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In adipocytes, cortisol inhibits lipid mobilization in the presence of insulin, thus leading to triglyceride accumulation and retention.

Since the density of GC receptors is higher in intra-abdominal (visceral) fat than in other fat depots, the activity of cortisol leading to accumulation of fat is accentuated in visceral adipose tissue (24, 158), providing a mechanism by which excessive endogenous or exogenous GC lead to abdominal obesity and IR

obese patients generally have normal or subnormal plasma cortisol concentrations

This may be explained by an increased intratissular/cellular concentration of cortisol in adipose tissues

Intracellular GC may be produced from recycling of GC metabolites such as cortisone in adipose tissues

Local GC recycling metabolism is mediated by 11β-hydroxysteroid dehydrogenase enzymes (11β-HSD1 and 11β-HSD2

Cortisol also increases 11β-HSD1 expression in human adipocytes

In humans, elevated 11β-HSD1 expression in visceral adipose tissue is also associated with obesity

even if obese patients generally have normal or subnormal plasma cortisol concentrations (131, 158), triglyceride accumulation in visceral adipose tissue may be due, at least in part, to the local production of GC in insulin- and GC-responsive organs such as adipose tissue, liver, and skeletal muscle

kisspeptin has emerged as one of the most potent secretagogues of GNRH release

Consistent with the hypothesis that obesity-mediated inhibition of kisspeptin signalling contributes to the suppression of the HPT axis, infusion of a bioactive kisspeptin fragment has been recently shown to robustly increase LH pulsatility, LH levels and circulating testosterone in hypotestosteronaemic men with type 2 diabetes

Figure 4

Interestingly, a recent 16-week study of experimentally induced hypogonadism in healthy men with graded testosterone add-back either with or without concomitant aromatase inhibitor treatment has in fact suggested that low oestradiol (but not low testosterone) may be responsible for the hypogonadism-associated increase in total body and intra-abdominal fat mass

A smaller study with a similar experimental design found that acute testosterone withdrawal reduced insulin sensitivity independent of body weight, whereas oestradiol withdrawal had no effects

Obesity and dysglycaemia and associated comorbidities such as obstructive sleep apnoea (Hoyos et al. 2012b) are important contributors to the suppression of the HPT axis

This is supported by observational studies showing that weight gain and development of diabetes accelerate the age-related decline in testosterone

Weight loss can reactivate the hypothalamic–pituitary–testicular axis

The hypothalamic–pituitary–testicular axis remains responsive to treatment with aromatase inhibitors or selective oestrogen receptor modulators in obese men

Kisspeptin treatment increases LH secretion, pulse frequency and circulating testosterone levels in hypotestosteronaemic men with type 2 diabetes

Several observational and randomised studies reviewed in Grossmann (2011) have shown that weight loss, whether by diet or surgery, leads to substantial increases in testosterone, especially in morbidly obese men

This suggests that weight loss can lead to genuine reactivation of the gonadal axis by reversal of obesity-associated hypothalamic suppression

There is pre-clinical and observational evidence that chronic hyperglycaemia can inhibit the HPT axis

in those men in whom glycaemic control worsened, testosterone decreased

successful weight loss combined with optimisation of glycaemic control may be sufficient to normalise circulating testosterone levels in the majority of such men

weight loss, optimisation of diabetic control and assiduous care of comorbidities should remain the first-line approach.

In part, the discrepant results may be due to the fact men in the Vigen cohort (Vigen et al. 2013) had a higher burden of comorbidities. Given that one (Basaria et al. 2010), but not all (Srinivas-Shankar et al. 2010), RCTs in men with a similarly high burden of comorbidities reported an increase in cardiovascular events in men randomised to testosterone treatment (see section on Testosterone therapy: potential risks below) (Basaria et al. 2010), testosterone should be used with caution in frail men with multiple comorbidities

The retrospective, non-randomised and non-blinded design of these studies (Shores et al. 2012, Muraleedharan et al. 2013, Vigen et al. 2013) leaves open the possibility for residual confounding and multiple other sources of bias. These have been elegantly summarised by Wu (2012).

Effects of testosterone therapy on body composition were metabolically favourable with modest decreases in fat mass and increases in lean body mass

This suggests that testosterone has limited effects on glucose metabolism in relatively healthy men with only mildly reduced testosterone.

it is conceivable that testosterone treatment may have more significant effects on glucose metabolism in uncontrolled diabetes, akin to what has generally been shown for conventional anti-diabetic medications.

the evidence from controlled studies show that testosterone therapy consistently reduces fat mass and increases lean body mass, but inconsistently decreases insulin resistance.

Interestingly, testosterone therapy does not consistently improve glucose metabolism despite a reduction in fat mass and an increase in lean mass

the majority of RCTs (recently reviewed in Ng Tang Fui et al. (2013a)) showed that testosterone therapy does not reduce visceral fat

testosterone therapy decreases SHBG

testosterone is inversely associated with total cholesterol, LDL cholesterol and triglyceride (Tg) levels, but positively associated with HDL cholesterol levels, even if adjusted for confounders

Although observational studies show a consistent association of low testosterone with adverse lipid profiles, whether testosterone therapy exerts beneficial effects on lipid profiles is less clear

Whereas testosterone-induced decreases in total cholesterol, LDL cholesterol and Lpa are expected to reduce cardiovascular risk, testosterone also decreases the levels of the cardio-protective HDL cholesterol. Therefore, the net effect of testosterone therapy on cardiovascular risk remains uncertain.

data have not shown evidence that testosterone causes prostate cancer, or that it makes subclinical prostate cancer grow

compared with otherwise healthy young men with organic androgen deficiency, there may be increased risks in older, obese men because of comorbidities and of decreased testosterone clearance

recent evidence that fat accumulation may be oestradiol-, rather than testosterone-dependent

Thus, while glucose metabolism is negatively regulated by phosphofructokinase, fructose can continuously enter the glycolytic pathway. Therefore, fructose can uncontrollably produce glucose, glycogen, lactate, and pyruvate, providing both the glycerol and acyl portions of acyl-glycerol molecules. These particular substrates, and the resultant excess energy flux due to unregulated fructose metabolism, will promote the over-production of TG (reviewed in [53]).

Glycemic excursions and insulin responses were reduced by 66% and 65%, respectively, in the fructose-consuming subjects

reduction in circulating leptin both in the short and long-term as well as a 30% reduction in ghrelin (an orexigenic gastroenteric hormone) in the fructose group compared to the glucose group.

A prolonged elevation of TG was also seen in the high fructose subjects

Both fat and fructose consumption usually results in low leptin concentrations which, in turn, leads to overeating in populations consuming energy from these particular macronutrients

Chronic fructose consumption reduces adiponectin responses, contributing to insulin resistance

A definite relationship has also been found between metabolic syndrome and hyperhomocysteinemia

the liver takes up dietary fructose rapidly where it can be converted to glycerol-3-phosphate. This substrate favours esterification of unbound FFA to form the TG

Fructose stimulates TG production, but impairs removal, creating the known dyslipidemic profile

the effects of fructose in promoting TG synthesis are independent of insulinemia

Although fructose does not appear to acutely increase insulin levels, chronic exposure seems to indirectly cause hyperinsulinemia and obesity through other mechanisms. One proposed mechanism involves GLUT5

If FFA are not removed from tissues, as occurs in fructose fed insulin resistant models, there is an increased energy and FFA flux that leads to the increased secretion of TG

In these scenarios, where there is excess hepatic fatty acid uptake, synthesis and secretion, 'input' of fats in the liver exceed 'outputs', and hepatic steatosis occurs

Carbohydrate induced hypertriglycerolemia results from a combination of both TG overproduction, and inadequate TG clearance

fructose-induced metabolic dyslipidemia is usually accompanied by whole body insulin resistance [100] and reduced hepatic insulin sensitivity

Excess VLDL secretion has been shown to deliver increased fatty acids and TG to muscle and other tissues, further inducing insulin resistance

the metabolic effects of fructose occur through rapid utilization in the liver due to the bypassing of the regulatory phosphofructokinase step in glycolysis. This in turn causes activation of pyruvate dehydrogenase, and subsequent modifications favoring esterification of fatty acids, again leading to increased VLDL secretion

High fructose diets can have a hypertriglyceridemic and pro-oxidant effect

Oxidative stress has often been implicated in the pathology of insulin resistance induced by fructose feeding

Administration of alpha-lipoic acid (LA) has been shown to prevent these changes, and improve insulin sensitivity

LA treatment also prevents several deleterious effects of fructose feeding: the increases in cholesterol, TG, activity of lipogenic enzymes, and VLDL secretion

Fructose has also been implicated in reducing PPARα levels

PPARα is a ligand activated nuclear hormone receptor that is responsible for inducing mitochondrial and peroxisomal β-oxidation

decreased PPARα expression can result in reduced oxidation, leading to cellular lipid accumulation

fructose diets altered the structure and function of VLDL particles causing and increase in the TG: protein ratio

LDL particle size has been found to be inversely related to TG concentration

therefore the higher TG results in a smaller, denser, more atherogenic LDL particle, which contributes to the morbidity of the metabolic disorders associated with insulin resistance

High fructose, which stimulates VLDL secretion, may initiate the cycle that results in metabolic syndrome long before type 2 diabetes and obesity develop

A high flux of fructose to the liver, the main organ capable of metabolizing this simple carbohydrate, disturbs normal hepatic carbohydrate metabolism leading to two major consequences (Figure 2): perturbations in glucose metabolism and glucose uptake pathways, and a significantly enhanced rate of de novo lipogenesis and TG synthesis, driven by the high flux of glycerol and acyl portions of TG molecules coming from fructose catabolism

Incretins are gut-derived hormones, members of the glucagon superfamily, released in response to nutrient ingestion (mainly glucose and fat)

They exert a wide range of effects, including stimulation of pancreatic insulin secretion in a glucose-dependent manner and play an important role in the local gastrointestinal and whole-body physiology

Laboratory studies have demonstrated anticancer effects of vitamin D metabolites on three critical phases in the development of breast tumors: differentiation, apoptosis, and angiogenesis

The substitution of l-T3 for l-T4 caused a significant weight loss

The substitution of l-T3 for l-T4 caused a significant reduction in lipid parameters

TH action is increased in the liver, and the SHBG increase supports this hypothesis

The changes in serum lipid metabolism parameters are similar to the effects observed with drugs approved for the treatment of dyslipidemia

This differential response appears to be limited to the lipid metabolism and SHBG, whereas no differences in indices of insulin resistance were detected. This is remarkable because hyperthyroid states are associated with an increase in hepatic gluconeogenesis (37), and overt thyrotoxicosis is a known cause of secondary diabetes.

Despite the increase in serum T3, the l-T3 treatment did not cause major changes in cardiovascular or musculoskeletal function, as indicated by the echocardiographic and maximal exercise tolerance tests and DXA studies.

Similarly, no significant differences were observed in blood pressure, heart rate, or endothelial vascular function

In conclusion, the results of this pharmacology, proof-of-concept study indicate that replacement therapy of hypothyroidism with l-T3, compared with l-T4 causes weight loss and favorable changes in the lipid profile without appreciable side effects

C. scindens and a small number of species belonging to the genus Clostridium are responsible for significant alterations in the human bile acid pool composition through bile acid 7α/β dehydroxylation

bile acids play an important role in maintaining intestinal barrier function as antimicrobial agents in the small bowel (37, 38) and inducers of antimicrobial peptides

Perturbations in the biliary bile acid pool composition can be indicative of hepatogastrointestinal diseases such as fat malabsorption (40), gallstones (3), gastrointestinal cancers (41), and possibly type II diabetes