Because a complex web of hormones interacts in your body, weight loss and changes in hormone levels is not a simple relationship but part of a system of physiological processes. Depending on the amount and speed of weight loss and your original weight, your hormone levels will either decrease or increase accordingly. Fat cells generate estrogen, testosterone and leptin, a hormone which regulates appetite and metabolism. When body fat decreases, it impacts the availability of these hormones and can hinder reproductive processes, including sperm production, menstrual cycles and follicular development.
Moderate Weight Loss
The optimal percentage of body fat depends on several factors, such as your body type, heredity, gender, age, diet and exercise regimen. For women, a certain amount of body fat is critical to sustain reproductive processes. Weight loss that amounts to greater than 10 to 15 percent of your normal body weight will result in decreased levels of estrogen. As you’re losing pounds, your brain, or hypothalamus, registers a shortage of energy and transmits signals to your body to conserve energy. The energy that would normally be used for reproductive functions must be diverted to other physiological processes. If you gain back the weight to within 95 percent of your original weight, your hormones will return to normal levels.
Athletes and Caloric Restriction
Certain types of athletes, such as gymnasts, wrestlers and dancers, seek to maintain either a lower percentage of body fat or a significantly low body weight. These athletes may dramatically restrict their energy consumption and disrupt their endrocrine function. Levels of various hormones, such as growth and sex hormones, plummet. Symptoms of a lowered metabolism include hypotension, sluggish reflexes, a slower heartbeat and rate of respiration, according to Sari Edelstein’s “Life Cycle Nutrition: An Evidence-Based Approach.”
Eating Disorders
Women with eating disorders, such as anorexia nervosa and bulimia, may be so underweight that their bodies cease to produce estrogen. The severe deficiency of fat blocks cells from converting cholesterol into sex hormones. Dramatic weight loss also reduces male fertility. During World War II, starvation experiments revealed that men who endured a 50-percent decrease in energy intake lost their libido, according to Judith E. Brown’s “Nutrition through Life Cycle.” When more than 25 percent of body weight was shed from normal levels, men stopped making sperm.
Obesity
In contrast to men of average weight, obese men tend to exhibit lower levels of testosterone and higher levels of leptin and estrogen. They tend to suffer from erectile dysfunction and lower sperm production. Obese women typically have higher levels of leptin, estrogen and testosterone, which leads to irregularities in their menstrual cycles. Significant weight loss can restore hormone levels to normal in both obese men and women.
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