinical and
experimental studies in obese subjects
demonstrate sympathetic hyperactivity,
accelerated regional kinetics of catecholamines,
excited neuromuscular activity, dysregulation in
sodium-modulator hormones and pre-clinical left
ventricular dysfunction (29-32).
The biological effects of adiponectin in humans
The first indication that adiponectin might have a
role in human obesity derives from the report of
Hu et al., indicating that the expression of
adiponectin using Northern blots is reduced in
the adipose tissue of obese mice and humans
(33,34). Adiponectin is the only fat protein that is
down regulated with weight gain, and it is
possible that an accumulation of visceral fat
might produce inhibiting factors for adiponectin
synthesis or secretion, such as TNF-alpha (35, 36).
Plasma adiponectin levels are higher in women
than in men, and in non-obese compared to
obese subjects [37]. Lower plasma levels of
adiponectin are predictive of type 2 DM and are
found in diabetic subjects, and in patients with
hypertriglyceridemia, low HDL-cholesterol and
hypertension (22, 37-39).
In fact, antiatherogenic effects of adiponectin
have also been demonstrated in some clinical
studies, indicating that higher adiponectin levels
are associated with a reduced risk of acute
myocardial infarction in men (40). However,
other prospective studies (41,42) have not
reported a significant cardioprotective effect of
adiponectin. These conflicting results raise the
possibility that adiponectin may have different
prognostic implications in populations with
different risk for vascular disease.
Link between Adiponectin, obesity and insulin
resistance
Animal experiments using an injection of
recombinant adiponectin proteins and studies on
adiponectin KO mice have demonstrated that
adiponectin produces effects on both body
weight and insulin sensitivity in the liver and
muscle (37). In a prospective human study
hypoadiponectinemia did not predict obesity
but did predict the development of type 2 DM.

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Interestingly, significant body weight reduction in
humans is shown to raise plasma adiponectin
levels and improve insulin sensitivity (43).
Patterns of genetic investigation of Obesity and
Type 2 Diabetes
It is difficult to identify a genetic influence on
continuous traits in humans, because it is difficult
to separate environmental factors from genetic
effects. This “multiple-factor hypothesis” (a large
number of genes, each with a small effect,
producing quantitative variation) is known as
quantitative genetics. Obesity may be dissected
into several partial phenotypes, which may be
called "descriptive traits" (DT) for genetic
analyses. This approach has been widely used in
genome scan research for new loci. Traits can be
anthropometric or body composition
parameters. For example, instead of being
defined as obese, a young individual can be
characterized as having a BMI of 39 kg/m2, a fat
mass of 49 kg, a subcutaneous depot of 41 kg, a
waist-to-hip ratio of 1.21, etc. The status of obesity
can also be characterized biologically in such
patients with fasting serum levels of leptin at 45
pmol/l, insulin at 33 µU/ml, FFAs at 0.73 mmol/l,
etc., with the hope that these DT’s could partially
reflect disease pathogenesis. The difficulty is that
most physiological systems have hierarchical
components, leading from the gene to its
product, to intermediate phenotypes of greater
complexity, to the ultimate phenotypes used to
diagnose disease, in this case obesity as shown
in figure 3. Thus high leptin, insulin, or FFA levels
can be implicated in both the causal
mechanisms and the consequences of the
obese status (Figure 3).
Measuring "thrifty" pathogenic traits during youth
The study of young subjects is generally
recommended for the genetic study of complex
diseases (44). There are several reasons why this
could be particularly important in the obesity-
diabetes field.
First, evolutionary forces may have shaped the
human genome according to mechanisms that
are now directly involved in the pathophysiology
of juvenile obesity and associated changes in
insulin-fuel homeostasis, such as the fact that
these physiological functions were of major
importance for our ancestors. It is likely that
prehistoric metabolic genes welcomed new
mutations that favoured storage of calories (fat
storage and mobilization, insulin secretion and
sensitivity, leptin signalling, weight and body
composition regulation, availability of glucose to
the brain) (9, 45-46).
Figure 3. Schematic diagram of the relationships
of genes and their products to intermediate
phenotypes concurring with weight regulation
and body adiposity AR: adrenergic receptor; HL,
high-level phenotypes; IL, intermediate
phenotypes; LL, low-level phenotypes. From:
Pierre Bougnères Diabetes.
Now these genes are exerting deleterious effects
on modern subjects because of an unexpected
caloric richness and sedentary environment.
Similarly, it is possible that insulin sensitivity
underwent evolutionary changes toward
increased channeling of glucose to the large
human brain rather than to the insulin-sensitive
muscle mass. Measuring these phenomena early
in life rather than in adulthood may more closely
reflect their evolutionary tendencies. In addition,
the life span of early humans was limited, and
evolution may have mostly worked on the
physiology of young people.
Studying young individuals meets the goals of
predictive genetic epidemiology because it
allows the follow-up of genotyped patients
through later phenotype evolution as well as
conducting clinical trials. It has also been
observed that the motivation and sampling of
siblings and parents for genetic analysis are
facilitated when medical traits, such as obesity,
are detected in children and adolescents. The
study of nuclear families with young sib-ships
favours the analysis of sibling pairs in a
comparable environment, as well as transmission
disequilibrium tests (47).

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Conclusion
Visceral obesity plays an important role in the
development of diabetes by mobilizing free fatty
acids and certain inflammatory cytokines
promoting insulin resistance. Studies on molecular
mechanism for visceral fat support this notion.
Adiponectin which is secreted from adipose
tissues is inversely correlated with weight gain. To
study the genetics of obesity and diabetes, it has
been suggested that a systematic approach to
common juvenile obesity or Type 2 DM genetics
based on measurement of related traits in a
pathogenic rather than descriptive perspective
may be more appropriate. To do so, individuals
should be studied long before the disease starts,
in a situation when traits can be measured
"intact". Further investigations are needed to
study the molecular mechanism of obesity for
insulin resistance and diabetes in different
populations since genetic predisposition plays an
important role in the genesis of insulin resistance.
Physician, Jean de Meyer, who in 1909 isolated
glucose lowering hormone from the pancreas
and gave it the name insulin (Latin, insula= Island,
as it was produced by islet cells [14].
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