Back to the Basics: Why a Low BMI can Cause Infertility

This is part three in a series on how leptin influences reproduction. Part 1  Part 2

A quick recap of the beginning of this series

Leptin is the hormone produced in your adipose (fat) tissue that tells your brain whether you have enough food. It controls appetite and metabolic rate.

If you are low in leptin, your brain urges you to find food and turns down metabolic process. If you are low in leptin (e.g. in a time of famine), your brain also puts the brakes on the reproductive hormones, signaling that it isn’t a good time to get pregnant.

If all is going well with your leptin signaling, when you have enough fat mass, your brain tells you that you don’t need to eat quite so much (no longer hungry) and signals to the hormones needed for reproduction.

Why does a low BMI cause infertility?

Low leptin is a STOP sign for fertility hormones.

Your fat tissue produces the hormone leptin, and women who do not have enough fat tissue simply may not produce enough of this hormone.

Leptin not only controls appetite, but low levels of leptin also cause a lack of reproductive hormones.

This is nature’s way of putting the brakes on reproduction in times of famine! When there isn’t enough energy stored (fat), your body knows that it isn’t the right time for “the energetically expensive reproductive processes such as pregnancy and lactation”.  (study)

Leptin signals to nerves in the hypothalamus to control appetite, causing you to want to eat more when leptin is low.

But leptin also acts on different nerves in the hypothalamus that signal to the pituitary gland, controlling the release of reproductive hormones.  When there is sufficient leptin, the signal reaches the pituitary gland, telling it to release and luteinizing hormone (LH) and follicle stimulating hormone (FSH) at the right time in the correct amounts.(study)

When there is not enough leptin, that initiating signal doesn’t reach the pituitary gland, preventing release of adequate LH and FSH levels.

Animal studies clearly show the link between leptin and fertility. Female mice that produce no leptin are infertile. And giving those mice leptin restores their fertility.(study)(study)

Women who are underweight often stop having their periods. This is known as amenorrhea and is the primary cause of infertility among women with low BMI.

Researchers discovered this connection between leptin and amenorrhea over twenty years ago. Hundreds of studies now conclusively show that low leptin is the cause of menstruation stopping in women who lose too much weight. (study)

What does ‘low BMI’ mean, exactly?

BMI stands for Body Mass Index and is a way to estimate your body fat.  You can calculate your BMI using this Body Mass Calculator.

In the studies on women with low BMI and infertility, a BMI of less than 18.5 is the cut-off for being underweight.

A healthy BMI range is from 18.5 to 24.9. This is the range that seems to be ideal for infertility.

Causes for low BMI

It is so easy for women to get the idea that ‘thinner is better’. Everything you read or see on TV tends to reinforce the idea that being as lean as possible is healthy.


Chronic dieting

We are bombarded with ads for diets, our Instagram feed is filled with photos of thin women in bikinis, and everything you read gives you the impression that you are one slip-up away from being part of the obesity epidemic.

Dieting is a way of life for many women from a young age.  But constant dieting may be at the root of your infertility problems. If you are always hopping from one new diet to the next, it may be time to take a realistic look at your BMI as well as the nutritional content of your diet.


Eating disorders

Women with anorexia or other eating disorders often stop having their periods. This is due to the low levels of body fat causing a lack of leptin.  (study)

Studies show that women who have lost their period due to anorexia usually regain menstruation after weight restoration. Other studies show that along with weight restoration, prescription supplemental leptin may also help restore your period. (study)(study)


Intense exercise

Women who exercise excessively or who are competitive athletes will often find that they stop having their periods.  This is due to low leptin levels, which cause low levels of hypothalamic stimulation of gonadal hormones. The medical term for this is ‘hypothalamic amenorrhea’.

A study in the Journal of Endocrinology found that the menstrual problems of women athletes varied by the athletic discipline. Women who are involved in leanness sports (e.g. runners, gymnasts, bikers, dancers, and swimmers) simply may not produce enough leptin. (study)

A study of women athletes participating in college sports found that almost 25% of women competing in leanness sports had menstrual dysfunctions.(study)

This isn’t an uncommon problem or one that is only found in elite athletes. Studies show that for about  30% of women with amenorrhea the cause is chronic energy deficiency due to over-exercising and undereating. Restoring leptin levels in these women will cause them to regain normal menstrual cycles, allowing them to become pregnant. In addition to the lack of menstruation, over-exercising is also tied to thyroid dysfunction and bone loss.  (study)

Solutions for a low BMI

If you have been constantly dieting for years, it is time to take stock of your eating habits and calculate your BMI.  If your BMI is below 18.5, it is a good indication that gaining a little weight may raise your leptin levels.

If you are unsure whether low leptin is the problem, you can get a simple blood test done to check your leptin levels.    Leptin levels are highest in the morning, so get the blood test done in the afternoon for a true measure of your status. Normal leptin levels for women with a BMI between 18 - 25 are 4.1 to 25 ng/mL. (study)(study)

A healthy diet that includes enough fat from wild fish, grass-fed beef, and pasture-raised eggs may be all that you need to increase your leptin levels.

If you exercise a lot or are training as an athlete, cutting back on exercise to a more moderate amount may restore your leptin levels and your fertility.

If you suspect you have an eating disorder, it is time to seek some professional help.  Start with your doctor or OB/GYN. They should be able to refer you to a nutrition specialist trained in eating disorders.