Did intermittent fasting ruin your metabolism?

What actually changes in your body when you fast and lose weight?

Go back to Neha's June. She was 9 kg lighter, and a lighter body simply needs less fuel, the way a small car uses less petrol. She was also tired, so without meaning to she took the lift more, sat through lunch at her desk and skipped the evening walk. Both of those lower the calories you burn, and neither has anything to do with fasting itself.

The cold desk and the 4 pm slump fit a body that has been short of food for months. They usually ease within a few weeks of eating enough. Muscle can matter too, if the single daily meal was light on protein, and that is covered below.

Then came breakfast. Some of the 5 kg that returned was water, and the rest was real regain from eating her old portions, not a sign of a broken metabolism. Neha could have checked what was going on with a step count on her phone, a tape round her waist and a week of writing down what she ate.

Does intermittent fasting slow your metabolism more than normal dieting?

Not in the trials that measured it. In 2005 Heilbronn and colleagues had 16 lean adults fast every other day for three weeks. Their resting metabolic rate did not change significantly, and they burned more fat on fasting days. Hunger stayed high on those days the whole way through, which tells you more about why people stop than about metabolism.

Trepanowski and colleagues in Chicago ran the longest alternate-day trial, with 100 adults with obesity followed for a year. The fasting group and the daily calorie cut group both ended about 6% lighter. People on the fasting plan ate more than planned on fast days and less on feast days, and more of them dropped out.

Time-restricted eating, the 16:8 style most people in India try, has a year-long trial too. In Guangzhou, Liu and colleagues gave 139 adults the same calorie target, and half also ate only between 8 am and 4 pm. After 12 months the two groups had lost similar weight, body fat and lean mass, with similar metabolic results. Across these patterns, fasting behaves like any other way of eating less.

Sources: [1], [2], [9]

Why does your body burn less after losing weight?

Any weight loss lowers daily energy use, because a smaller body costs less to run. There is also an extra drop on top of that, called metabolic adaptation. In a 1995 study, Leibel and colleagues held people at 10% below their usual weight. Their daily energy use fell by between 6 and 8 calories per kilo of lean mass beyond what their size explained.

For an average adult with about 50 kg of lean mass, that works out to roughly 300–400 calories a day, and this is what most people mean by a ruined metabolism. Martins and colleagues in Norway then asked how long it lasts. After 71 adults with obesity lost about 14 kg, the extra slowdown was around 90 calories a day. Four weeks at a steady weight roughly halved it, and at one year it was no longer measurable in those followed throughout.

The adaptation is largest while you are still in a deficit and fades once you eat at maintenance. The one group with a lasting, large slowdown were contestants on an American weight-loss television show, who lost close to 60 kg over 30 weeks with extreme exercise and dieting. Fothergill and colleagues found their resting rate still about 500 calories below expected six years later. A 16:8 window is nothing like that.

Sources: [3], [4], [5]

Is lost muscle the real reason your metabolism feels slow?

It is often part of the story, and the part you can rebuild. In the TREAT trial, Lowe and colleagues asked 116 adults to eat only between noon and 8 pm for 12 weeks. In the group that was scanned, the fasting arm lost more lean tissue from the arms and legs than the arm eating three meals. Lean tissue on a scan includes water as well as muscle, but much of it was likely muscle.

Templeman and colleagues at the University of Bath found something similar in lean adults. Over three weeks, alternate-day fasting and a daily calorie cut took off similar weight. The daily cut came almost entirely from fat. The fasting group lost less than half as much fat for the same change on the scale.

Muscle burns only a modest amount at rest, so losing a kilo of it will not explain a big slowdown by itself. You notice it in other ways. The stairs to a third-floor flat feel steeper, your arms look softer even at a lower weight, and sugar after a meal is cleared more slowly. One meal a day makes this worse, since very few people fit a day's protein onto one plate.

Sources: [6], [7]

Why does the weight come back so fast after stopping?

Part of the first jump is water. The body stores carbohydrate as glycogen in the liver and muscles, bound to water, and a fasting phase with fewer rotis and less rice usually leaves those stores lower. Going back to regular meals refills them within days, along with more salt and more food in the gut. A kilo or two can appear in a week without any fat being added.

The rest is usually food. Appetite after a diet tends to run higher than before, and the portions that held you at your old weight now feed a smaller body. Neha went straight back to two aloo parathas with butter for breakfast, chai with biscuits at 11 and her usual dinner. That menu was sized for someone 9 kg heavier.

The fix is to find your new maintenance level and eat close to it for a month. The energy needs calculator under Read next gives a first figure. Weigh yourself each morning for two or three weeks, look at the weekly average, and trim a little if it keeps rising.

How do you get your metabolism back on track after fasting?

Start with regular meals at set times, three a day or two meals and a snack, at your new maintenance calories. Put a protein food in each one. Two eggs give about 12 g of protein, 100 g of paneer about 18 g, a katori of thick dal roughly 8 g, and 100 g of chicken about 25 g. A vegetarian day might be a besan chilla with curd, rajma with rice and a bowl of curd at lunch, and paneer bhurji with roti at night.

Strength training rebuilds what fasting may have cost. Two or three sessions a week of squats, lunges, push-ups and rows are enough at first, at home with a band or in a gym. Add a little weight or a few more repetitions each week for the first two months. Walking counts too. Going from about 3,000 steps a day up towards 8,000 adds real energy use without feeling like exercise.

Before you try to lose more, hold your weight steady for a while. In the MATADOR trial, Byrne and colleagues had men with obesity alternate two weeks of dieting with two weeks at maintenance. They lost more fat than men who dieted straight through, and their resting energy use fell less once body size was allowed for.

When you do restart, aim for a plan that protects muscle from the first week. In doctor-led metabolic care the goal for many people is to lose up to 20% of body weight and keep their muscle. That takes protein targets, strength work and regular checks that the weight coming off is fat, with a fasting window as one optional piece.

Sources: [8]

When is it something other than fasting?

Sometimes a tired, slow body after a diet has a medical cause that turns up at the same time. An underactive thyroid brings tiredness, feeling cold, dry skin, constipation and weight gain, and it is common in Indian women. Low iron or vitamin B12, both frequent in vegetarians, can leave you breathless and drained. A long, restrictive diet can itself run iron and B12 low. Rising blood sugar can bring heavy tiredness after meals.

A blood panel of thyroid, haemoglobin and iron stores, B12, fasting sugar and HbA1c usually sorts this out. Some symptoms should not wait for a month of new habits. Fainting, a racing heart, chest pain, severe weakness, or shakiness and sweating on fasting days need a doctor promptly, especially if you take treatment for diabetes.