Can intermittent fasting slow down hair growth?

Intermittent fasting, the practice of alternating between periods of fasting of varying lengths and normal eating times, is gaining increasing popularity. When done in moderation, this method could be beneficial for health, allowing for better management of blood sugar levels and type 2 diabetes by increasing the body's sensitivity to insulin and fat breakdown. Intermittent fasting may also have positive effects on cardiovascular health, as it could help reduce body fat, blood pressure, and heart rate.

Depending on the fasting duration, we can distinguish different types of intermittent fasting, the main ones being the 24-hour fast, carried out once or twice a week; alternate-day fasting; and the so-called 5/2, 16/8, 20/4, and 36/12 methods. The 5/2 method involves limiting calorie intake to approximately 600 calories on two non-consecutive days per week. The 16/8, 20/4, and 36/12 methods are understood as follows: the first number indicates the number of fasting hours and the second the number of hours during which it is possible to eat. It should be noted that the 20/4 and 36/12 methods are quite radical and often lead to weight loss, which can be dangerous for individuals who are already slim.

If you're considering starting intermittent fasting, it's recommended to first seek medical advice.

While most scientists agree that intermittent fasting is beneficial for health, some caution against a potential unexpected side effect: disruption of the hair cycle and slowing of hair growth. This hypothesis is based on previous studies that have shown that a low-calorie diet could lead to a hair loss increase. To test this, a group of scientists conducted a ten-day clinical study with 49 healthy volunteers, randomly divided into three groups: the first subjected to an 18/6 intermittent fasting (TRD), the second following a diet with energy restriction (ERD) of 1,200 to 1,500 kcal/day, and a control group with normal food intake (ND).

Just before the study, the researchers shaved a 1 cm² area on the participants' scalp. They then measured the length of the regrown hair three days later. They did the same at the end of the period to compare the hair growth rate for each participant, before and after the fasting period. The results indicate that, compared to the control group, the group subjected to intermittent fasting (TRD) had hair growth deemed significantly reduced by the researchers (p = 0,0028). For your information, a result is generally considered statistically significant when its p-value is less than 0,05.

More specifically, it was assessed that the average hair regrowth speed had been reduced by 18% in the TRD group. Blood tests were also conducted and showed that 18 hours of fasting led to an increase in lipolytic hormones in the participants' blood. These hormones, involved in the breakdown of fats to provide energy to cells, could divert available resources at the expense of hair follicles. Intermittent fasting could thus indeed impact hair follicles and hair growth.

To better understand the underlying biological mechanisms, researchers studied the effect of 16/8 intermittent fasting, referred to as TRF in the study, and alternate fasting, named ADF, on 60-day-old mice whose hair follicles were in the telogen phase. As a reminder, the hair cycle follows three phases: the anagen phase (growth phase), the catagen phase (resting phase), and the telogen phase (shedding phase). The mice were shaved before the treatments, and their hair regrowth was monitored over a period of 96 days.

Three groups of mice were formed, the number of which was not disclosed: the first group consisted of mice fed ad libitum (AL), the second group included those following a 16/8 intermittent fasting (TRF), and the third group comprised animals subjected to alternate day fasting (ADF). The scientists observed a regrowth of the dorsal hair of the AL mice from day 80. However, the TRF and ADF animals showed significantly altered hair follicle regeneration, with only partial hair regrowth observed on day 156. Nevertheless, the lack of information on sample sizes complicates the analysis of these results, as this factor could greatly influence the robustness of the findings. This robustness could also be strengthened by repeating the experiment several times with other groups of mice.

Furthermore, a haematoxylin and eosin stain revealed that the hair follicles of mice following intermittent fasting were locked in a prolonged telogen phase, resulting in a lack of new hair shaft production. This staining technique, widely used in histology, allows for the detection of changes in cellular structures and here enabled the identification of follicle abnormalities. Indeed, follicles in the telogen phase are characterised by miniaturisation and partial atrophy of follicular cells. Follicles in the anagen phase, on the other hand, show expansion and the presence of actively proliferating cells, allowing for their identification.

Intermittent fasting appears to slow down hair growth in rodents.

Furthermore, researchers used RNA sequencing techniques to compare gene expression between mice that were fed or had undergone a fasting period of 24 or 48 hours, and to determine which genes are activated or repressed. They discovered a differential regulation of several metabolic pathways, including those involved in fatty acid oxidation, which were strongly activated after 24 hours of fasting. Concurrently, genes associated with glucose synthesis, such as those regulating gluconeogenesis, showed a decrease in expression, indicating a shift in energy metabolism towards a preferential use of lipids. Additionally, certain markers of oxidative stress were upregulated. Finally, some genes related to inflammation were also less expressed, which could explain the anti-inflammatory effects reported in other studies on intermittent fasting.

Two major hypotheses have been proposed by researchers to explain this observation.

• An inter-organ communication between the adrenal glands and adipocytes in hair follicles.

  One of the hypotheses proposed by the authors is that intermittent fasting activates an inter-organ communication mechanism, involving the adrenal glands and adipocytes in hair follicles. In response to fasting, the adrenal glands increase the production of cortisol, a hormone that aids in mobilising energy reserves by breaking down fats and proteins. However, this hormone could have negative effects on hair growth by inhibiting the proliferation of stem cells, thus reducing their ability to multiply and renew hair follicles.

  Cortisol is also suspected of disrupting the hair cycle and promoting the entry into the telogen phase. Concurrently, during fasting, adipocytes release free fatty acids into the bloodstream as part of the fat mobilisation process. However, this rapid release of fatty acids is suspected to slow down the metabolism of stem cells in hair follicles. This initial hypothesis of inter-organ communication between the adrenal glands and adipocytes in hair follicles was formulated when researchers observed a correlation between the activation of the adrenal glands and that of the stem cells in the hair follicle. While it seems plausible, it needs to be validated in future studies.

• An increase in oxidative stress within the hair follicles.

  The researchers' second key hypothesis is that intermittent fasting induces increased oxidative stress in the stem cells of hair follicles. Indeed, energy metabolism changes during fasting and lipolysis intensifies. However, when fat metabolism increases, the production of reactive oxygen species also increases. These unstable molecules are known for their ability to damage cellular structures, including DNA, proteins, and cell membranes. The stem cells of hair follicles are no exception and are sensitive to oxidative stress. This stress, due to the accumulation of free radicals, can cause the apoptosis of stem cells in hair follicles, that is, their cellular death. This hypothesis has been supported by preliminary work carried out in vitro on hair follicles by the researchers but still requires further investigation.

While these mechanisms are plausible, they currently remain as simple hypotheses.

While these studies suggest that intermittent fasting could slow hair growth, it is important to exercise caution. Indeed, to date, only one clinical study has been conducted and it involved a relatively small number of volunteers. This calls for caution and not to draw hasty conclusions: even if a difference with a solid p value was observed, the results could be influenced by the size of the group or interindividual variability. Therefore, these initial results need to be confirmed in further studies to validate their robustness.

Another limitation of the study, highlighted by the researchers themselves: it was only conducted on healthy adults, thus excluding individuals suffering from obesity, i.e., those who most often adopt intermittent fasting diets. Moreover, the long-term effects of intermittent fasting on hair growth have not been evaluated, as the study was only conducted over ten days. Therefore, while it's possible that a correlation exists between intermittent fasting and the rate of hair growth, this needs to be validated in the course of further studies, carried out over longer periods and with more volunteers.

Although these initial findings provide intriguing insights, further research is required to confirm a potential link between intermittent fasting and the slowing of hair growth.

• SEARS D. & al. Metabolic effects of intermittent fasting. Annual Review on Nutrition (2017).

• DAMIANI G. & al. Fasting and its impact on skin anatomy, physiology, and physiopathology: A comprehensive review of the literature. Nutrients (2019).

• VARADY K. & al. Effect of intermittent fasting on reproductive hormone levels in females and males: A review of human trials. Nutrients (2022).

• ZHANG B. & al. Intermittent fasting triggers interorgan communication to suppress hair follicle regeneration. Cell (2024).