Losing Muscle, Gaining Belly Fat in Your 40s: Is It Low T?

In short: losing a little muscle and gaining fat around the middle in your 40s is common, and it's driven mostly by less activity, more alcohol, poorer sleep and the slow ageing of muscle. Low testosterone can play a part, but the link runs both ways: belly fat itself lowers testosterone, and losing weight raises it. Strength training and fat loss come first. Testing testosterone makes sense when the body changes come with low sex drive or fewer morning erections.

Plenty of men notice it around the same time: the waistband gets tighter even though you're eating the same, the bar feels heavier, and recovery takes longer. Testosterone gets the blame online. Sometimes it's involved. More often it's a passenger. This guide explains what's driving the change, how to tell whether hormones are part of it, and what to do about it.

Why am I losing muscle and gaining belly fat in my 40s?

Several slow changes stack up:

• Muscle ages. A quantitative review of the research found that men lose muscle mass at a median of about 0.5% a year in adulthood, faster later in life, and that strength falls faster than size (Mitchell et al., 2012).
• Activity drops. Desk jobs, kids and injuries cut the amount you move and lift, and muscle that isn't used isn't kept.
• Alcohol and sleep. Both drift in the wrong direction for many men in midlife, and both affect appetite, fat storage and recovery.
• Testosterone drifts down slowly. In the Massachusetts Male Aging Study, total testosterone fell by about 1.6% a year within the same men, but men in good health (no chronic illness, regular prescription medicines, obesity or heavy drinking) had levels of several androgens 10–15% higher (Feldman et al., 2002).

None of these is dramatic on its own. Together, over a decade, they're enough to change your shape. So yes, some belly fat at 40 is common. Common isn't the same as harmless, though, which is where your waist measurement comes in.

Low testosterone and belly fat: which causes which?

Both. Excess body fat lowers testosterone: in moderate obesity mainly by lowering SHBG, the protein that carries testosterone in the blood, and in more severe obesity also by suppressing the brain signals that tell the testes to make testosterone (Fui et al., 2014). In the European Male Ageing Study, men with a waist of 94 cm or more were more likely to develop low testosterone (Rastrelli et al., 2015). Low testosterone in turn promotes more fat, especially around the abdomen, so the two can feed each other.

The encouraging part is that the effect reverses. A meta-analysis of 24 studies found that weight loss, by diet or by surgery, raised testosterone, and the more weight men lost, the bigger the rise (Corona et al., 2013). The European Male Ageing Study saw the same pattern over four years in ordinary men: those who lost weight had rising testosterone, and those who gained weight had falling testosterone (Camacho et al., 2013).

That's why doctors work on weight before they consider testosterone treatment, and why BPAC advises identifying and addressing modifiable causes, including excess weight, before treatment (BPAC NZ 2024). Healthify's advice for men with low testosterone starts the same way: lose weight if you're overweight (Healthify).

Why the waistline matters: how much belly fat is too much?

Your waist measurement is a better guide than your weight. The Heart Foundation recommends men keep their waist under 94 cm, or under 90 cm for men of South Asian, Chinese or Japanese origin (Heart Foundation NZ). Measure midway between your lowest rib and the top of your hip bone, at the end of a normal breath out.

A waist above those numbers matters for heart and diabetes risk whether or not your testosterone is low, so it's worth acting on either way.

Is it low testosterone or just ageing?

The body changes alone don't tell you. What helps is the rest of the picture.

The three sexual symptoms (low desire, fewer morning erections and erectile dysfunction) are the ones most specifically linked to low testosterone (Wu et al., 2010). Belly fat and lost muscle are common in men with normal testosterone too.

When to get your testosterone checked

Get tested if the body changes come with the sexual symptoms above. If you've done the work for three to six months and nothing is shifting, see a doctor to look for other causes, such as thyroid problems, sleep apnoea or a medicine. A proper test means:

• two morning total testosterone results, about four weeks apart (BPAC advises 7 to 11 am, ideally fasting; shift workers two to three hours after waking) (BPAC NZ 2024)
• SHBG, so free testosterone can be calculated, which matters more in men carrying extra weight because their SHBG is often low (Fui et al., 2014)
• LH and FSH, plus HbA1c and lipids, because the same waistline raises diabetes and heart risk

Enhanced Men diagnoses low testosterone when a man has symptoms and two morning results, three to four weeks apart, both below 12 nmol/L total testosterone (the European Association of Urology threshold) or 225 pmol/L free testosterone. Above that line is not low testosterone.

If low testosterone is confirmed, treatment can shift body composition modestly: a meta-analysis of 59 randomised trials found more lean mass and less fat mass with testosterone than in control groups, but no significant change in body weight, waist size or BMI (Corona et al., 2016). It isn't a weight-loss treatment, it doesn't replace training and diet, and it isn't used for men whose levels are normal.

How men over 40 rebuild muscle and lose belly fat

Age-related muscle loss has a medical name, sarcopenia, and the same things slow it at 45 as at 75, though gains from training are a little smaller the later you start (Peterson et al., 2011).

Strength training. A meta-analysis of 49 studies in adults over 50 found that resistance training added an average of about 1.1 kg of lean body mass, with bigger gains from higher-volume programmes (Peterson et al., 2011). New Zealand's activity guidelines recommend muscle-strengthening on at least two days a week, working all five major muscle groups: arms, legs, chest, abdominals and back (Health New Zealand). Progressively heavier full-body sessions are a solid way to meet that. If you've had a long break or have joint problems, a physiotherapist can set you up.

Fat loss. A steady calorie deficit, built on meals with enough protein and vegetables, reduces belly fat and, as above, tends to raise testosterone at the same time. Cutting back on alcohol is often the single easiest change.

Sleep. Short sleep raises appetite and lowers testosterone. Protect it, and get loud snoring checked.

Patience. Body composition changes over months, not weeks. Measure your waist and track your lifts rather than weighing yourself daily.

What to do next

1. Measure your waist and write down your current lifts.
2. Start or restart strength training, and cut back on alcohol for eight weeks.
3. Take the free AMS self-check if you also have low drive or fewer morning erections. It gives you a symptom score to take to a doctor.
4. Get tested properly. Through your GP, or see where to get your testosterone checked, below.

For the bigger symptom picture, see male menopause: is it real?, tired all the time with no motivation and low testosterone symptoms.

Where to get your testosterone checked in NZ

Testing makes sense when the body changes come with low sex drive or fewer morning erections. The two usual routes are below, and neither needs a referral.

Your GP. Your GP can request the same morning tests, repeat them and start treatment when low testosterone is clearly there. If two morning results fall between 8 and 12 nmol/L, BPAC suggests your GP consider discussing them with an endocrinologist.

A private doctor by video. Enhanced Men, which publishes this guide, is a New Zealand telehealth clinic run by Dr Jack Yeoman, who does every consult himself. He's registered with the Medical Council of New Zealand in general scope (MCNZ 89436). General scope isn't a specialist registration, and it isn't general practice either: he isn't vocationally registered as an endocrinologist, a urologist or a GP, and he refers you on when your case needs a specialist. You see him by video wherever you live in New Zealand, with no GP referral. Your bloods are taken at your local community lab; if you're eligible for publicly funded healthcare, funded tests cost you nothing, and the lab bills you for any test that isn't funded. He diagnoses low testosterone only when you have symptoms and two morning results, three to four weeks apart, both below 12 nmol/L total or 225 pmol/L free testosterone. Above that line is not low testosterone, and he doesn't prescribe for it. Testosterone treatment usually suppresses sperm production. If you may want children, say so before you start.

Whichever route you choose, take copies of any earlier results, with the time each sample was taken, and what your GP made of them.

FAQ

Why am I gaining belly fat in my 40s when I'm eating the same? Usually because you're moving less, sleeping less, drinking a bit more and slowly losing muscle, which lowers the energy you burn. Testosterone drifts down slowly with age too, but excess belly fat is as much a cause of lower testosterone as a result of it.

Does low testosterone cause belly fat? It can contribute, and the link runs both ways: belly fat also lowers testosterone, and that's the half you can most easily change. Weight loss raises testosterone in men carrying extra weight, which is why fat loss is the first step before considering treatment.

Can you rebuild muscle after 40? Yes. Resistance training builds lean mass at any age. A meta-analysis of studies in adults over 50 found an average gain of about 1.1 kg of lean mass, with more from higher-volume programmes.

What waist size is a health risk for men? The Heart Foundation recommends men keep their waist under 94 cm, or under 90 cm for men of South Asian, Chinese or Japanese origin. A larger waist raises heart and diabetes risk whether or not testosterone is low.

Should I get my testosterone tested if I'm losing muscle? Test it if the muscle loss comes with low sex drive or fewer morning erections. If training and fat loss haven't helped after a few months, see a doctor to look for other causes. A proper test is two morning results about four weeks apart, with SHBG, LH and FSH.

Can I get my testosterone checked without seeing my GP in NZ? Yes, you don't need a referral. Testing makes sense when the body changes come with low sex drive or fewer morning erections. Enhanced Men, Dr Jack Yeoman's telehealth clinic, assesses testosterone by video anywhere in New Zealand and gives you a lab request form for your local community lab, and your GP can do the same tests. If you're eligible for publicly funded healthcare, funded tests cost you nothing; the lab bills you for any test that isn't funded.

References

1. Mitchell WK, et al. Sarcopenia, dynapenia, and the impact of advancing age on human skeletal muscle size and strength; a quantitative review. Front Physiol. 2012;3:260. https://doi.org/10.3389/fphys.2012.00260
2. Feldman HA, et al. Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts Male Aging Study. J Clin Endocrinol Metab. 2002;87(2):589–98. https://doi.org/10.1210/jcem.87.2.8201
3. Fui MN, Dupuis P, Grossmann M. Lowered testosterone in male obesity: mechanisms, morbidity and management. Asian J Androl. 2014;16(2):223–31. https://doi.org/10.4103/1008-682X.122365
4. Corona G, et al. Body weight loss reverts obesity-associated hypogonadotropic hypogonadism: a systematic review and meta-analysis. Eur J Endocrinol. 2013;168(6):829–43. https://doi.org/10.1530/EJE-12-0955
5. Camacho EM, et al. Age-associated changes in hypothalamic-pituitary-testicular function in middle-aged and older men are modified by weight change and lifestyle factors: longitudinal results from the European Male Ageing Study. Eur J Endocrinol. 2013;168(3):445–55. https://doi.org/10.1530/EJE-12-0890
6. BPAC NZ. Prescribing testosterone in ageing males. 25 November 2024. https://bpac.org.nz/2024/testosterone.aspx
7. Heart Foundation NZ. Measure your waist. https://www.heartfoundation.org.nz/wellbeing/body-size-and-heart-health/measure-your-waist
8. Wu FCW, et al. Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med. 2010;363(2):123–35. https://doi.org/10.1056/NEJMoa0911101
9. Peterson MD, Sen A, Gordon PM. Influence of resistance exercise on lean body mass in aging adults: a meta-analysis. Med Sci Sports Exerc. 2011;43(2):249–58. https://doi.org/10.1249/MSS.0b013e3181eb6265
10. Corona G, et al. Testosterone supplementation and body composition: results from a meta-analysis study. Eur J Endocrinol. 2016;174(3):R99–116. https://doi.org/10.1530/EJE-15-0262
11. Rastrelli G, et al. Development of and recovery from secondary hypogonadism in aging men: prospective results from the EMAS. J Clin Endocrinol Metab. 2015;100(8):3172–82. https://doi.org/10.1210/jc.2015-1571
12. European Association of Urology. EAU Guidelines on Sexual and Reproductive Health, 2026 edition: Male hypogonadism (section 3.3.5). https://uroweb.org/guidelines/sexual-and-reproductive-health/chapter/male-hypogonadism
13. Health New Zealand | Te Whatu Ora (HealthEd). Be active every day: physical activity for adults (HE2311). Updated May 2025. https://healthed.govt.nz/resource/be-active-every-day-physical-activity-adults
14. Healthify NZ. Testosterone deficiency. Reviewed 13 December 2024. https://healthify.nz/health-a-z/t/testosterone-deficiency
15. Healthify NZ. Obstructive sleep apnoea. https://healthify.nz/health-a-z/o/obstructive-sleep-apnoea

This article is general health information, not personalised medical advice. Talk to a doctor before starting a new exercise programme if you have heart disease, joint problems or other health conditions.