Who intermittent fasting does not suit, and the signs it is time to stop

"Doctor, I only eat one meal a day, so why am I gaining weight?" It is what I hear most often in weight consultations, and the book I co-wrote, Eat to Succeed (published in Korean, 2026), opens with it. These days some people skip meals on purpose, as a method. That method is intermittent fasting, which has spread widely over the past few years.

Intermittent fasting limits the hours in which you eat. With 16:8 you go 16 hours without food and eat within an 8-hour window, which means regularly skipping either breakfast or dinner. With 5:2 you eat normally five days a week and only 500 to 600 kcal on the other two. The idea behind it is that insulin falls during the fast and more fat gets broken down.

Yet two people on the same 16:8 can have very different days. One carries on comfortably, while the other fights hunger all afternoon and gives in at night. As I wrote in the book, the same method can be perfect for one person and torture for another.

How well a body copes with a 16-hour fast differs from person to person. For some people a long fast does more harm than good, so the book lists who it does not suit. Put together, there are six groups.

• People prone to low blood sugara long fast is hard to get through, especially with diabetes or hypoglycaemia.
• People under heavy stressgoing without food adds one more stress and pushes up the stress hormone cortisol.
• People who skip dinner and then sleep badlyhunger keeps them awake.
• People who have had an eating disorderextreme restriction can tip over into bingeing.
• Pregnant or breastfeeding women, growing teenagers and athletes in heavy trainingintermittent fasting is not recommended for them either.
• People whose work needs sharp concentrationfor surgeons, pilots and drivers, losing focus to hunger can be dangerous.

Stress is the one people tend to brush aside. When cortisol stays high, blood sugar rises, insulin resistance grows, fat builds up around the belly and organs, and muscle shrinks. So while you go hungry to lose weight, the body is busy storing fat. Sleep works the same way. After a night of hungry, broken sleep, appetite is harder to rein in the next day.

Once you start, the scale becomes the first stop every morning. But whether the method suits you shows in how your day went well before it shows in the numbers.

In the book I put it this way: "If intermittent fasting leaves you tired, irritable, unable to concentrate and bingeing, it is not the method for you." Of the four, bingeing is the one people most often blame on their own willpower.

When hunger peaks, the limbic system, which runs on instinct, takes over from the frontal lobe, which handles reasoning, and orders you to eat a lot, right now. A big meal after a long fast sends blood sugar up all at once, insulin floods in and the leftover energy is stored as fat. Then blood sugar drops and fierce hunger comes back. That is why the book calls this a problem of the brain, not of willpower.

Among people who say they gain weight on one meal a day, many were eating heavily in the evening. So when the book explains 3.6.9, much of it is about not sitting down to dinner ravenous.

Does that mean intermittent fasting is never recommended? The book covers the other side too. For people with insulin resistance, a long fast may help improve insulin sensitivity. Freelancers, people who work from home, and people who like a simple rule such as "I eat in these hours and not in those" also find it easier to keep up.

One study looked at insulin sensitivity directly. Eight overweight men with prediabetes spent five weeks on each of two eating schedules. On one they ate within six hours and finished dinner before 3 pm. On the other they ate across twelve hours. Their food was matched so their weight would not change, yet insulin sensitivity and blood pressure improved during the six-hour period.

It was a small study of only eight men, and the researchers name that as a limitation themselves. The fast also fell in the evening, unlike the common 16:8 where breakfast is skipped. It would be going too far to say any 16:8 gives the same result for anyone with insulin resistance.

Either way, if you feel fresh, have energy, focus well and find portions easy to manage, you can carry on. Many people skip breakfast because they are not hungry in the morning. That can be a sign the body has adapted to going without.

Does 16:8 help you lose more on the same amount of food? A 2020 US study split 116 adults with overweight or obesity into two groups for 12 weeks. One group ate three meals at set times. The other ate as much as they liked, but only between noon and 8 pm.

After 12 weeks the difference in weight between the groups was not statistically significant. But among the 50 people whose body composition was measured in person, the 16:8 group had lost more lean mass in the arms and legs. Lean mass is body weight minus fat, and most of it is muscle. The researchers concluded that limiting eating hours alone, without other changes, does not take off more weight than eating throughout the day.

Muscle is built all day, but especially in the three to four hours after you eat protein. Two meals a day means only two of those chances, and in the hours without protein, muscle is broken down instead. So even if you cut down on meals, put protein in every meal within your eating window.

Lose weight fast and lose muscle with it, and your basal metabolic rate drops. The weight then comes back more easily on the same food, and the face goes through another round of change along the way.

If you fail to last 16 hours more often than you manage it, try shortening the fast before giving up on the method altogether. It does not have to be all or nothing. With 14:10 you go 14 hours without food and eat within 10. With 12:12 you finish dinner at 8 pm and have breakfast at 8 am the next day.

Just cutting out late-night snacks brings the overnight fast close to 12 hours. It helps to finish a light dinner two to three hours before bed.

"Won't eating more often make me gain weight?" The question comes up whenever 3.6.9 does, and the book calls it the most common misunderstanding. 3.6.9 means eating five times a day, three hours apart. The times follow when you wake up. On a day you get up at 9 am, you eat at 9, 12, 3, 6 and 9.

The aim is to stop the metabolism slowing and to stop bingeing. When a fast drags on, the body treats it as a food shortage and lowers its basal metabolic rate to spend less and store more. Eating regularly avoids that. You never get ravenous, so you overeat less, and muscle is not lost any faster.

More meals does not mean more food in total. The daily total and what you eat matter more than how often, so put protein such as eggs, yoghurt or tofu into every meal and snack, and fill half the plate with vegetables at lunch and dinner. The 9 pm snack is only for when you are hungry.

That said, if you already eat three regular meals and do not binge, there is no need to go up to five. 3.6.9 is for people in the habit of going a long time without food and then eating heavily.

Your weight may go up a little in the first two to three weeks. That is the body settling as it comes out of starvation mode, so there is no need to quit then. If nothing has changed after six weeks, keep eating often and cut the daily total.

With any method, if the numbers have not moved as hoped after a month or so, people start looking for something else. In the book I wrote: "It does not matter which method, but keep it up for at least three months." Whether you choose diet changes or medication, this is the one condition I always give in clinic.

Stop before a habit forms and you cannot tell whether the method was wrong or you quit too soon. The book says sticking to it on 70 of 90 days is enough. Rather than dwelling on a day or two that slipped, start again from the next meal.