Intermittent fasting: what it does, and whether it suits training

The main forms

• Time-restricted eating. All food within a daily window, typically eight hours, such as noon to 8 pm, followed by sixteen hours without calories. Windows of ten or twelve hours are gentler versions.
• 5:2. Normal eating on five days and about a quarter of usual intake on two non-consecutive days.
• Alternate-day fasting. A fast or a very low intake every other day. Harder to sustain and less studied.

Water, black coffee and unsweetened tea are taken freely during the fasting period in all of them.

What the evidence shows, and what it does not

The 2019 review by de Cabo and Mattson set out the biological case: after ten to twelve hours without food, the body shifts from using glucose to using fatty acids and ketones, and this metabolic switch triggers cellular responses that, in animals, improve resistance to stress and disease. In rodents the effects on lifespan and health are striking.

In humans, the picture is more modest.

• Short trials show weight loss and improvements in insulin sensitivity, blood pressure and blood lipids.
• Where intermittent fasting has been compared directly with daily calorie restriction providing the same deficit, the two have generally produced similar weight loss.
• No trial has shown that it extends human life.
• Long-term adherence is no better than with other diets: people drift back.

A fair summary is that intermittent fasting is one effective way of eating less, with possible additional metabolic effects that remain to be confirmed in long human trials.

Why it suits some people with demanding jobs

The appeal is practical.

• One rule. Nothing before noon, or nothing after 8 pm. No counting, no weighing.
• Fewer decisions. A skipped breakfast is one meal that does not need to be found or thought about.
• It removes the weakest meals. Late-evening eating and mid-morning pastries are often the lowest-quality calories of the day.

And the drawbacks are equally practical.

• Business dinners end at eleven. A window that closes at 8 pm does not survive a week of entertaining. Choose a window that fits your actual life.
• Hunger and irritability in the first two weeks, usually passing.
• Overeating in the window. A restricted window does not reduce intake if the meals grow to compensate.

Combining it with training

Training in a fasted state is safe for healthy people at low and moderate intensities. Performance in hard or long sessions is usually better with food in the system.

Three arrangements work.

1. Train at the end of the fast, then eat. A session at 11 am followed by the first meal at noon. The calculator above shows the period this leaves you.
2. Train inside the window. Eat two to three hours before a demanding session.
3. Train early and widen the window on those days. A rule that bends for training is better than a session done badly.

The risk specific to people who lift is under-eating protein. Fitting 120 to 160 grams into two meals is difficult. Three meals in a ten-hour window make it far easier. See protein needs.

Total energy matters too. A large deficit together with hard training leads to poor recovery, disturbed sleep and loss of muscle. See calorie needs.

A sensible way to try it

1. Start with a twelve-hour overnight fast, for example 8 pm to 8 am, for two weeks. Many people are already close to it.
2. Extend to fourteen hours if it feels easy.
3. Keep three meals in the window, each with a protein source.
4. Watch your sleep, mood and training for a month. If any of them deteriorate, widen the window.
5. Do not combine a new fasting routine with a new hard training block. Change one thing at a time.

Ramadan

Fasting from dawn to sunset during Ramadan differs from the patterns above in one essential respect: fluids are also withheld. Guidance for people who train is to keep sessions shorter and lighter, to place them shortly before the fast is broken or a couple of hours after, and to rehydrate deliberately through the evening. In summer months in the Gulf, outdoor training during the fast is best avoided. See training in the heat.

Who should not fast without medical advice

• People with diabetes treated with insulin or other glucose-lowering medicines, because of the risk of hypoglycaemia.
• Pregnant or breastfeeding women.
• Anyone with a current or past eating disorder.
• People who are underweight, frail or over 70.
• Those taking medicines that must be taken with food.

Intermittent fasting is a way of organising meals, not a treatment. If it makes your days harder rather than simpler, it is not the right tool for you.