Yes, at the top of the BMI range, and it is partly reversible. In a 2013 collaborative meta-analysis of 21 studies and 13,077 men, compared with men of normal BMI the odds of a low or absent sperm count were 1.11 times higher in overweight men, 1.28 times higher in obese men and 2.04 times higher in men with a BMI of 40 or over; underweight men showed a small rise that was not statistically significant. In a 2022 Danish trial, 47 men with obesity who took off an average of 16.5 kg in eight weeks saw sperm concentration rise 1.49-fold and total count 1.41-fold, and a year later the gain remained only in the men who had kept the weight off. Because sperm take about 74 days to develop, count this as a three-month change that has to last.
What the BMI numbers mean
Body mass index is your weight in kilograms divided by your height in meters squared. The NHS ranges for most adults are 18.5 to 24.9 for a healthy weight, 25 to 29.9 for overweight and 30 or more for obese; the studies below use 40 as the cut-off for the highest band. The thresholds are lower for people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds (overweight from 23, obese from 27.5), and BMI cannot tell muscle from fat, so a heavily muscled man can sit in the overweight band with little body fat. Treat the number as a rough guide to the studies rather than a verdict on you.
The J-shaped curve
Sermondade and colleagues pooled 21 studies from general populations and fertility clinics, 13,077 men in total, and looked at the odds of oligozoospermia (a low count) or azoospermia (no sperm) by BMI band. The curve was J-shaped: 1.15 in underweight men (a confidence interval that crossed 1, so not a clear effect), 1.11 in overweight men, 1.28 in obese men and 2.04 in men with a BMI of 40 or over. In plain terms, the risk climbs with weight, gently through the overweight band and sharply at the top. Being underweight looks slightly worse than normal weight, but the evidence is too thin to call it.
A separate 2015 meta-analysis of 30 studies and 115,158 men (Campbell and colleagues) looked at outcomes rather than counts. Obese men were 1.66 times as likely to be infertile, and in couples having IVF or ICSI the odds of a live birth per cycle were about a third lower when the man was obese. That review found no clinically meaningful difference in the ordinary semen measures (volume, concentration, motility) but did find more DNA fragmentation and more abnormally shaped sperm, which suggests weight can act on sperm quality in ways a standard analysis does not always show.
Why fat tissue is thought to matter
Three mechanisms are proposed, none proven in a trial. Fat tissue carries the enzyme that converts testosterone to estrogen, so more of it means lower testosterone and higher estrogen, and both hormones feed back on the signals that drive sperm production. And fat around the groin and thighs holds heat against the testes, which work best a couple of degrees below core temperature; the heat guide covers why that matters. The DNA fragmentation finding above fits an oxidative stress story too. "Linked to" is the right verb for all of it.
The weight-loss trial
Andersen and colleagues (Human Reproduction, 2022) put 56 men with obesity, average BMI 37 and average age 41, on an 800-calorie-a-day diet for eight weeks under medical supervision; 47 gave a semen sample before and after. They took off 16.5 kg on average, bringing BMI to 32. Sperm concentration rose 1.49-fold and total sperm count 1.41-fold; motility did not change. The men were then randomized to a year of exercise, the GLP-1 drug liraglutide, both, or neither. What mattered at 52 weeks, among the 37 men still giving samples, was whether the weight stayed off: men who maintained most of the loss kept a concentration 1.71 times and a count 1.97 times their starting point, while men who regained the weight drifted back toward baseline. Exercise and liraglutide each helped men hold the loss.
It is one trial, small by the end, on a supervised very-low-calorie program rather than something to copy from a website. The lesson is the shape of the result: a large, sustained loss moved sperm count, and the effect went away when the weight came back.
How long until it shows in a test
Sperm take about 74 days to develop and then one to two weeks to pass through the epididymis, so a semen analysis two to three months after a change is the first one that mostly reflects it. Weight sits at the slow end of the factors, because the loss itself takes months for most men and the gain depends on keeping it. The how-long guide compares the timeframes, and the timeline tool turns a start date into a retest date.
If your BMI is normal, or low
This factor is not yours to work on. Men in the healthy band had the lowest odds in the meta-analysis, and the underweight finding was small and not significant, so gaining weight is not a fertility strategy. Put the effort into the factors that apply to you: the main guide ranks them.
What to do
- Work out your BMI and where it sits. Over 30 (or over 27.5 for the groups the NHS lists) is where the evidence is clearest.
- Aim for gradual loss toward the healthy range, through food first. A GP can refer you to a weight management service, and the diet guide covers what to eat.
- Plan for keeping it off, not just losing it. The trial's gains lasted only in men who maintained the loss, and exercise was one of the two things that helped.
- Pair it with the fast levers: no saunas or hot tubs, looser underwear, most days alcohol-free. Those show in a test sooner.
- Book a semen analysis for three months after the weight has come off, and compare it with the first. The Motily app tracks each value across reports and folds your height and weight into your Overall score.
Related guides
Sources
- Sermondade N, et al. BMI in relation to sperm count: an updated systematic review and collaborative meta-analysis. Human Reproduction Update, 2013.
- Campbell JM, et al. Paternal obesity negatively affects male fertility and assisted reproduction outcomes: a systematic review and meta-analysis. Reproductive BioMedicine Online, 2015.
- Andersen E, et al. Sperm count is increased by diet-induced weight loss and maintained by exercise or GLP-1 analogue treatment: a randomized controlled trial. Human Reproduction, 2022.
- NHS. Obesity. (BMI ranges and the lower thresholds for some ethnic groups.)
- Heller CG, Clermont Y. Spermatogenesis in man: an estimate of its duration. Science, 1963.