When we talk about testosterone, most people think about muscles, mood, and libido. But did you know that body fat in men—particularly visceral fat—can actually suppress your body’s ability to make this important hormone? As the rates of obesity rise, so too does a lesser-known consequence: low testosterone levels in men. It’s not just about the numbers on the scale — it’s about what that extra weight is doing behind the scenes

A Growing Concern: Obesity and Testosterone Are Deeply Linked

In New Zealand, over 30% of adult men are classified as obese, with even more falling into the overweight category. International studies suggest that men with obesity are up to 2.4 times more likely to have low testosterone levels compared to men of a healthy weight.

That’s a big deal — because low testosterone isn’t just about sexual health. It affects everything from energy and motivation to muscle strength, mental clarity, and long-term metabolic health.

The Science Behind It: How Fat Tissue Affects Hormones

Here’s the short version of a complicated hormonal feedback loop:

1. Fat tissue converts testosterone into estrogen
   • This happens through an enzyme called aromatase, which is more active in adipose (fat) tissue.
   • As a result, testosterone levels drop while estrogen levels rise.
2. Excess fat also leads to insulin resistance and inflammation
   • These changes interfere with the signals between the brain (pituitary gland) and the testes, further reducing natural testosterone production.
3. Low testosterone can lead to more fat gain and muscle loss
   • It’s a vicious cycle — low testosterone makes it harder to maintain lean muscle, and muscle is metabolically protective.

What Might This Look Like in Real Life?

Men with low testosterone due to obesity might notice:

1. Low energy and persistent fatigue
2. Reduced libido or erectile difficulties
3. Loss of muscle mass or difficulty building strength
4. Increased belly fat, even with exercise
5. Low mood, motivation, or even symptoms of depression
6. Difficulty concentrating or memory changes

It’s often subtle — and easy to dismiss as “just getting older” — but in many cases, these changes are reversible.

Why Testosterone Supplements Aren’t the Answer (for weight-related low testosterone)

It’s tempting to go straight to testosterone replacement therapy (TRT), especially with the rise of low-T clinics and online ads promising instant results. But here’s the catch:

1. Testosterone therapy suppresses your own production
   • Once you start external testosterone, your body reduces its natural production — potentially making you reliant long term.
2. It doesn’t treat the underlying problem
   • If low testosterone is caused by excess weight, TRT is like putting a plaster (bandaid) on a leaky pipe.
3. It carries real risks
   • These include red blood cell thickening (which can increase clot risk), sleep apnoea, acne, testicular shrinkage, and fertility issues.

What Works Better? Treating the Cause

The most effective way to restore testosterone in this scenario is to reduce the visceral fat that’s interfering with your body’s own hormone production.

That doesn’t mean extreme dieting or unrealistic fitness goals. In fact, at the other end of the weight or calorie balance spectrum, calorie deficit can also cause low testosterone (see our separate article on this).  Instead we recommend gradual, sustainable lifestyle changes to consistently improve your body composition — your muscle:fat ratio.

This can involve regular exercise and working with a dietitian to ensure that, whilst you don’t over-consume calories (so that you can lose fat), you also fuel adequately with protein and other nutrients (particularly around your training sessions) so that you gain — rather than lose — lean muscle mass.  

Improving sleep and identifying and treating sleep apnoea are also important. In fact, sleep apnoea is a common cause of fatigue and other symptoms that can mimic low testosterone. Also important is a good overall medical assessment — looking for metabolic health issues like diabetes and considering causes other than obesity for low testosterone.

A Note on Testing: It’s Not Just the Number That Matters

In men with obesity, interpreting testosterone blood tests can be tricky. Obesity lowers levels of a protein called sex hormone-binding globulin (SHBG). SHBG carries testosterone in the bloodstream but also locks some of it away. Only a small portion — the free or bioavailable testosterone — is usable by the body.

When SHBG is low, total testosterone might appear falsely low, even when free testosterone is sufficient. That’s why in some cases, we recommend checking free or calculated bioavailable testosterone, not just total testosterone.

We do measure testosterone when symptoms suggest a problem — but it’s just one part of the picture. Equally important is having the results interpreted by a clinician who understands the full hormonal and metabolic context, so that you get the right diagnosis — and the right plan.

Awareness is the First Step — And the Sooner, the Better

Low testosterone doesn’t need to be a life sentence, and treating it effectively means looking beyond a quick fix. If you’ve noticed changes in your energy, mood, strength, or sexual health — especially if you’re carrying extra weight around the middle — it’s worth having a medical assessment.

An assessment with your doctor, and a clear plan that targets the cause — not just the symptoms — can make a world of difference not only to your symptoms but your long term health.