Should I try Intermittent Fasting? Understanding the Effects of the Diet on Hormones

Intermittent fasting and autophagy

Any new fad or trend should be taken with a grain of salt. This is particularly true when it comes to intermittent fasting. The popular diet can have a significant effect on our hormones. Curious to learn more ? Keep reading!

What is Intermittent Fasting ?

Intermittent fasting (IF) involves alternating between regular eating and fasting on a regular schedule. There are several Intermittent Fasting schedules, but all start with deciding on regular eating and fasting windows of time. You may, for instance, try eating only for eight hours each day and fasting the other sixteen hours. Alternatively,  you could decide to only eat one meal each day on two days per week. During the fasting period, one abstains from food, but can drink water and other calorie-free beverages like black coffee or tea. Although Intermittent Fasting has well-established benefits on weight loss, studies have revealed mixed effects of the dietary pattern on  hormone levels. 

Intermittent fasting is a double-edged sword with both beneficial and detrimental effects on hormones

How does Intermittent Fasting affect hormone levels? What are the range of hormones it targets?

While Intermittent Fasting is predominantly used as a weight-control strategy, it has complex effects on the body’s hormones. There is a myriad of hormones that Intermittent Fasting targets, with the core ones being human growth hormone (HGH), cortisol, insulin, estrogen, progesterone, follicular stimulating hormone (FSH), luteinizing hormone (LH), kisspeptin, insulin, glucagon, and thyroid hormone.  “There is substantial research that suggests that Intermittent Fasting may help to improve hormonal balance. This can enhance metabolism and sleep, leaving us feeling more energized and also help us more effectively burn caloriesreports James Roche, Registered Dietitian (RD) and Nutritionist with Sisters of Charity of Leavenworth (SCL) Health.

As an example, fasting for at least 16 hours can result in a considerable drop in insulin. A decrease in insulin assists not only in fat burning, but also lessens one’s risk of contracting certain illnesses, like pre-diabetes and diabetes. Likewise, it can elevate levels of HGH, which in turn actively combats age-related drops in bone density and muscle mass. However, effects may also be detrimentalit all depends on your health status. For instance, thyroid hormones regulate metabolism and energy levels. While fasting, the synthesis of these hormones decreases.

This is great news for people who have an overactive hyperthyroid, but perilous for those who have a sluggish hypothyroid. In a nutshell, the effects of Intermittent Fasting on hormones are complex, and largely depend on the health status of the patient.

Intermittent Fasting and Insulin Levels

Can Intermittent Fasting improve insulin resistance and hormonal balance?

The hormone insulin regulates glucose levels in the blood. When glucose levels in the blood are high, such as after a meal, insulin stimulates uptake and storage of excess glucose in the form of a sugar known as glycogen, and fat. The hormone also blocks glucose production in the liver. Collectively, these effects help lower blood glucose post meal consumption to maintain normal levels. If you constantly eat all day, you are continually producing insulin. When there is too much insulin, the cells of our musclesliver and fat tissue become less responsive to the hormone. As a result,  glucose cannot enter the cells as easily, and it builds up in the blood. This is known as insulin resistance, and can increase susceptibility to type 2 diabetes.

Studies have  revealed that Intermittent Fasting can improve insulin sensitivity. Fasting for at least 16 hours allows the body to rest, and re-balances insulin to normal levels in the blood. This not only aids in fat loss, but also reduces one’s risk of disease, particularly diabetes and pre-diabetes. Indeed, Victoria Della Rocca, RD and Nutrition Director of the Weight and Metabolic Health Program, reports that  “when fasting decreases insulin levels, it can be very helpful to people with diabetes and pre-diabetes, and people who struggle with their blood pressure and blood sugar levels.”


Intermittent Fasting and Cortisol levels

What impact does Intermittent Fasting have on cortisol and stress hormones?

Cortisol is a steroid hormone produced and released by the  adrenal glands, which are located on top of your kidneys. Cortisol exerts many effects on the body, primarily regulating your body’s response to stress. In addition to regulating stress, cortisol also regulates blood pressure and sugar, as well as the breakdown of fat, proteins and carbohydrates (in other words, your metabolism).

In addition to cortisol, our body also releases other  hormones like epinephrine (adrenaline), and norepinephrine during times of stress. Collectively, these stress hormones increase our heart rate, blood pressure and sugar levels in order to either ‘fight’ or flight’ the stressor. During the fasting stage of intermittent fasting, you are depriving your body of solid food for an extended period of time. This can trigger an increase in both the  levels and frequency of cortisol.

Indeed, several studies have confirmed this finding. Bergendahl et al (1996) reported that five-day fasting can increase cortisol levels and shift peak production of cortisol from the morning to the afternoon. Likewise, Johnstone et al (2004) showed that fasting for 6 days dramatically elevates blood cortisol levels from baseline levels. Similar findings have also been shown for the remainder of stress hormones. In addition to cortisol, epinephrine (adrenaline), and norepinephrine are also elevated during Intermittent Fasting.  Indeed, Michalsen (2010)  reported increased blood levels of both stress hormones during the first 7 days of intermittent fasting schedule.

Intermittent Fasting and Leptin levels

How does intermittent fasting affect leptin levels and appetite control?

Leptin is a hormone produced by fat tissue that helps your body maintain a healthy weight over time. This is accomplished by creating a feeling of  satiety (feeling full) to regulate appetite.

In addition to suppressing appetite, the hormone also elevates caloric expenditure, and hence helps burn fat. Similar to insulin, our body can develop resistance to this hormone as well. In leptin resistance, leptin levels become very high. This causes target cells located in a region of the brain called the hypothalamus, to become desensitized to the hormone. Without the satiety effects of leptin, our body is tricked into constantly feeling hungry which makes it want to eat and store more fat.

Such resistance can develop from:

  • too little sleep
  • too much stress
  •  too much of the wrong foods. 

Fortunately, intermittent fasting can decrease levels of leptin and re-sensitise hypothalamic cells to the hormone.

Several studies have provided evidence for the same. In fact, a recent meta-analysis of 545 people, the majority of whom were overweight or obese, discovered that intermittent fasting schedules are related with lower body mass index (BMI), leptin blood levels, improved leptin sensitivity and decrease in appetite.  

Intermittent Fasting and the Menstrual Cycle

Can intermittent fasting help regulate menstrual cycles, and hormonal imbalances?

As a heads up, the menstrual cycle describes the series of events that occur in your body each month as it prepares for the possibility of pregnancy each month. On average, it lasts for 28 days, from the first day of your monthly period till the first day of your next monthly period. 

During the cycle, your body experiences many hormonal fluctuations, especially in the reproductive hormones estrogen, progesterone, FSH and LH. While intermittent fasting is overall a a healthy dieting strategy, it may exert unwanted side effects on your menstrual cycle. This is because the female reproductive system is particularly sensitive to calorie consumption and levels.

During fasting periods, you limit your caloric intake for a prolonged time period. This can lead to lower progesterone levels, the female hormone that peaks during the second half of the menstrual cycle. The hormone plays a very important role in ovulation, a critical step involving the release of the egg for fertilization by sperm. By decreasing progesterone levels, intermittent fasting can reduce chances of ovulation, and hence contribute to menstrual irregularities. 

By restricting calorie consumption during fasting windows, you also expose your body to high levels of stress, as evident by the increase in stress hormones that comes hand in hand with intermittent fasting. With heightened stress, release of FSH and LH from the pituitary gland, a structure in the brain, is decreased. These are also critically involved in the menstrual cycle, as they regulate release of progesterone and estrogen during the menstrual cycle. If the fasting window is extended too far, it can trigger menstrual irregularities including amenorrhoea, which is a temporary, intermittent or permanent absence of periods.

Hence, while intermittent fasting schedules are useful in weight loss, they have side effects which extend to your menstrual cycle. Women, especially those who are beginners to fasting, should regularly consult their health care provider to best optimize meal plans and intermittent fasting strategies.  


Conclusively, intermittent fasting is all about when you eat, rather than what you eat. It involves restricting eating to certain time frames, and fasting in between. While it has become extremely popular as a weight loss strategy, its effects on hormones are complex, and depend on the health status of the individual. As an example, intermittent fasting may be  particularly beneficial for those predisposed to diabetes, or who have diabetes, as it lowers insulin and burns fat. A similar landscape exists for obese individuals, where intermittent fasting lowers leptin levels and increases sensitivity to the hormone for satiety. For individuals with hyperthyroidism, intermittent fasting is once again beneficial, as it lowers thyroid hormones to baseline levels.

However, intermittent fasting has also got a fair set of repercussions. Fasting for a prolonged time frame is a major stressor, and elevates stress hormone levels which are bad for the body. For individuals with hypothyroidism, intermittent fasting may exacerbate already low levels of thyroid hormones, and hence worsen symptoms.

Finally, for women on their menstrual cycle, intermittent fasting can mess up their schedule,  if not implemented appropriately. Overall, intermittent fasting is a double-edged sword with both benefits and side effects. It is in your best interest to consult a healthcare provider before unpacking the quest of this famous nutritional trend! 



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