How Does Obesity Affect Male Fertility?

When a couple struggles to conceive, the first questions usually land on her. Her age, her cycle, her health. What rarely gets asked is whether his waistline has anything to do with it. Yet about one in six people worldwide deal with infertility, and a male factor is involved in roughly half of those cases.[1][2] So here is a question worth sitting with: could carrying extra weight affect your male fertility or be quietly lowering your chances of becoming a father?

What counts as obesity, and how common is it?

The World Health Organization (WHO) defines obesity as a body mass index (BMI) of 30 or above. BMI is your weight in kilograms divided by your height in metres squared, and a score of 25 or more is classed as overweight.[3] By those measures, plenty of men are affected. In the United States, 40.3 per cent of adults had obesity from 2021 to 2023.[4] In England, 69.2 per cent of men were overweight or living with obesity in 2022 to 2023.[5] In Australia, about 67 per cent of adults fall into the same range.[6] Among men and women of typical parenting age, the numbers are just as striking. The 2025 review cited later in this article estimates that obesity affects about 40 per cent of Americans aged 20 to 39, and between 23 and 30 per cent of Australians aged 25 to 45.[2] If your BMI sits above 25, you are in very large company.

What the research on male fertility says about sperm

Researchers have compared the semen of men at different body weights for years, and the pattern is fairly steady. A 2024 analysis regarding male fertility found that men with obesity had, on average, about 20 million fewer sperm in total than men below a BMI of 25. Their progressive motility, which is the share of sperm swimming forward in a straight line, was also nearly 6 percentage points lower.[7] A 2021 review of published studies linked overweight and obesity with lower semen volume, sperm count, sperm concentration, vitality, total motility and normal shape. It also found lower total testosterone in men carrying extra weight.[8]

Another 2024 review on male fertility reached a similar view, concluding that the potential fertility of men with excess weight is reduced compared with men of normal weight.[9] Men who were overweight but not obese showed worse results in only two measures.[9]

Does this change how long a couple waits for a pregnancy? A 2025 review in Human Reproduction Update reports that infertility rates are about 50 per cent higher in men with obesity than in men of normal weight, and that conception tends to be delayed.[2]

Part of the reason this topic stays quiet is that sperm problems rarely announce themselves. A man can feel fit, have a normal sex life and still have a semen sample that falls short. Without a test, nobody knows, and weight becomes one of the few visible clues that something may be worth checking.

Why extra weight gets in the way

Nobody has one tidy explanation. Researchers think several things happen together. Body fat contains an enzyme called aromatase, which turns testosterone into oestrogen. More fat means more of that conversion, and testosterone can fall as a result.[10] Fat around the scrotum can also raise the temperature of the testicles.[10] On top of that, obesity is associated with more damage to the genetic material inside sperm (DNA, short for deoxyribonucleic acid), along with signs of oxidative stress, which is a kind of chemical wear and tear on cells, and inflammation.[2]

None of this means every man with obesity will have trouble. Plenty of men at a healthy weight do, and plenty of heavier men father children without a hitch. The studies describe averages across groups, and they cannot tell you what will happen to you.

Can losing weight help in improving male fertility?

Probably, although the evidence is thinner than you might hope. The same 2025 review pooled studies on weight loss and sperm. In men who changed their eating and exercise habits, progressive motility rose by about 10.6 percentage points, and normally shaped sperm increased by about 0.6 of a percentage point.[2] Those studies followed men for roughly three to six months. The authors rated the certainty of this evidence as low, and few studies checked whether couples actually conceived.[2]

Weight loss surgery was less clear. The review found no meaningful change in sperm measures afterwards, and a few small case reports described men whose sperm counts fell sharply. The authors raised the possibility that very rapid weight loss has its own downsides.[2] Preconception guidelines for men do recommend a healthy weight, but they offer little detail on how to reach one.[2] Advice for women with overweight or obesity rests on clearer evidence of better fertility and pregnancy outcomes, so research on men is still catching up.[2]

What about the newer injectable weight loss medicines? The review found too little data to say how they affect fertility, and it listed the effects of this drug class on sperm as a gap that still needs proper study.[2] Metformin, an older diabetes medicine, was tested in only two small studies, so no firm conclusion can be drawn about it either.[2]

The surgery reports deserve a closer look. In one set of case reports, men who had fathered children before bariatric surgery (an operation that shrinks the stomach) later had no sperm at all in their semen, and three men who tried assisted reproduction did not achieve a pregnancy.[2] These were small reports and cannot show how common the problem is. They do show why a doctor should be part of any big weight decision.

So what should you do? The review's authors suggest that improving nutrition and physical activity may help semen quality.[2] That points toward steady changes rather than crash diets. If you are thinking about medication or surgery, speak with your general practitioner (GP) first. Keep in mind that BMI is a blunt tool. It says nothing about where fat sits or how much of your weight is muscle, which is one reason studies that rely on BMI alone can miss differences between men.[2]

Finding out where you stand

Weight is only one piece of the picture. The only way to know about your sperm is to test them. A GP can arrange a semen analysis, and Sapyen offers an at home kit that measures sperm count, concentration, motility and morphology (shape) from a sample you collect privately and post to a laboratory. You can visit the Sapyen website to purchase the male fertility analysis kit.

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Since the lifestyle studies above followed men for three to six months, retesting after a similar gap is a fair way to see whether your changes have shown up. Take male fertility results to your GP, who can read them alongside your health.

REFERENCES

1. https://www.who.int/news/item/04-04-2023-1-in-6-people-globally-affected-by-infertility
2. https://academic.oup.com/humupd/article/32/2/154/8279592
3. https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
4. https://www.cdc.gov/nchs/products/databriefs/db508.htm
5. https://www.gov.uk/government/statistics/update-to-the-obesity-profile-on-fingertips/obesity-profile-short-statistical-commentary-may-2024
6. https://www.aihw.gov.au/reports/overweight-obesity/overweight-and-obesity/contents/how-many
7. https://www.nature.com/articles/s41366-024-01580-w
8. https://onlinelibrary.wiley.com/doi/10.1111/obr.13082
9. https://onlinelibrary.wiley.com/doi/10.1111/andr.13460
10. https://pmc.ncbi.nlm.nih.gov/articles/PMC7884171