Fasting: Who It Helps, Who It Harms

One camp says fasting is the key to weight loss and long life. The other says it wrecks your hormones and shortens your life. Both are selling one rule for everyone. Here’s what the trials actually show, graded, and why the better question is who’s doing the fasting.

The grades in this post: Strong means good randomised trials agree. Mixed means the trials are few, small, short or unblinded. Mechanistic means the biology or laboratory work makes sense but it hasn’t been shown to change outcomes in people. None means there’s no usable evidence in people. Observational studies are labelled as such: they show a link, not proof of cause.

What “fasting” means here

• Time-restricted eating: all food within a daily window, often 8 hours (“16:8”).
• 5:2 and alternate-day fasting: very low intake on some days, normal on others.
• Longer fasts: 2 days or more with no food.

They’re different things, and most arguments go wrong by treating them as one.

The case for, graded

• It helps people lose weight, mostly by eating less. Across 11 meta-analyses of 130 trials, intermittent fasting was linked to lower weight, BMI and fat mass, and some improvements in cholesterol, blood sugar, insulin and blood pressure. But most of those findings were low or very low quality, the trials were short (a median of 3 months), and fasting was also linked to losing some lean mass (Patikorn 2021). Mixed One high-quality finding: modified alternate-day fasting, short-term BMI.
• Head to head with ordinary calorie cutting, it’s no better. When people with obesity cut calories for 12 months, eating within an 8-hour window added no significant extra weight loss, fat loss or metabolic benefit (Liu 2022, NEJM, 139 people). A 16:8 window without calorie advice did no better than three meals a day over 12 weeks, and the fasting group lost more lean mass (Lowe 2020). Strong Good randomised trials.
• Timing may matter on its own, in some people. Men with prediabetes who ate all their food before 3 pm, with enough to keep their weight steady, improved insulin sensitivity and blood pressure compared with a 12-hour window (Sutton 2018). Mixed One small, tightly controlled trial.

So: fasting is a way of eating less that suits some people. It isn’t magic.

The case against, graded

• Testosterone can fall, especially with longer fasts. In 10 healthy men of normal weight, an extra 48 hours without food lowered testosterone from 8.7 to 5.7 µg/L (about 30 to 20 nmol/L), and LH, the brain’s signal to the testes, fell too. Men given small amounts of glucose through a 56-hour fast didn’t show the fall (Röjdmark 1987). Another study of 8 men found 48 hours of fasting slowed LH pulses, with no rise in cortisol (Cameron 1991). Mixed Small, short physiology studies, consistent.
• In lean, training men on 16:8, testosterone fell, but strength didn’t. Resistance-trained men eating the same calories in an 8-hour window for 8 weeks lost fat, and their testosterone, IGF-1 and T3 fell. Muscle and strength were maintained (Moro 2016, 34 men). A review of human trials found the same pattern: lower testosterone in lean, active young men, without loss of muscle or strength (Cienfuegos 2022). Mixed Small trials; the long-term meaning is unknown.
• In heavier men it may be different. In massively obese men, 56 hours of fasting didn’t change the testosterone response to a hormone stimulation test, whereas in normal-weight men the response rose by 180% (Röjdmark 1989). Mixed One small physiology study.
• Inflammation. In mice, fasting sent immune cells (monocytes) back into the bone marrow, and refeeding released a surge of older cells that changed the response to infection (Janssen 2023). Mechanistic Animal. It hasn’t been shown that a daily eating window does this in people.
• Shorter lives? In 33,052 US adults followed for about 8 years, eating within less than 8 hours a day went with 34% higher death from any cause, and 15 hours or more with 25% higher, compared with a 12–13 hour window. The lowest risk was around 11–12 hours. In adults under 65, the link wasn’t significant once other factors were allowed for (Mao 2025). Observational. The eating window came from two days of food recall, and people with short windows were poorer, more often short of food and had worse diets. A short window in a national survey often means illness, shift work, poverty or chaotic eating, not a deliberate 16:8. It can’t show that fasting caused the deaths.

So: the harms are real in some people, under some conditions, and overstated as a rule for everyone.

Where it goes wrong: stacking the stressors

The person who comes to me after fasting “stopped working” is rarely just fasting. It’s usually fasting, plus very low carbohydrate, plus hard training most days, plus poor sleep. Each one is a stressor. Together, in a lean or busy person, they add up to too little energy for what the body is being asked to do. That’s the pattern described in my under-fuelling article: thyroid hormones down, sex hormones down, energy and recovery down.

And after hard dieting, the body adapts. Six years after an extreme TV weight-loss competition, contestants’ resting metabolism was still about 700 kcal (2.9 MJ) a day lower than at the start, and on average they had regained 41 of the 58 kg they lost (Fothergill 2016). Observational, 14 people, an extreme case. It’s not what happens after a 16:8 window, but it shows how hard the body defends against deep, fast restriction.

Who might do well, and who should be careful

Might suit: someone carrying extra weight, sleeping well, not training hard, who finds a shorter eating window makes eating less easier.

Be careful, or don’t:

• lean people training hard, especially men worried about testosterone and women whose periods have changed;
• anyone with a history of disordered eating;
• pregnancy and breastfeeding; under 18s;
• type 1 diabetes, or diabetes treated with insulin or tablets that can cause low blood sugar: talk to your diabetes team first;
• anyone already exhausted, sleeping badly or under heavy stress.

Where testing fits, honestly

If you’re fasting and stalling, or wondering whether you should start, these are the things I’d want to know: blood sugar control (fasting glucose, insulin, HbA1c), testosterone, LH and SHBG in men, cycle changes in women, a full thyroid panel, the cortisol pattern across the day, training load and body composition.

A plain grade on that: no trial has used these tests to decide who should fast. It’s my clinical reasoning: they show who’s already short of energy or under strain, and they give you a baseline to see whether fasting changes things. Mechanistic Reasoning, untested as a selection tool.

The bottom line

Fasting is a tool. For some people it’s an easy way to eat less, and that’s most of what the trials show. For lean, hard-training, stressed people stacking it on top of everything else, it can be one stressor too many. Neither “fast for everything” nor “fasting will wreck you” survives the evidence.

How strong is the evidence?

Sources

1. Cameron JL et al. Slowing of pulsatile luteinizing hormone secretion in men after forty-eight hours of fasting. J Clin Endocrinol Metab 1991;73:35–41. PMID 1904451. doi:10.1210/jcem-73-1-35
2. Cienfuegos S et al. Effect of intermittent fasting on reproductive hormone levels in females and males: a review of human trials. Nutrients 2022;14:2343. PMID 35684143. doi:10.3390/nu14112343
3. Fothergill E et al. Persistent metabolic adaptation 6 years after “The Biggest Loser” competition. Obesity 2016;24:1612–9. PMID 27136388. doi:10.1002/oby.21538
4. Janssen H et al. Monocytes re-enter the bone marrow during fasting and alter the host response to infection. Immunity 2023;56:783–96. PMID 36827982. doi:10.1016/j.immuni.2023.01.024
5. Liu D et al. Calorie restriction with or without time-restricted eating in weight loss. N Engl J Med 2022;386:1495–504. PMID 35443107. doi:10.1056/NEJMoa2114833
6. Lowe DA 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;180:1491–9. PMID 32986097. doi:10.1001/jamainternmed.2020.4153
7. Mao Z et al. Association of eating window with mortality among US adults: insights from a nationally representative study. Aging Cell 2025;24:e70230. PMID 40944446. doi:10.1111/acel.70230
8. Moro T et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med 2016;14:290. PMID 27737674. doi:10.1186/s12967-016-1044-0
9. Patikorn C et al. Intermittent fasting and obesity-related health outcomes: an umbrella review of meta-analyses of randomized clinical trials. JAMA Netw Open 2021;4:e2139558. PMID 34919135. doi:10.1001/jamanetworkopen.2021.39558
10. Röjdmark S. Influence of short-term fasting on the pituitary-testicular axis in normal men. Horm Res 1987;25:140–6. PMID 3106181. doi:10.1159/000180645
11. Röjdmark S et al. Pituitary-testicular axis in obese men during short-term fasting. Acta Endocrinol 1989;121:727–32. PMID 2686332. doi:10.1530/acta.0.1210727
12. Sutton EF et al. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab 2018;27:1212–21. PMID 29754952. doi:10.1016/j.cmet.2018.04.010