Crash diet, then hair loss

In the spring you lost eight kilos in six weeks: no bread, no sugar, a salad at lunch and next to nothing in the evening. You were pleased with the result. Three months later the shower drain clogs and your brush fills up. You don’t connect the two, yet it’s a familiar sequence. The American Academy of Dermatology puts it plainly: eating too few calories every day can, on its own, cause significant hair loss¹.

Here’s why, how long it lasts, and what your plate can put right.

Why dieting makes hair fall out

At the root of every hair are cells that divide constantly to build the hair shaft. They are among the fastest-dividing cells in the whole body². They need energy, protein and iron. When a diet cuts energy too far, they no longer get enough: that’s the explanation given by the doctors who described this kind of shedding³.

In practice, lots of hairs leave their growth phase at the same time and switch to the resting phase. Normally about 15% of hairs are resting; during this kind of shedding, that rises to 30% or more⁴. Dermatologists call it telogen effluvium, and British dermatologists list marked weight loss and extreme dieting among its common triggers, alongside major stress or surgery⁴.

The follicle, the small pocket the hair grows from, isn’t destroyed. It takes a break, then starts again.

How long after the diet

That’s the catch: the shedding doesn’t happen during the diet, but months after it starts. A small case series published in 1976, and still cited, described it. Nine people had lost between 11.7 and 24.75 kg on an intensive weight-loss programme. All of them saw heavy hair loss two to five months after starting the diet, with 25 to 50% of hairs resting instead of the usual 15%. Their hair grew back within several months³.

Nine patients is very few, and the study is fifty years old. But the delay it describes matches what dermatologists give today for any telogen effluvium: 2 to 3 months according to the French Society of Dermatology⁵, 2 to 4 months according to DermNet⁶. Three of the nine had already lost hair after earlier successful diets³: for them, it had happened again.

How much weight loss does it take? The AAD lists losing 20 pounds (about 9 kg) or more among the events that commonly trigger this kind of shedding⁷. It isn’t an exact threshold: how fast you lose it and what you eat matter too.

What weight-loss surgery shows

Weight-loss surgery is a useful vantage point: the weight comes off fast and in large amounts, and patients are closely followed. A 2021 meta-analysis pooled 18 studies and 2,538 patients⁸.

Zhang et al., 2021, 18 studies. The timing figures come from a single one of these studies.⁸

Patients who lost hair were younger on average, and had lower levels of zinc, ferritin (the body’s iron store) and folate⁸. The authors set out the limits themselves: hair loss was self-reported with no shared criteria, most studies were retrospective, and results varied hugely from one study to another, from 4.5% to 80%⁸.

What people who’ve been through it say

On a hair-loss forum, several men describe what followed their diets⁹.

Testimonials published on the HairLossTalk forum⁹.

The three-month delay fits the studies. The last line is worth pausing on: shedding concentrated on the top of the head that never fully recovers suggests something else. Telogen effluvium can unmask pattern hair loss that was already developing⁴.

Protein and iron: what most often runs short

It isn’t only about calories. Too little protein can on its own make hair thinner and cause it to fall out², and the same goes for too little iron¹. Very restrictive diets often cut meat, eggs or dairy first.

If you’re eating less, keep a source of protein every day: meat, fish, eggs, dairy, pulses. If your diet is restricted, vegetarian or vegan for example, British dermatologists suggest considering a supplement⁴. And to find out whether you’re actually short of something, a blood test will tell you¹: that’s what shows low iron, vitamin D or B12, not guesswork.

Too many vitamins cause shedding too

After heavy shedding, the reflex is to reach for “hair” capsules. Go carefully. The AAD points out that these supplements aren’t regulated, that they can do more harm than good, and that getting too much selenium, vitamin A or vitamin E has been linked to hair loss¹.

The Guo and Katta review goes further: too much vitamin A has a strong known link to hair loss, and selenium toxicity can cause hair loss all over the scalp². For vitamin E, the effect was seen in volunteers taking around thirty times the recommended intake². Before you buy, read what a capsule actually contains and check whether yours includes them.

What a balanced diet does, and doesn’t do

What it does: it removes the trigger. DermNet advises a nutritious diet with plenty of protein, fruit and vegetables, and notes that regrowth usually follows once the trigger is removed⁶. The British leaflet says a balanced diet rich in iron and protein may be helpful⁴. Once you’re eating enough again, the shedding stops on its own, and fullness usually returns within six to nine months⁷.

What it doesn’t do: speed regrowth beyond the hair’s own pace. New hairs take months to show. Taking medication doesn’t speed up regrowth after telogen effluvium either⁴. And eating better won’t regrow established pattern hair loss: if the top of your head or your parting is still thin after a year, your doctor can look for another cause and may suggest a treatment.

When shedding lasts more than six months, it’s called chronic telogen effluvium⁴. In that case, or if you see no improvement, see a doctor: untreated iron deficiency can set the shedding off again⁴.

Seeing the shedding stop

After a diet, everything plays out over months: the shedding arrives late, stops without warning, and regrowth comes as short little hairs the mirror doesn’t show. Counting hairs in your brush doesn’t help either: the number you lose in a single day says very little.

The most reliable approach is the same photo every week: same room, same light, same distance, dry hair. On the forum, one of the men says he’s taking pictures to document everything⁹. Calvap makes this easy: your previous photo appears as a transparent overlay so you line up in the same spot, across 7 scalp zones. The journal lets you log when the diet started and when you went back to normal eating, on the same timeline as your photos. After a few months, you’ll know whether the top of your head is filling in like the rest. Your photos stay on your phone.