Intermittent fasting means eating only within set hours, or eating very little on set days, and going without food the rest of the time. In the human trials, intermittent fasting helped people lose about as much weight as cutting calories every day, but not more. In one small trial, eating early in the day improved blood sugar control and blood pressure even without weight loss. Most of the claims about slower aging come from animal studies.
This guide explains the main methods and what the trials found for weight, blood sugar and blood pressure. It covers who should skip fasting or ask a doctor first, and how to time supplements around an eating window. For more on weight, see our Weight and Metabolism page; for blood sugar, how to lower blood sugar naturally.
Intermittent Fasting Methods at a Glance
Each method below was tested against eating as usual or against cutting calories every day.
| Method | How it works | What the trial showed |
|---|---|---|
| 16:8 (time-restricted eating) | Eat within 8 hours, fast for 16, every day | Over 12 weeks in 116 adults, weight fell 0.94 kg, no more than with three meals a day; more muscle was lost (Lowe 2020) |
| Time-restricted eating with fewer calories | Eat from 8 a.m. to 4 p.m. and cut calories | Over 12 months in 139 adults, weight fell 8.0 kg against 6.3 kg with the same calorie cut alone; the difference could be chance (Liu 2022) |
| Alternate-day fasting | About a quarter of usual calories every other day | Over 12 months in 100 adults, weight fell 6.0% against 5.3% with a daily calorie cut, but 38% dropped out against 29% (Trepanowski 2017) |
| 5:2 and similar | Two low-calorie days a week, normal eating on the rest | Over 6 months in 107 women, weight fell 6.4 kg against 5.6 kg with a daily calorie cut, with slightly larger falls in fasting insulin (Harvie 2011) |
| Early time-restricted eating | Eat within 6 hours, with dinner before 3 p.m. | Over 5 weeks, improved insulin sensitivity and blood pressure in men with blood sugar above normal, with food set to keep their weight the same (Sutton 2018) |
Pooling 11 trials of 8 to 24 weeks, intermittent and daily calorie cutting led to the same weight loss (Cioffi 2018).
What Intermittent Fasting Does, and What It Doesn't
Weight
Intermittent fasting works for weight mainly because people eat less, and it works about as well as other ways of eating less. On 16:8 alone, without a calorie target, the TREAT trial found no more weight loss than three meals a day, and the fasting group lost more lean mass, which includes muscle (Lowe 2020). If fasting makes it easier for you to eat less, it can help; if it leads to overeating in the window, it won't.
Blood sugar and insulin
In the 5:2 trial, fasting insulin fell slightly more than with a daily calorie cut (Harvie 2011), and the pooled analysis found the same small edge (Cioffi 2018). The early-eating trial is the strongest sign that timing matters on its own: an eating window that ended at 3 p.m. improved insulin sensitivity with no weight loss (Sutton 2018). It was small and short, and most people find dinner before 3 p.m. hard to keep.
Blood pressure and cholesterol
The early-eating trial lowered blood pressure. In the longer trials, blood pressure, cholesterol and fasting glucose changed no more with fasting than with a daily calorie cut (Trepanowski 2017; Liu 2022).
Aging
In animals, fasting switches the body to burning fat, raises ketones and turns on repair processes in cells, and it lengthens life in several species (de Cabo 2019). Whether it slows aging in people has not been shown in trials. Our Healthy Aging page ranks what the human studies show for aging markers.
Who Should Skip Intermittent Fasting or Ask First
Anyone taking meds that lower blood sugar or blood pressure: long gaps without food can push either too low. Ask your doctor before you start.
Anyone pregnant or breastfeeding, or under 18.
Anyone with a history of disordered eating.
Older adults trying to keep muscle: the 16:8 trial saw more lean mass lost, so protein in the eating window and strength training matter more.
Supplements and Intermittent Fasting
What breaks a fast. Anything with calories: gummies with sugar, protein powders, collagen, and amino acid drinks such as BCAA and EAA. Plain capsules and tablets carry almost no calories.
Take fat-soluble vitamins with a meal. In a study of 50 older adults, vitamin D3 taken with a meal containing fat reached a 32% higher peak in the blood than with a fat-free meal (Dawson-Hughes 2015). Vitamins A, D, E and K and fish oil belong in the eating window, not the fast.
Electrolytes. Long fasts and low-carb eating can shift how much sodium and water the body holds. Electrolyte products without sugar carry no calories; our electrolytes page ranks them.
Creatine and protein. If you train, keep protein and creatine in the eating window to protect muscle. Our creatine page shows the dose that worked in the studies.
If you take meds: meds taken with food may need moving into the eating window. Check timing with your doctor or pharmacist, and see our guide to supplement and med interactions.
Fit Intermittent Fasting Into One Daily Protocol
Rala, the AI behind Reverse Age Lab, builds each member one Daily Protocol covering food, exercise and supplements. It times each supplement to your day and eating window, and checks every product against your meds. Build my protocol, timed to my eating window.
Is this medical advice? No. This guide sets out the published research on intermittent fasting; it is not a diagnosis, and Rala isn't a medical device or a clinician. Talk to your doctor before you start fasting, especially if you take meds.
Frequently Asked Questions
Does intermittent fasting work for weight loss?
Yes, about as well as cutting calories every day, and no better. In trials of 6 to 12 months, weight loss with intermittent fasting and with a daily calorie cut was close, and pooled across 11 trials it was the same.
Is 16:8 fasting better than eating three meals a day?
Not on its own. In a 12-week trial of 116 adults, 16:8 without a calorie target led to no more weight loss than three meals a day, and more lean mass was lost.
What is time-restricted eating?
Eating all your food within the same set hours every day, such as 8 or 10 hours, and fasting for the rest. 16:8 is the most common version.
Does intermittent fasting slow aging?
It lengthens life in several animal species, but no trial has shown it slows aging in people.
Can I take supplements while fasting?
Plain capsules and tablets carry almost no calories, but gummies, protein and amino acid drinks do. Take vitamins A, D, E and K and fish oil with a meal, because fat helps absorb them.
Sources
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: the TREAT randomized clinical trial. JAMA Internal Medicine, 2020. PubMed.
Liu D, Huang Y, Huang C et al. Calorie restriction with or without time-restricted eating in weight loss. New England Journal of Medicine, 2022. PubMed.
Trepanowski JF, Kroeger CM, Barnosky A et al. Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection: a randomized clinical trial. JAMA Internal Medicine, 2017. PubMed.
Harvie MN, Pegington M, Mattson MP et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic risk markers: a randomized trial in young women. International Journal of Obesity, 2011. PubMed.
Sutton EF, Beyl R, Early KS et al. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with blood sugar above normal. Cell Metabolism, 2018. PubMed.
Cioffi I, Evangelista A, Ponzo V et al. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. Journal of Translational Medicine, 2018. PubMed.
de Cabo R, Mattson MP. Effects of intermittent fasting on health and aging. New England Journal of Medicine, 2019. PubMed.
Dawson-Hughes B, Harris SS, Lichtenstein AH et al. Dietary fat increases vitamin D-3 absorption. Journal of the Academy of Nutrition and Dietetics, 2015. PubMed.
