Fertility of Overweight and Underweight Men

BMI and Male Fertility Potential

Maintaining a healthy body mass index (BMI) can help maximize male fertility potential.  Both being overweight as well as underweight can have a negative impact on sperm production and quality.  I recommend that men aim for a normal body mass index (BMI), somewhere between 20 and 25. The most beneficial way to do this from a fertility standpoint is through regular exercise and healthy dietary choices.

Overweight/Obesity (High BMI)

Obesity is a growing problem in the United States and a well-known risk factor for diabetes and heart disease, among other health problems. Being overweight can also have a negative impact on a man’s fertility, impacting both sperm counts and motility. [Gurayah AA. Urol 2023]  This is due to a combination of hormonal impacts and increased oxidative stress on the body.

Excessive fat can have a negative effect on a man’s hormone levels, increasing estradiol and decreasing FSH and LH. Levels of both free and total testosterone also are known to decrease with obesity, with the size of the decrease being proportional to the degree of obesity. Excess fat around the groin and inner thigh areas can also insulate the testicles and increase the temperature within the scrotal region; numerous research studies have shown a correlation between decreased sperm counts/sperm quality and being overweight or obese. Sexual and ejaculatory dysfunction, as well as increased levels of sperm DNA fragmentation, have also been found to be associated with carrying excessive weight.

On the other end of the spectrum, extremely thin men may have problems with sperm production as well, due to hormonal and nutritional issues. Being underweight has been associated with lower testosterone levels, increased FSH, and decreased sperm density.

From a medical standpoint, body mass index (BMI), which takes into account both height and weight, is the tool used to determine whether someone is of normal weight, overweight or obese, or underweight. You can calculate your own BMI using one of the many BMI calculators available online. An ideal BMI value is usually defined as being between 18.5 and 24.9. Because BMI measures weight in relation to height, BMI may not be accurate in heavily muscled men, so individual body characteristics do need to be taken into account. For the majority of men, however, BMI is a fairly good indicator of whether they are at a healthy weight.

Underweight (Low BMI)

As discussed previously, men with a very low BMI can have fertility problems due to abnormalities of hormones and nutritional deficiencies. Many men with a low BMI have very fast metabolisms and just cannot seem to gain weight no matter how many calories they consume. If a man has any symptoms of hyperthyroidism (sweating, palpitations, etc.) then a TSH level can be drawn to see if he has hyperthyroidism (see "Hormones" section later in Boot Camp). Otherwise, the best strategy that these men can take is to eat a healthy, well-balanced diet and avoid overexercising (see previous Boot Camp section on "Health, Diet, and Exercise").

Managing an Elevated BMI to Improve Sperm Quality

From a fertility standpoint, gradual weight loss through a combination of increased exercise and dietary modification is generally the best way to improve fertility. 

Exercise

Engaging in regular exercise—vigorous enough to moderately increase your heart rate—is a good way to help decrease weight. Men with significant health problems should check with their primary care physician before starting on an exercise regimen. See the "Health, Diet, and Exercise" Boot Camp section for more detailed information on exercise recommendations.

Diet

There are hundreds (if not thousands) of weight loss diets out there. Two particular weight loss diets that I find reasonable include the DASH diet and the Mediterranean diet.

DASH Diet

The Dietary Approaches to Stop Hypertension (DASH) diet (www.dashdiet.org) was developed by the National Institutes of Health in order to help people with hypertension better control their blood pressure, but it’s also a great example of a well-balanced approach to eating for anyone. The DASH diet emphasizes fruits, vegetables, whole grains, and low-fat dairy foods. It includes lean meat, fish, poultry, nuts, and beans as protein sources, and it recommends limiting sugar-sweetened foods and beverages, red meat, and added fats.

Mediterranean Diet

The Mediterranean diet has been shown to decrease the risk of heart disease, cancer, Parkinson’s disease, and Alzheimer’s disease. This eating plan emphasizes plant-based food (nuts, fruits, vegetables, whole grains, legumes), healthy fats (such as olive oil) instead of less healthy fats (such as butter), limiting red meat to no more than a few servings a month, avoiding excessive salt in favor of herbs and spices, eating fish and poultry at least twice a week, and (optionally) drinking red wine in moderation. It also emphasizes the importance of staying physically active.

Weight Loss Strategies That Can Harm Fertility

Crash diets that feature extreme calorie restriction and lead to rapid changes in body weight can result in nutritional deficiencies and increased oxidative stress on the body. These changes can have a negative impact on the pituitary’s secretion of gonadotropins, decrease testosterone levels, and impair sperm production and quality.

Fad diets can also pose a problem. Dieting fads come and go—in the 1990s, for example, fats were bad and carbohydrates were good, while recently the Atkins and Paleo diets have identified carbohydrates as the enemy and labeled fats as either not unhealthful or something to be encouraged. I am certainly not going to wade into the controversies surrounding the efficacy and safety of each of these diets for the general population. However, from a fertility standpoint I do have some concerns about diets that exclude entire categories of food, such as carbohydrates. Studies have not conclusively shown whether some carbohydrates are needed for decent sperm production and there are a few case studies showing improved semen parameters on these extremely low carb diets in people who lost significant weight.  There is no evidence to strictly avoid keto and paleo diets for men trying to conceive at this point in time.  However, if your diet is causing significant stress on the body (chronic fatigue, etc.) then you may want to weight the cost-benefit analysis of weight loss/strict adherence to a stringent diet vs. potential temporary decrease in fertility potential.   Men on very strict vegetarian diets avoid many of the pitfalls of the Western Diet (processed food, saturated and trans animal fats) and these diets are also usually high in antioxidants.  However, strict vegetarians can suffer from iron and zinc deficiencies, exposure to estrogenic compounds, and pesticide contamination, all of which have the potential to negatively affect a man’s fertility.

Weight loss surgery, also called bariatric surgery, can lead to substantial and sustained weight loss in many obese patients. This type of surgery has been shown to have a potentially positive impact on rates of diabetes, hypertension, and life expectancy. Some studies have shown improvements in hormone profiles in obese men following bariatric surgery, but the overall impact on fertility has not been well studied. Immediately following bariatric surgery, men often have significant decreases in sperm counts (even dropping to zero) and sperm quality, despite normal hormone profiles.  Potential reasons for this include the immediate stress of surgery on the body, problems absorbing nutrients as a result of surgical techniques that reduce the size of the stomach or bypass it, as well as the release of toxic substances as large amounts of fat are burned in a relatively short period. Over the long term, these men may well rebound and have an improved fertility status, but ongoing problems with nutrient absorption abnormalities can lead to longstanding sperm quality problems in some men.