Obesity, Insulin Resistance and Testosterone: the Vicious Cycle
Belly fat lowers testosterone, low testosterone promotes fat. In between sit insulin and SHBG. This creates a cycle that drives itself. Yet this is exactly where the opportunity lies, because a cycle like this can be addressed at several points.
When a man with a belly and a low testosterone value sits in front of me, I often hear the quiet self-reproach: "I just need to be more disciplined." I see it differently. Obesity and low testosterone are not purely a matter of willpower, but a biological cycle in which belly fat, insulin and hormone control can pull each other downward. This is not an excuse. It is a map. Because once you understand how the circle turns, you find the points where it can open up again.
Perhaps you know this. The belly grows, energy falls, drive flattens, desire fades. At the doctor the words low testosterone come up, and it sounds as if that were the cause. Yet often testosterone is less the trigger than a passenger in a cycle that has been running for a while. In this article we look at exactly this cycle. We understand how belly fat, insulin and SHBG interact, and why weight is so often the first lever for the male hormone.
Here is the honest message up front: this is not about blame. It is about mechanisms. And mechanisms can be understood and acted upon, instead of condemning yourself for a supposed failure.
The vicious cycle: how fat, insulin and testosterone pull each other
Picture three gears meshing together: belly fat, insulin and testosterone. If one turns the wrong way, the others follow. More belly fat means more conversion of testosterone into estrogen and more inflammation. This can lower testosterone. Low testosterone in turn makes it harder to maintain muscle and can promote fat storage. And excess insulin, which arises in insulin resistance, drives fat storage and lowers the transport protein SHBG. So the circle closes.
This interweaving is well described. Low testosterone and the metabolic syndrome, the combination of belly fat, raised blood sugar, high blood pressure and unfavorable blood fats, often occur together. Obesity is considered the condition most strongly associated with low testosterone in men.
Obesity lowers testosterone more than the reverse
Review Mathis Grossmann described in 2018 in Clinical Endocrinology how obesity lowers testosterone in men. With moderate obesity, the low total value mainly reflects reduced SHBG. With marked obesity, a genuine dampening of the hypothalamic-pituitary-testicular axis comes on top, mediated through inflammatory messengers and disturbed leptin signalling, and the aromatase in fat tissue converts testosterone into estrogen. Importantly, the effect of weight on testosterone is greater than the reverse, and substantial weight loss can reactivate the axis.
Grossmann M. Clin Endocrinol (Oxf). 2018;89(1):11-21. doi:10.1111/cen.13723 · PMID: 29683196
A vicious cycle sounds hopeless, but it is not. Precisely because the gears drive each other, it is sometimes enough to brake one gear so the others follow. Anyone who addresses weight does not only act on fat, but indirectly on insulin and testosterone too. A cycle that reinforces itself can also turn in the other direction.
Belly fat is a hormone organ, not a dead store
For a long time fat tissue was seen as a mere energy store. Today we know it is an active organ that releases messengers and intervenes in the hormone balance. Belly fat in particular, the fat between the organs, is metabolically active. This is where the enzyme aromatase sits, which converts testosterone into estrogen. The more belly fat, the more conversion can take place, and the more the testosterone level can come under pressure.
On top of this comes silent inflammation. Inflammatory messengers stream out of belly fat that, together with disturbed leptin signalling, can dampen the upstream control system in the brain. The result: less signal to the testes, less of the body's own testosterone production.
One third of men with obesity have too low free testosterone
Review Sandeep Dhindsa and colleagues summarized the state as diabesity in 2018 in Diabetes Care. About one third of men with obesity or type 2 diabetes have low free testosterone with inappropriately normal control hormones, that is, a hypogonadotropic hypogonadism. Notably, estrogen is rather low in these men, not high. Disturbed insulin and leptin signalling in the brain are discussed as mechanisms. These men had more fat mass and were more insulin resistant than men with normal testosterone.
Dhindsa S, Ghanim H, Batra M, Dandona P. Diabetes Care. 2018;41(7):1516-1525. doi:10.2337/dc17-2510 · PMID: 29934480
And now it becomes clear why the usual expectation does not hold for these men. One might think a lot of fat tissue means a lot of estrogen. Yet in diabesity the picture is often more complex, and exactly the low values across the board show that the control system itself is affected here, not just the conversion.
Insulin and SHBG: the quiet mediators in the cycle
Insulin is not only the hormone that lowers blood sugar. It is a central mediator between metabolism and hormones. In insulin resistance the cells respond less well to insulin, so the body releases more and more of it. These high insulin levels have consequences that reach far beyond sugar.
One of them concerns SHBG, the sex hormone-binding globulin. This transport protein is made in the liver and binds testosterone in the blood. High insulin levels and a fatty liver throttle SHBG production. If SHBG falls, total testosterone often falls too, without a true deficiency necessarily being present. This is why low SHBG is an important clue that reaches beyond testosterone.
Low SHBG is an early marker of insulin resistance
Review Chen and colleagues summarized in 2010 in Minerva Endocrinologica that low SHBG levels are an early sign of insulin resistance and can predict the development of type 2 diabetes in men and women. Raised insulin does throttle SHBG production in the liver. Yet genetic studies suggest that SHBG may also play an independent role in sugar metabolism and is not merely a passive marker. SHBG is therefore more than a side note in the lab, it is a junction between metabolism and hormones.
Chen C, Smothers J, Lange A, et al. Minerva Endocrinol. 2010;35(4):271-280. PMID: 21178921
The link between testosterone and insulin is also well supported clinically. A review by Kapoor and colleagues from 2005 in Clinical Endocrinology described that testosterone appears to be an important regulator of insulin sensitivity in men and that low values occur more often with diabetes, visceral fat and metabolic syndrome (doi:10.1111/j.1365-2265.2005.02299.x, PMID: 16117808). And now you know why good diagnostics never look at testosterone alone.
The four KPNI lenses on the vicious cycle
In clinical psychoneuroimmunology, or KPNI for short, we do not look at a single value. We look at four interwoven levels that together explain why the cycle of fat, insulin and testosterone arises and persists. Each lens describes a part at the cellular level.
Metabolism and insulin
In insulin resistance the cells respond less well to insulin, and the body releases more of it. At the cellular level, excess insulin promotes fat storage and throttles the production of SHBG in the liver. Less SHBG can lower total testosterone. So metabolism becomes the engine that pulls the hormone system down with it. A calm blood sugar therefore eases both sides at once.
Immune system and inflammation
Belly fat is metabolically active and releases inflammatory messengers. At the cellular level, these cytokines can disturb signalling pathways and, together with disturbed leptin signals, dampen the control system in the brain. This silent inflammation is a link between a lot of fat tissue and low testosterone production. The gut also belongs here, because an irritated gut barrier can keep the immune system permanently busy.
Hormone system and aromatase
The enzyme aromatase, which converts testosterone into estrogen, sits in fat tissue. More fat means more conversion capacity. At the control level, the hypothalamus and pituitary send the signal to the testes through LH. With marked obesity this signal can be dampened, so the testes produce less testosterone. Both levels mesh and explain why the value falls.
Nervous system and behavior
Low testosterone and a burdened metabolism can affect drive, mood and sleep. At the cellular level, stress shifts the energy balance toward fat storage through cortisol. Fatigue and low drive make movement harder, which in turn lets weight rise. So the nervous system closes the cycle from the behavioral side. Sleep and stress regulation are therefore not a side issue.
These four lenses are not a theoretical model. They show that the vicious cycle does not arise at a single point but from the interplay. And that is exactly why there are several points at which it can be interrupted.
What the data show: weight loss can raise testosterone
The most important practical question is: can the cycle be reversed? The data suggest that weight loss can raise testosterone in men again. This is one of the better-supported statements in this whole field.
Weight loss raises testosterone and SHBG, the more the stronger
Meta-analysis, 24 studies Giovanni Corona and colleagues analyzed 24 studies on diet and bariatric surgery in 2013 in the European Journal of Endocrinology. Both a calorie-reduced diet and surgery were associated with a marked rise in bound and free testosterone. Surgery worked more strongly than diet. The rise was greater the more weight was lost, and more pronounced in younger, non-diabetic men with a higher starting weight. At the same time estradiol fell and the control hormones rose. The degree of weight loss was the best predictor of the testosterone rise.
Corona G, Rastrelli G, Monami M, et al. Eur J Endocrinol. 2013;168(6):829-843. doi:10.1530/EJE-12-0955 · PMID: 23482592
It is also important that no particular diet label is what matters. A controlled study compared two paths to the same goal.
Protein-heavy or carbohydrate-heavy: the weight loss counts
RCT, n=118 Lisa Moran and colleagues studied 118 overweight men over 52 weeks in 2016 in PLoS One. One group ate protein-heavy, the other carbohydrate-heavy, both calorie-reduced. In both groups total testosterone, SHBG and free testosterone rose markedly, with no difference between the diets. Testosterone and SHBG increased within the first twelve weeks, and total testosterone continued to rise over the year. This suggests that it is not the composition of the diet that makes the difference, but the weight loss itself.
Moran LJ, Brinkworth GD, Martin S, et al. PLoS One. 2016;11(9):e0161297. doi:10.1371/journal.pone.0161297 · PMID: 27584019
A word on testosterone therapy at this point. For the common obesity-associated form, reviews advise first focusing on weight loss and treating the accompanying conditions. Giving testosterone does not lower body weight and can impair fertility, even though it did reduce fat mass somewhat and increased muscle mass in studies (Grossmann 2018, doi:10.1111/cen.13723). A later review by Corona and colleagues in 2024 in Expert Review of Clinical Pharmacology likewise stresses that lifestyle weight loss should come first in overweight men (doi:10.1080/17512433.2024.2366505, PMID: 38853775).
"My testosterone is low, so I need testosterone." For the obesity-associated form this is often too short-sighted. Here weight is usually the driving force, and the hormone value follows it. Giving testosterone does not change weight and can dampen a man's own fertility. The first step therefore rarely lies in an injection, but in the cycle itself, which runs through weight, insulin and movement.
Three levers that can open the cycle
Before adjusting individual hormones, it is worth looking at the basics that act exactly on the gears of the cycle. These three levers are a beginning, not a treatment plan. You will find the individual path with medical guidance.
Go for belly fat and blood sugar, not the scale alone
Because belly fat presses on testosterone through aromatase and inflammation, sustained weight loss can ease the whole cycle. What matters is a diet that keeps blood sugar calm, with enough protein and fiber and fewer fast carbohydrates. Even a moderate but lasting weight loss could reactivate the control of the testes and at the same time improve insulin sensitivity.
Build muscle, because it is metabolic tissue
Strength training and regular movement can improve insulin sensitivity, build muscle and lower belly fat, that is, exactly the factors that drive the cycle. Muscle uses sugar and so eases the insulin system. Acute hormone peaks after training matter less than the long-term reshaping of body composition. You do not have to become an athlete, even regular demanding movement could help.
Take sleep and stress seriously as part of metabolism
Poor sleep and chronic stress shift the energy balance toward fat storage through cortisol and can worsen the insulin state. So they intervene directly in the cycle. A fixed rhythm, a dark, cool bedroom and taking snoring and breathing pauses seriously can make a difference. Sleep apnea is common in overweight men and treatable and should be assessed.
And if the complaints remain despite good basics, diagnostics that look at the whole picture belong with it. It makes sense to measure SHBG and free testosterone alongside total testosterone, plus the control hormones LH and FSH and blood sugar and possibly insulin. This helps to determine whether a true deficiency is present and which form. Good diagnostics take your complaints seriously instead of attributing them prematurely to a hormone.
A cycle that reinforces itself can also loosen
Belly fat, insulin and testosterone pull each other. At first this sounds discouraging. Yet the same interweaving is your opportunity. Anyone who addresses weight and blood sugar intervenes in insulin and in the hormone system at the same time. You do not have to solve everything at once. Even a first, held step can push the circle in the other direction.
Frequently asked questions about obesity, insulin and testosterone
How are obesity and testosterone connected?
Obesity is the condition most strongly associated with low testosterone in men. The link runs in both directions. Belly fat contains the enzyme aromatase, which converts testosterone into estrogen, so more fat means more conversion. With marked obesity, a genuine dampening of the upstream control system in the brain comes on top, mediated through inflammatory messengers and disturbed leptin signalling. At the same time, low testosterone can make it harder to build muscle and can promote fat storage. This creates a cycle that can reinforce itself. The encouraging part: studies suggest that substantial weight loss can raise testosterone again.
What is the role of insulin resistance in testosterone?
Insulin is itself a hormone, and in insulin resistance the cells respond less well to it, so the body releases more and more insulin. This state is closely linked to low testosterone. High insulin levels lower the production of the transport protein SHBG in the liver, which can push down total testosterone. At the same time, excess insulin promotes fat storage, and more fat means more conversion of testosterone into estrogen through aromatase. Observational studies show that low testosterone and insulin resistance often occur together and can reinforce each other. A calm blood sugar therefore eases not only the metabolism but may also support hormone production.
What is SHBG and why does it matter?
SHBG stands for sex hormone-binding globulin, a transport protein made in the liver that binds testosterone in the blood. Only the unbound, free testosterone is biologically active. SHBG therefore matters when interpreting lab values. With moderate obesity, a low total testosterone often mainly reflects reduced SHBG rather than a true deficiency. High insulin levels and a fatty liver lower SHBG. Low SHBG is considered an early sign of insulin resistance and an increased risk of type 2 diabetes. Anyone who wants to understand testosterone values therefore looks not only at the total value but also at SHBG and free testosterone.
Does belly fat really lower testosterone?
Yes, there is good evidence for this. Belly fat is not just a store but an active organ. It contains the enzyme aromatase, which converts testosterone into estrogen. The more fat tissue, the more conversion can take place. In addition, inflammatory messengers stream out of belly fat that, together with disturbed leptin signalling, can dampen the upstream control system in the brain, so that less signal reaches the testes. Reviews describe that with marked obesity a genuine suppression of the hypothalamic-pituitary-testicular axis comes on top. Belly fat is therefore a central player in low testosterone in men, not just a side effect.
Can weight loss increase testosterone?
The data point clearly in this direction. A meta-analysis of studies on diet and bariatric surgery found that weight loss was associated with a marked rise in total and free testosterone. The effect was greater the more weight was lost, and more pronounced in younger, non-diabetic men. A controlled study in overweight men showed that a calorie-reduced diet over twelve months raised testosterone and SHBG, regardless of whether the diet was protein-heavy or carbohydrate-heavy. This suggests that it is not a particular diet label that counts, but the weight loss itself. Weight is therefore often the first lever for testosterone in men.
What is hypogonadotropic hypogonadism in obesity?
About one third of men with obesity or type 2 diabetes have low free testosterone, while the control hormones from the pituitary stay inappropriately normal instead of rising as expected. This pattern is called hypogonadotropic hypogonadism. Interestingly, estrogen tends to be low rather than high in these men, as one might assume with a lot of fat tissue. Disturbed insulin and leptin signalling in the brain are discussed as mechanisms. This form differs from a classic testicular failure and is often partly reversible with weight loss. A medical assessment determines which form is present.
Which blood values make sense for obesity and testosterone?
More than a single value makes sense. Testosterone should be measured in the morning and ideally more than once, since it fluctuates over the day. It is important to measure SHBG alongside total testosterone in order to estimate the free, biologically active testosterone. This also includes the control hormones LH and FSH, to distinguish a testicular from a control problem, as well as fasting blood sugar or long-term sugar and possibly insulin to capture the metabolic state. Blood count, thyroid and iron also belong here, because they can cause similar complaints. This produces a picture of the whole system rather than an isolated number. The interpretation belongs in medical hands.
Is testosterone therapy the answer for obesity?
Testosterone therapy is rarely the first step in obesity. For the common obesity-associated form, reviews advise first focusing on weight loss and treating the accompanying conditions, because giving testosterone does not lower body weight and can impair fertility. In studies, testosterone did reduce fat mass somewhat and increased muscle mass, but it does not replace the basics. In a clearly confirmed deficiency with symptoms, therapy can be considered medically, but it belongs in experienced hands with information about benefits and risks. This text does not replace medical advice and is not a guide to self-treatment.
How quickly does testosterone change after weight loss?
Part of the change can become visible quite early. In a controlled diet study, testosterone and SHBG rose within the first twelve weeks of weight loss, and total testosterone continued to increase over the year, while free testosterone rose mainly in the second half. With marked weight loss after bariatric surgery, the increases are larger than with diet alone. This suggests that it is not a fast effect that counts, but the sustained reshaping of body composition. Patience and consistency may matter more here than speed. Individual courses vary greatly and belong under medical supervision.
When should I see a doctor with low testosterone and obesity?
You should have a medical assessment for persistent fatigue and low drive, a marked loss of libido, newly arising erection problems, an unfulfilled wish for children, and signs of a disturbed sugar metabolism such as strong thirst, frequent urination or unwanted weight changes. Treatable causes can hide behind such complaints, from the thyroid through sleep apnea and iron deficiency to a true hormone deficiency or a pre-stage of diabetes. Erection problems can also be an early sign of vascular disease and should be taken seriously. Good diagnostics look at the whole system rather than just a number. If you have thoughts of no longer wanting to live, please get help immediately.
All topics in the cluster "Hormones in Men"
This article is a spoke in the cluster. Here is the way back to the pillar and on to the other topics.
- Hormones in Men (overview/pillar)
- Testosterone is falling worldwide (less each generation)
- Testosterone deficiency: symptoms in men
- Raising testosterone naturally
- Testosterone test: understanding your values
- TRT: testosterone replacement therapy
- Erectile dysfunction: causes
- Loss of libido in men
- Hypogonadism: forms and causes
- Gynecomastia: hormonal causes
- Sperm quality and fertility
- Testosterone boosters: what do they do
- Andropause: the male change
- Micronutrients for testosterone
- DHT, hair loss and testosterone
- Estrogen in men and aromatase
- Cortisol, stress, sleep and testosterone
- Obesity, insulin and testosterone
- Xenoestrogens in men
- Sport, strength training and testosterone
- Prolactin and thyroid in men
Connections to other topics
The deeper context of when a low value is really a deficiency and which causes can lie behind it.
How female hormones function as a networked system, with many parallels to the insulin and metabolic axis in men.
Why chronic stress can shift fat storage and the insulin state through cortisol, right in the cycle of weight and hormone.
Iron deficiency amplifies many complaints that look like a pure hormone problem, from exhaustion to reduced exercise capacity.
Why a borderline thyroid can slow the metabolism and co-influence weight, drive and energy.
The gut co-influences, through the immune system and silent inflammation, how well metabolism and hormone balance stay in equilibrium.
Sources and further reading
- Grossmann M. Hypogonadism and male obesity: Focus on unresolved questions. Clin Endocrinol (Oxf). 2018;89(1):11-21. doi:10.1111/cen.13723 · PMID: 29683196 [Review]
- Dhindsa S, Ghanim H, Batra M, Dandona P. Hypogonadotropic Hypogonadism in Men With Diabesity. Diabetes Care. 2018;41(7):1516-1525. doi:10.2337/dc17-2510 · PMID: 29934480 [Review]
- Corona G, Rastrelli G, Monami M, et al. Body weight loss reverts obesity-associated hypogonadotropic hypogonadism: a systematic review and meta-analysis. Eur J Endocrinol. 2013;168(6):829-843. doi:10.1530/EJE-12-0955 · PMID: 23482592 [Meta-analysis]
- Moran LJ, Brinkworth GD, Martin S, et al. Long-Term Effects of a Randomised Controlled Trial Comparing High Protein or High Carbohydrate Weight Loss Diets on Testosterone, SHBG, Erectile and Urinary Function in Overweight and Obese Men. PLoS One. 2016;11(9):e0161297. doi:10.1371/journal.pone.0161297 · PMID: 27584019 [RCT]
- Chen C, Smothers J, Lange A, Nestler JE, Strauss JF 3rd, Wickham EP 3rd. Sex hormone-binding globulin genetic variation: associations with type 2 diabetes mellitus and polycystic ovary syndrome. Minerva Endocrinol. 2010;35(4):271-280. PMID: 21178921 [Review]
- Kapoor D, Malkin CJ, Channer KS, Jones TH. Androgens, insulin resistance and vascular disease in men. Clin Endocrinol (Oxf). 2005;63(3):239-250. doi:10.1111/j.1365-2265.2005.02299.x · PMID: 16117808 [Review]
- Cheung KK, Lau ES, So WY, et al. Low testosterone and clinical outcomes in Chinese men with type 2 diabetes mellitus. Diabetes Res Clin Pract. 2016;123:97-105. doi:10.1016/j.diabres.2016.11.012 · PMID: 27997863 [Cohort, prospective]
- Travison TG, Araujo AB, O'Donnell AB, Kupelian V, McKinlay JB. A population-level decline in serum testosterone levels in American men. J Clin Endocrinol Metab. 2007;92(1):196-202. doi:10.1210/jc.2006-1375 · PMID: 17062768 [Cohort, prospective]
- Corona G, Rastrelli G, Sparano C, et al. Pharmacological management of testosterone deficiency in men: current advances and future directions. Expert Rev Clin Pharmacol. 2024;17(8):665-681. doi:10.1080/17512433.2024.2366505 · PMID: 38853775 [Review]
- Dhindsa SS, Irwig MS, Wyne K. Gonadopenia and Aging in Men. Endocr Pract. 2018;24(4):375-385. doi:10.4158/EP-2017-0131 · PMID: 29658833 [Review]
- Stokes VJ, Anderson RA, George JT. How does obesity affect fertility in men, and what are the treatment options? Clin Endocrinol (Oxf). 2015;82(5):633-638. doi:10.1111/cen.12591 · PMID: 25138694 [Review]