Intermittent Fasting: 16:8, 5:2 and What Actually Happens

The short version

• Intermittent fasting limits when you eat, not what you eat.
• Trials suggest it isn't better than ordinary calorie reduction, but it can be easier for people who like its simplicity.
• Protein and strength training matter, because you can lose muscle if your overall protein is low.
• It isn't safe for everyone, particularly people on diabetes medication or with an eating disorder history.

In a sentence

Eat only during set hours or days. What you eat is up to you.

How restrictive

Counting needed?

No, just the clock

Quality of evidence

Moderate

Typical cost

Low

Main drawback

Not better than plain calorie cutting, and risky with some medications

See how this compares with every other diet →

The main types

During the fast, water, black coffee and plain tea are generally allowed.

What the evidence says

Weight. It works because you tend to eat less when there's less time to eat. But it doesn't seem to do more than that. In a year-long trial of 139 people, all on the same calorie target, those who also ate within an 8-hour window lost about the same as those who ate at any time (about 18 versus 14 pounds, not a statistically significant difference). In another trial, 16:8 without any calorie guidance made little difference at 12 weeks, and the group lost some muscle.

Metabolism. Claims about autophagy and "resetting" your cells come largely from animal and short-term studies. Short fasts don't damage your metabolism, and they don't supercharge it either.

Timing. Some research hints that eating earlier in the day may be better for blood sugar than eating late, but the evidence isn't conclusive.

The takeaway is that intermittent fasting is a practical tool for eating less. If it fits your life, it's a great choice. If it makes you miserable, you aren't missing a secret.

How to start

1. Start gently, with a 12-hour overnight window, such as 8 p.m. to 8 a.m.
2. Extend it by an hour every few days, stopping at whatever feels manageable.
3. Pick a window that suits your life. Many people prefer to skip breakfast and keep dinner.
4. Don't use your window as a free pass. A big late binge cancels the fast.
5. Prioritize protein and vegetables. Aim for at least 25 to 30 grams of protein per meal, because it's harder to get enough in less time.
6. Lift weights a couple of times a week to protect your muscle.

Pros and cons

Good: Simple, free, no food rules, fewer decisions.

Harder: Mornings can be rough at first. It clashes with family meals, social events and some training schedules, and it can lead to overeating in the window. Some people get headaches or irritability.

Sources

1. Harvard T.H. Chan School of Public Health: Diet Review: Intermittent Fasting
2. Liu et al., NEJM (2022): Calorie restriction with or without time-restricted eating
3. Lowe et al., JAMA Internal Medicine (2020): Time-restricted eating and weight loss
4. Johns Hopkins Medicine: Intermittent Fasting, What Is It and How Does It Work?

This guide is general information, not medical advice. Please check with a clinician before making big changes, especially if you have a health condition, take medication, or are pregnant. Read the full disclaimer.