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The Challenges Women Face with Intermittent Fasting and Keto Diets 

When it comes to eating to improve your health, I always tell my clients that there is no one-size-fits-all dietary approach. This is particularly true for popular eating patterns like the ketogenic (keto) diet and intermittent fasting (IF). As you may be aware, the keto diet is a high-fat, very low-carb diet that prompts the body to enter into a metabolic state called ketosis, where fat is burned as the primary fuel source instead of glucose. Intermittent fasting, on the other hand, focuses on restricting food intake to certain times of the day and fasting for the remainder of the time. Similar to the keto diet, intermittent fasting can shift the body’s preferred fuel source from glucose to fat.  While they are both touted for their positive effects on weight loss, appetite control, and insulin sensitivity, these dietary approaches may not be suitable for everyone—specifically women of childbearing age. 

In fact, you may be surprised to learn that many of the studies on the benefits of keto and IF were conducted using male participants. As a result, the differences between the male and female physiology are often unaccounted for. For example, women have higher levels of estrogen and progesterone, while males have a higher level of testosterone, which gives men the advantage of increased muscle mass and a greater ability to burn fat. But more importantly, the dietary constraints associated with IF and keto may have a greater impact on women’s hormone levels and reproductive function, which may negatively impact women’s health in the following ways.

Heightened Stress Response 

Research shows that both intermittent fasting and the keto diet activate the body’s stress response system, prompting the adrenal glands to release the stress hormone cortisol. This rise in cortisol occurs as the body adapts to the time-specific absence of food and the transition from using carbohydrates to fat as its primary energy source. While this can be a beneficial mechanism in the short-term, long-term heightened cortisol levels can potentially lead to weight gain around the midsection. Therefore, women with a history of elevated cortisol levels often have a harder time shedding excess weight when they adopt these dietary regimens. But guess what? The nutrient associated with reduced cortisol levels is none other than carbohydrates! For this reason, some women may find it beneficial to add more nutrient-dense, fiber-rich carbohydrates to their meals instead of strictly adhering to a low-carbohydrate diet.

Decreased Thyroid Function

Women following the keto diet or IF are also at a greater risk of reduced thyroid function. One reason this occurs is because the ketogenic diet is known to lower the appetite-suppressing hormone leptin. Leptin is a hormone that is produced by adipose (fat) tissue; therefore, women typically have higher circulating levels of leptin than their male counterparts. When leptin levels fall too low, it may lower thyroid hormone levels and increase appetite. As a result of reduced thyroid function and increased hunger, many women experience weight gain instead of the weight loss they desire. Another reason this occurs is because the ketogenic diet is low in carbohydrates, and the thyroid relies on glucose to make thyroid hormones.  

Regarding intermittent fasting, research highlights that fasting rapidly decreases thyroid hormones. However, thyroid function is restored once the fasting period ends and food is re-introduced. Therefore, prolonged fasting can significantly slow thyroid function if the time window of food consumption isn’t balanced with a sufficient amount of calories. This is why I always tell women who are interested in intermittent fasting that it’s not about calorie restriction; it’s about reducing the hours in the day when you are actively digesting food to reap the benefits of mental clarity and increased longevity.

Altered Menstrual Cycle 

Over the course of the menstrual cycle, estrogen and progesterone levels are tightly regulated by a hormone called gonadotropin-releasing hormone (GnRH). However, prolonged fasting can halt the release of GnRH, subsequently disrupting the production of estrogen and progesterone. Similarly, restricting carbohydrate intake on keto diets can impair women’s hormones. As a result of altered sex hormones, women often experience irregular or absent periods. Therefore, prolonged fasting may be too stressful for some women’s bodies, causing menstrual cycle irregularities. For these individuals, I recommend switching to a 12-hour fasting window and a 12-hour eating window instead of fasting for the typical 18 hours. 

Infertility and Birth Defects 

One of the concerning risks associated with strict intermittent fasting and keto diets is potential infertility. As mentioned above, women are more hormonally sensitive to changes in nutrient timing and balance. This is due, in part, to having higher levels of a protein called kisspeptin than men. Kisspeptin is a key component of reproduction, as it is the molecule that stimulates the release of GnRH—which, again, influences the production and secretion of sex hormones. As a result, kisspeptin plays a pivotal role in maintaining fertility. Therefore, stress and shifts in energy status, like those that accompany overly restrictive keto diets and fasting, may greatly affect kisspeptin levels and fertility. 

In addition, the keto diet may contribute to an increased risk of birth defects. More specifically, research reveals that women who reported following low-carb diets the year before trying to conceive were 30% more likely to give birth to an infant with neural tube defects, such as spina bifida or anencephaly (a condition that affects the development of the baby’s brain and skull). This is particularly alarming because 40% of pregnancies are unplanned. Therefore, dietary habits during the childbearing age should be taken into serious consideration. 

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Nutrition science is ever-evolving, highly nuanced, and requires an individualized approach. This is why no single diet works for everyone—especially women. However, the above risks don’t mean that these diets are completely ineffective and unsafe for every woman. After all, we all respond to dietary changes differently. And I am a huge proponent of intermittent fasting for those who benefit. If you need help navigating or tweaking either of these dietary approaches to meet your unique hormonal needs, feel free to book a one-on-one consultation with me here so I can safely guide you along your health journey.

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