Transcript:
Your BMI is lying to you if you're a guy trying to have a baby or if your testosterone is just low. We've always looked at BMI as a risk factor for fertility in men. A brand new study just dropped in Human Reproduction, and with pretty strong evidence, it suggests that BMI might not be the best metric to assess the overweight risk to low T and fertility. Researchers measured body fat percentage in over a thousand young men, not just BMI. They measured that too, but men with high body fat had a 23% lower sperm count than men with normal body fat. And this held true even for men with a normal BMI. I'll put the interpretation of body fat readings in the description to check yours against.. First, a quick refresher. BMI is Body Mass Index and it's just weight in kilograms divided by height in meters squared. It was invented in the 1830s as a way to compare populations. It was never intended to measure any individual's health. It can't tell fat from muscle, doesn't account for bone density, and doesn't reflect where fat is stored. So a big, healthy athlete can rank the same as a sedentary person of the same height and weight but with lots of fat and classify both as obese. Body fat percentage is a different measure entirely: the actual proportion of your body that's fat tissue, versus muscle, bone, water, and organs. That distinction is important, because fat tissue itself is hormonally active. It shifts hormone signaling. This study also found men with higher body fat had lower testosterone and higher estradiol. That's not what a guy trying to build a family wants. But this is not just about fertility. Excess body fat is part of the downward spiral of low T, decreased motivation, poor exercise, loss of muscle mass, and more fat. Testosterone is kind of the juice of life. A weight scale alone will not catch this. Body fat can be measured cheaply. Bioimpedance scales run under $50. Skinfold calipers run under $20. DEXA scans are more expensive but the most accurate. Any of these gives more useful information than weight alone. You decide what to measure. Weight alone is only part of the story.
BMI misses this if you're a guy who wants to have kids or you just have low T. A new study from Aarhus University in Denmark just found that men with high body fat had 23% lower sperm counts than men with normal body fat — and this held true even in men with a normal BMI. The study, published in Human Reproduction on July 29, 2026, measured body fat percentage using bioimpedance in over 1,000 young men and compared it head-to-head with BMI. A 1-SD increase in body fat percentage was linked to a 13% lower sperm count in men with normal-weight BMIs, while a 1-SD increase in BMI alone was linked to only 8%. Higher body fat also went with lower testosterone and higher estradiol, suggesting a shift in the hormonal balance that supports male reproductive function.
Body fat percentage categories (American Council on Exercise):
Men:
Essential fat: 2-5%
Athletes: 6-13%
Fitness: 14-17%
Acceptable: 18-24%
Obese: 25%+
Women:
Essential fat: 10-13%
Athletes: 14-20%
Fitness: 21-24%
Acceptable: 25-31%
Obese: 32%+
Things to know:
BMI is Body Mass Index and it's just weight in kilograms divided by height in meters squared. It was invented in the 1830s as a way to compare populations. It was never intended to measure any individual's health. It can't tell fat from muscle, doesn't account for bone density, and doesn't reflect where fat is stored.
Two men with the same weight can have very different body compositions. Muscle and fat weigh the same on the scale but do very different things to hormones and fertility.
Body fat can be measured cheaply. Bioimpedance scales run under $50. Skinfold calipers run under $20. DEXA scans are more expensive but the most accurate. Any of these gives more useful information than weight alone.
One caveat: the men in this study were 18 to 19 years old. Whether the same effect size holds in men in their 30s and 40s hasn't been directly tested. But the mechanism (adipose tissue converting testosterone to estradiol via aromatase) is well established across adult men, so the direction of effect is likely to hold.
You decide what to measure.