Very restrictive diets and hair loss: what is known

Very restrictive diets can make hair fall out, and this has been known for decades. As early as 1976, a physician described in JAMA cases of hair loss in people who had followed "crash diets": the shedding began 2 to 5 months after they started, and the hair came back when they returned to normal eating. Since then, the mechanism has been well established: losing weight very fast, eating very little or cutting out whole food groups acts as a "shock" to the body, which saves resources by sacrificing the hair.

More recently, a 2024 study in the journal Cell showed that intermittent fasting slowed hair growth in mice and, in a small trial, in people as well.

In this guide we explain which diets carry more risk, what studies say about fasting and the ketogenic diet, which signs show that a diet is going too far, and how to lose weight without harming your hair.

What counts as a very restrictive diet

There is no single definition, but there are clear risk patterns:

• Very-low-calorie diets: less than 800 kcal per day. European guidelines only accept them for short periods, in selected patients and under specialist supervision (Yumuk et al., 2015).
• Diets below 1,200 kcal per day without planning: the same guidelines warn of the risk of micronutrient deficiencies.
• Crash diets: promises of losing many kilos in a few weeks, with minimal meals, soups, shakes or teas.
• Mono-diets: basing your eating on a single food, such as fruit, soup or eggs.
• Cutting out whole groups without adequate replacement: dropping meat and fish without making up the protein, or cutting all carbohydrates in an extreme way.
• Prolonged or repeated fasting without supervision.
• "Detox" and juice diets, low in protein and fat.

The common denominator is always the same: low energy, low protein and few micronutrients for weeks.

What the studies say

Crash diets: the first warning, in 1976

An article published in JAMA described 9 people with severe hair loss after very restrictive diets, with weight losses of 11.7 to 24.75 kg. The shedding began 2 to 5 months after the diet started. In hair tests, 25% to 50% of the hairs were in the resting phase, compared with about 13% under normal conditions. The hair grew back when the people returned to eating enough calories (Goette, 1976).

It is a small and old series, but it describes exactly the pattern that doctors still see today.

Intermittent fasting: a recent study

In December 2024, researchers at Westlake University in China published in the journal Cell a study on intermittent fasting and hair (Chen et al., 2024):

• In mice, two fasting regimens (16 hours without eating per day, or eating on alternate days) improved metabolic health but greatly slowed hair growth. Mice with normal access to food regrew almost all their fur within 30 days; those subjected to fasting only partly recovered after 96 days.
• The mechanism: during fasting, free fatty acids increased around the follicles, which led to the death of follicle stem cells that were trying to activate. Vitamin E applied to the skin partly protected these cells.
• In people, a small trial with 49 healthy young adults, fasting 18 hours a day for 10 days, showed hair growth that was on average 18% slower than in the control group.

How to read this study: it is the first to show this effect, and the human part was small and short. The authors acknowledge that in people the effect is probably milder and that larger studies are needed. It does not mean that intermittent fasting causes baldness, but it suggests caution in those who already have hair loss.

Ketogenic diet and very low-carbohydrate diets

There are no quality studies dedicated to hair loss with the ketogenic diet. Reports of shedding are mostly associated with rapid weight loss at the start of the diet and with insufficient intake of calories, protein or micronutrients when the diet is poorly planned. A ketogenic diet followed under supervision and with good food variety has no demonstrated link to hair loss in itself.

Eating disorders

In anorexia nervosa and other eating disorders, malnutrition can cause telogen effluvium, dry and brittle hair and the appearance of lanugo, a fine down on the body. Dermatologists stress that signs in the skin and hair can be the first visible clue to an eating disorder and should be approached without judgement and with a multidisciplinary team (Zhang et al., 2023).

Why hair is one of the first things to suffer

The hair follicle is one of the structures in the body where cells divide fastest. It needs energy, protein to make keratin, and micronutrients such as iron and zinc.

When food is scarce, the body makes choices: it protects the brain, the heart and the muscles, and saves wherever it can. Hair is not essential to survival. The typical result is telogen effluvium:

1. Many hairs moveat the same time from the growth phase to the resting phase;
2. they stay anchored in the folliclefor about 3 months;
3. they all fall out together,2 to 4 months after the diet begins.

That is why the shedding usually appears when the person has already lost weight and has even gone back to eating better, which confuses many people.

The nutrients most often lacking

• Protein: hair becomes thinner and sheds more.
• Iron: low ferritin is associated with hair loss in women (Treister-Goltzman et al., 2022).
• Zinc and folic acid: lower in people with hair loss after significant weight loss.
• Essential fatty acids: diets with almost no fat can leave hair dry and brittle.
• B vitamins and vitamin D.

We explain each one in detail in Protein, iron and zinc: what hair needs.

Diet by diet: risks and precautions

This table is a general guide, based on known mechanisms and obesity management guidelines. For most fad diets there are no specific studies on hair.

Warning signs

In the body

• diffuse hair loss that begins weeks to months after starting the diet;
• hair that is drier, thinner or more brittle;
• brittle or ridged nails;
• persistent tiredness, dizziness, constantly feeling cold;
• dry skin, slow wound healing;
• missed or irregular periods;
• loss of muscle strength.

In the relationship with food

Some restrictive diets are one-off attempts to lose weight. Others can be a sign of a difficult relationship with food. It is worth paying attention if:

• food, weight or body shape take up a large part of your thoughts;
• there is an intense fear of gaining weight, even when underweight;
• meals with family or friends are avoided;
• there are episodes of binge eating followed by guilt, vomiting, laxatives or excessive exercise;
• the rules about food are getting stricter and stricter.

If you recognise yourself in these signs, or recognise someone close to you, talk to your family doctor. Eating disorders can be treated, and the sooner help is sought, the better.

How to lose weight while protecting your hair

If the goal is to lose weight, there are ways to do it that protect your hair and your health:

1. A moderate deficitEuropean guidelines recommend a deficit of about 600 kcal per day, which corresponds to about 0.5 kg per week, and consider it realistic to lose 5% to 15% of body weight in 6 months (Yumuk et al., 2015).
2. Protein at every mealIt is the factor that most protects hair and muscle during weight loss. See Losing weight without losing hair: protein and nutrients.
3. Do not cut out whole groupswithout replacing the nutrients they provide.
4. Variety:vegetables, fruit, pulses, whole grains, fish, eggs, dairy, olive oil, nuts.
5. Strength trainingtwo to three times a week, in addition to aerobic exercise.
6. If you do intermittent fasting,make sure the eating window has enough calories and protein, and reconsider if you notice hair loss or if you already have alopecia.
7. Follow-up by a doctor or dietitian,especially if the goal is to lose a lot of weight, if you have chronic conditions or if you have already had hair loss with previous diets.
8. Blood tests at the start and a few months later,if the diet is more restrictive. See Blood tests for hair loss: what to ask for.

If your hair is already falling out

You do not have to give up on your goal. What helps is correcting your diet: more protein, more variety, a smaller deficit. Hair tends to recover within 6 to 12 months once eating normalises. See how in Eating well again: where to start.

When to seek a medical assessment

At Luxmed, the Hair Medicine consultation assesses the cause of the hair loss with trichoscopy and, when indicated, blood tests, and helps work out whether diet is behind the shedding.

This article is for information only and does not replace the follow-up of your doctor or dietitian.

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