Fasting myths, busted

Fasting attracts strong opinions. Some say it will wreck your metabolism; others say it's a miracle. Both camps usually rely on anecdotes. Here are eight common beliefs, tested against randomized trials, the kind of study where people are assigned to one approach or another by chance, so the results aren't skewed by who chooses what.

Myth 1: Breakfast is the most important meal of the day

MYTH

Skipping breakfast slows your metabolism and makes you gain weight.

FACT

In a six-week trial, eating breakfast didn't change resting metabolism at all, and breakfast eaters ate more over the day ¹.

Myth 2: Fasting puts you in starvation mode

MYTH

When you stop eating, your body panics and slows down to save energy.

FACT

Over the first days of a fast, resting energy use actually rose, by about 14% by the third day in healthy volunteers ³.

Myth 3: You need to eat every few hours

MYTH

Small, frequent meals stoke your metabolism and help you lose weight.

FACT

On the same calories, eating six times a day led to no more weight loss than eating three times ⁶.

Myth 4: Fasting is a weight-loss shortcut

MYTH

Fasting melts fat in a way that ordinary dieting can't.

FACT

Head-to-head trials find that fasting and everyday calorie restriction lead to very similar weight loss ^4,5.

Myth 5: Fasting is easier for everyone

MYTH

Fasting is easier to stick to than any other diet.

FACT

It suits many people, but not all: in one trial, more people dropped out of alternate-day fasting than out of daily calorie cutting ⁴.

Myth 6: Fasting burns your muscle

MYTH

Fasting makes your body eat its own muscle.

FACT

Most of the weight you lose is fat, but some lean mass can go too, as with any diet ⁷. Protein and strength training help you keep it.

Myth 7: Black coffee breaks your fast

MYTH

Any drink other than water ends your fast.

FACT

Black coffee and plain tea have almost no calories. What breaks a fast is what you add: sugar, milk, cream or syrups.

Myth 8: Ketosis is dangerous

MYTH

Making ketones while fasting is the same as diabetic ketoacidosis.

FACT

Ketones naturally rise during fasting and exercise. Ketoacidosis is a different, dangerous state that happens when the body has too little insulin, mainly in diabetes ⁸.

Sources

1. Betts JA, Richardson JD, Chowdhury EA, et al. The causal role of breakfast in energy balance and health: a randomized controlled trial in lean adults. Am J Clin Nutr. 2014;100(2):539-47. doi.org/10.3945/ajcn.114.083402
2. Sievert K, Hussain SM, Page MJ, et al. Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials. BMJ. 2019;364:l42. doi.org/10.1136/bmj.l42
3. Zauner C, Schneeweiss B, Kranz A, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000;71(6):1511-5. doi.org/10.1093/ajcn/71.6.1511
4. Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-938. doi.org/10.1001/jamainternmed.2017.0936
5. Liu D, Huang Y, Huang C, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. N Engl J Med. 2022;386(16):1495-1504. doi.org/10.1056/NEJMoa2114833
6. Cameron JD, Cyr MJ, Doucet E. Increased meal frequency does not promote greater weight loss in subjects who were prescribed an 8-week equi-energetic energy-restricted diet. Br J Nutr. 2010;103(8):1098-101. doi.org/10.1017/S0007114509992984
7. Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Intern Med. 2020;180(11):1491-1499. doi.org/10.1001/jamainternmed.2020.4153
8. Laffel L. Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes. Diabetes Metab Res Rev. 1999;15(6):412-26. doi.org/10.1002/(sici)1520-7560(199911/12)15:6<412::aid-dmrr72>3.0.co;2-8