Intermittent Fasting in Men and Women

Biology has played a part in shaping male and female metabolic responses to exercise, carbohydrates, sleep deprivation, and (you’ve guessed it) fasting.

While intermittent fasting can benefit both sexes, it may affect men and women differently. The discrepancies may be the result of hormonal differences. Besides influencing insulin, norepinephrine, and HGH levels, intermittent fasting may affect female sex and hunger hormones in specific ways. There are possible risks for women to consider regarding reproductive health, bone health, and overall well-being. Research involving female intermittent fasting is in its early days, and large scale studies are sorely missing.

Studies performed on rodents suggest that intermittent fasting may cause changes in estrogen levels and negatively affect reproductive functions, such as menstrual regularity, fertility, pregnancy, and lactation [1]. Although the hormone levels of male rodents were affected, their reproductive functions were not affected as much. One possible reason for this is that some aspects of reproduction in women require more energy than those in men.

Intermittent Fasting Differences in Men and Women

Biology has played a part in shaping male and female metabolic responses to exercise, carbohydrates, sleep deprivation, and (you’ve guessed it) fasting.

While intermittent fasting can benefit both sexes, it may affect men and women differently. The discrepancies may be the result of hormonal differences. Besides influencing insulin, norepinephrine, and HGH levels, intermittent fasting may affect female sex and hunger hormones in specific ways. There are possible risks for women to consider regarding reproductive health, bone health, and overall well-being. Research involving female intermittent fasting is in its early days, and large scale studies are sorely missing.

Studies performed on rodents suggest that intermittent fasting may cause changes in estrogen levels and negatively affect reproductive functions, such as menstrual regularity, fertility, pregnancy, and lactation [1]. Although the hormone levels of male rodents were affected, their reproductive functions were not affected as much. One possible reason for this is that some aspects of reproduction in women require more energy than those in men.

The Similarities Between the Sexes

Though many claim otherwise, there is research that suggests that men and women respond similarly to alternate-day fasting (ADF) and time-restricted eating. Though the data are limited, scientific findings suggest that both men and women have similar adherence rates, weight loss, and body composition changes in both alternate-day fasting and time-restricted eating.

One key study found no notable difference between men and women that tried alternate day fasting for 12 weeks [2]. Though “bad” cholesterol (LDL) decreased slightly more in premenopausal women versus post-menopausal women, but there were no significant differences in weight loss, fasting glucose, HDL cholesterol, and triglycerides.

Another recent study focused on factors like age, sex, race/ethnicity, and baseline body weight/BMI [3]. It found that white males and people between the age of 50 and 59 years lost slightly more weight. Though 60 percent of all 121 participants lost 4 to 8 percent of their body weight, sex didn’t seem to make a difference.

The Differences Across Sexes

Though there are similarities between both genders when it comes to intermittent fasting, there are also a lot of differences. During alternate day fasting, researchers found that after three weeks, women had a slightly worse glucose response than men [4]. The men, on the other hand, had a worse insulin response [4, 5]. Interestingly, there were no changes in muscle-related genes.

Other studies also suggest that, overall, women may experience more issues with glucose clearance. Some experts think this may have to do with the difference in male and female body compositions, while others think women may be more prone to insulin issues because of differences in carbohydrate metabolism [6]. 

Evidence suggests that male and female livers may also perform differently when regulating glucose while fasting [7]. Women may release less glucagon (a hormone released to help raise blood sugar levels when they are low) to fuel them through exercise compared to men.

It is also common for women to feel more hungry during a fast than men [8].

Intermittent Fasting and the Male Body

Fasting has a huge role to play in testosterone levels. For men, testosterone supports and increases muscle mass and strength through improved protein synthesis. It blocks fat uptake and storage and burns fat instead. Testosterone also promotes healthy kidney function and bone growth and density. Testosterone also has positive effects on cognition and mental health.

Intermittent fasting has several ties with testosterone levels. Firstly, it elevates growth hormone levels [9]; supporting muscle growth, penile function, and better cognition [10-12]. Secondly, it also increases LH (luteinizing hormone) levels. And lastly, it regulates leptin levels [13], which in turn, stimulates testosterone secretion from the hypothalamus [14].

During short periods of fasting (12-24 hours), one study showed that men’s metabolism increases up to 14% [15]. In addition, another study reports that the male body improves testosterone utilization (10-200%), growth hormone (100-200%) production [16], and an improvement in blood lipids to support the increased hormonal production and decreased risk factors for cardiovascular disease.

Intermittent Fasting and the Female Body

Some evidence suggests that intermittent fasting may not be as beneficial for some women as it is for men. One study showed that, after three weeks of alternate-day fasting, the blood sugar control in women worsened [4]. Plus, many anecdotal stories claim that menstrual cycles may be affected.

Such shifts occur because female bodies are extremely sensitive to calorie restriction [17]. If your calorie intake is too low the hypothalamus (a small area in the brain that plays an important role in hormone production) is affected. 

This may disrupt the secretion of gonadotropin-releasing hormone (GnRH), a hormone that helps release two reproductive hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH) [18, 19]. If these hormones cannot communicate with the ovaries, you run the risk of irregular periods, infertility, poor bone health, and other health effects [18].

Studies in rats have shown that 3–6 months of alternate-day fasting caused a reduction in ovary size and irregular reproductive cycles [20, 21]. Similar studies have not been conducted on humans yet.

Why Do Women Respond Differently? A Few Theories

There are some theories as to why men and women respond differently to intermittent fasting. Firstly, the hypothalamic-pituitary-gonadal (HPG) axis, responsible for regulating key hormones that are involved in key functions, including appetite control and reproduction, is incredibly sensitive to energy intake. The effect of a negative energy balance is thought to work through kisspeptin. Kisspeptin is a protein-like molecule that neurons use to communicate with each other. It has an important role in the regulation of reproductive functions and, according to recent studies, metabolism [22].  Research suggests that women have more kisspeptin than males, meaning greater sensitivity to changes in energy balance.

Secondly, it also seems that male and female livers are different when it comes to energy metabolism. A study that observed the livers of mice following intermittent fasting indicates livers of male mice stopped producing energy storage molecules. The livers of the female mice, on the other hand, made use of all available resources in an effort to store the energy necessary to maintain their reproductive capacities.

Particular Considerations for Women

While there are many possible benefits of intermittent fasting, some women should avoid it unless cleared by their doctor. The first group of women that should consult their doctor is women who want to conceive, who are pregnant, or breastfeeding. Intermittent fasting is generally not recommended for women who are pregnant, as it may impact the growth of the baby and cause other negative side effects.

The other set of women that should take caution is those with medical conditions, including diabetes, women who are on medication that should be taken with food, and women with autoimmune diseases.

It is not advised to do intermittent fasting if you have a history of eating disorders. Fasting is also not considered safe for children or pre-adolescent girls.

Though males and females might be different when it comes to intermittent fasting, it doesn’t mean that females will not experience the benefits related to intermittent fasting. The fasting schedules that women should follow compared to men might just be different. Women are advised to follow less-strict intermittent fasting schedules with a well-balanced diet in their eating windows.

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