A comprehensive study examined the effects of intermittent fasting on reproductive hormone levels in men and women. The results suggest that intermittent fasting may affect androgen markers such as testosterone and the free androgen index (FAI) in premenopausal women who are overweight. In particular, it appears to lower testosterone levels and increase levels of sex hormone-binding globulin (SHBG), particularly when food intake is limited to the early hours of the morning and ends before 4 p.m.

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The changes in hormonal balance during fasting in both sexes are examined in more detail below:

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Men

• Testosterone
  ‍Fasting can lead to a slight drop in testosterone levels, especially in active men with low body fat. However, muscle mass and strength do not appear to be affected. Interestingly, some studies suggest that fasting may improve testosterone utilization, potentially leading to better muscle growth. On the other hand, reduced androgen levels in men could have effects on metabolic health and libido.

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Women

• Sex hormones
  ‍Fasting, particularly strict or prolonged fasting, can disrupt the delicate balance of sex hormones in women. That's because levels of luteinizing hormone (LH), which is important for ovulation, fall. As a result, menstrual periods may become irregular or even completely absent.
• PCOS
  ‍For women with polycystic ovary syndrome (PCOS), fasting can be a double-edged sword. While it can disrupt menstrual cycles, studies show that it can also be beneficial. Fasting can help regulate androgen levels, which are often elevated in PCOS, and improve insulin resistance, a common symptom of PCOS.
• Other hormones
  ‍Fasting may lower levels of dehydroepiandrosterone (DHEA) in women. DHEA plays a role in fertility, so this drop can be particularly problematic for premenopausal women who want to become pregnant.

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General hormonal considerations for both sexes

• Stress response
  ‍Fasting can trigger the release of the stress hormone cortisol. For women who already suffer from stress or hormonal imbalance, this increase in cortisol can be particularly stressful.
• Thyroid function
  ‍Prolonged or restrictive fasting can lead to reduced thyroid function in both men and women, which in turn can lead to tiredness, sluggishness, and difficulty losing weight.

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Key findings

Fasting is not a panacea. The individual hormonal situation must be taken into account, especially for women with hormone problems or who want to have children.

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• Listen to your body
  ‍If irregular menstrual periods, increased tiredness, or other worrying symptoms occur during fasting, fasting should be discontinued and a doctor should be consulted.
• Alternative approaches
  ‍If the hormonal effects of fasting are a cause for concern, other dietary approaches, such as conscious eating or controlling portion sizes, may be a more appropriate choice to lose weight or improve health.

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Fasting can be a valuable tool, but it is important to be aware of the potential hormonal effects and adapt fasting to individual needs, particularly with regard to hormone levels.

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Previous research suggests that fasting affects hormone levels differently in men and women, but further studies are needed to confirm these findings.