- Intermittent fasting means limiting when you eat, such as a 16:8 daily window or 2 low-calorie days a week (5:2).
- In year-long trials, fasting led to about the same weight loss as ordinary calorie cutting.
- A small study in men with prediabetes found that eating earlier in the day improved insulin sensitivity and lowered blood pressure by about 10 mm Hg, even without weight loss.
- Fasting is not safe for everyone, especially people on glucose-lowering medicines, pregnant women and anyone with a history of eating disorders.
Intermittent fasting is one of the most talked-about ways to lose weight and improve blood sugar. For some people with prediabetes, a shorter eating window is a simple rule that makes it easier to eat less, especially if late-night snacking is a problem. For others, it leads to hunger, overeating later or skipped nutrients. Here is what the research actually shows, and how to decide if it suits you.
The main types of fasting
Eat within 12 hours, fast for 12, such as 8 am to 8 pm. A gentle start that mainly cuts late-night snacking.
Eat within 8 hours, for example noon to 8 pm or 9 am to 5 pm. The most popular form of time-restricted eating.
A 6–8 hour window that ends by mid-afternoon, such as 8 am to 2 pm. Matches the body clock but is hard to fit into family and social life.
Eat normally 5 days a week and have about 500–600 calories on 2 non-consecutive days.
What the trials show
Most fasting studies are small or short. The strongest trials compare fasting with ordinary calorie cutting over a year, and they tell a consistent story: fasting works about as well, but not better, for weight loss.
| Study | Who and what | Main result |
|---|---|---|
| Sutton 2018 | 8 men with prediabetes; 6-hour early window (ending before 3 pm) vs 12-hour window for 5 weeks, same food | Better insulin sensitivity and lower blood pressure without weight loss |
| Liu 2022 | 139 adults with obesity; calorie cutting with or without an 8 am–4 pm eating window, 12 months | 8.0 kg vs 6.3 kg lost; difference not significant |
| Trepanowski 2017 | 100 adults with obesity; alternate-day fasting vs daily calorie cutting, 12 months | Similar weight loss; more dropouts with fasting (38% vs 29%) |
| Lowe 2020 | 116 adults with overweight; 16:8 without other advice vs 3 meals a day, 12 weeks | Very small weight loss, not different from 3 meals |
| Carter 2018 | 137 adults with type 2 diabetes; 5:2 vs daily calorie cutting, 12 months | Similar HbA1c drop; medicines were adjusted to avoid lows |
Sutton et al., Cell Metab 2018; Liu et al., NEJM 2022; Trepanowski et al., JAMA Intern Med 2017; Lowe et al., JAMA Intern Med 2020; Carter et al., JAMA Netw Open 2018.
The lesson: the eating window mainly helps if it leads you to eat less overall. A 16:8 pattern filled with large portions of refined snacks will not do much for blood sugar.
There is also a downside to watch for. In the 16:8 trial, part of the small amount of weight lost was muscle rather than fat, possibly because people ate less protein. Muscle is your body’s main store for blood sugar, so losing it works against you. In the alternate-day fasting trial, more people gave up, which suggests that strict fasting is hard to keep up for many. The best plan is the one you can follow for years, not weeks. A review in the New England Journal of Medicine concluded that fasting has promise, but that long-term human evidence is still limited.
The early-eating study in prediabetes
The Sutton trial is interesting because it tested timing alone. Men with prediabetes ate enough food to keep their weight stable, either within 6 hours ending before 3 pm or spread over 12 hours. With the early window, their bodies needed less insulin to handle food, their insulin-producing cells responded better, and blood pressure fell by about 10 mm Hg. Evening hunger also dropped. It was a tiny study of 8 men, so it needs confirming, but it fits with other research showing that the body handles food better earlier in the day.
Sutton et al., Cell Metab 2018.
A practical takeaway, even if strict early fasting is not for you: make breakfast and lunch your main meals and keep dinner lighter and earlier. See Breakfast Ideas for Prediabetes.
If you want to try it
- Start with 12:12 for 2 weeks, then move to 14:10 or 16:8 if it feels easy.
- Put the window earlier if your routine allows, finishing dinner by about 7 pm.
- Make meals count: vegetables, dal or beans, eggs, paneer, fish or chicken, and whole grains. See What to Eat with Prediabetes.
- Drink water, and choose unsweetened tea or coffee during fasting hours.
- Keep protein up and keep up strength training to protect muscle.
- Judge by results: after 3 months, check your weight, waist and HbA1c.
Not sure how much to eat in your window? Try our Calorie Calculator, or follow our Prediabetes Meal Plan.
Who should not fast
- Adults with prediabetes who are not on glucose-lowering medicines
- People who snack late at night
- People who like simple rules more than counting
- People taking insulin, sulfonylureas or other glucose-lowering medicines
- Pregnant or breastfeeding women
- Anyone with a current or past eating disorder
- People who are underweight, frail or under 18
Fasting and medicines can be a risky mix. If you take insulin or medicines that lower blood sugar, fasting can cause dangerous lows. Blood pressure medicines may also need adjusting. Always talk to your doctor before you start, and do not try fasting during pregnancy or if you have ever had an eating disorder.
MythFasting burns fat in a special way, so what you eat in the window does not matter.
FactIn long trials, fasting and ordinary calorie cutting gave similar results. Food quality and total amount still matter most.
Common questions
Is skipping breakfast bad for prediabetes?
Not necessarily, but skipping breakfast and eating a big late dinner goes against your body clock. If you use a 16:8 window, an earlier window is probably better.
Will fasting reverse my prediabetes?
It can help if it leads to weight loss, like any approach that cuts calories. Read Prediabetes and Weight Loss for why 5–7% matters.
Can I drink tea with milk during the fast?
A small splash of milk adds few calories, and many people find it helps them stick with the plan. Avoid sugar.
- Intermittent fasting limits when you eat; 12:12, 16:8, early eating and 5:2 are common patterns.
- Over a year, fasting gives similar weight loss to ordinary calorie cutting.
- Eating earlier in the day may help insulin sensitivity, based on a small study.
- It is not safe for everyone: check with your doctor if you take medicines, are pregnant or have had an eating disorder.
- Next: Fiber and Protein for Blood Sugar: How Much and Where to Get It.
Sources
- 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 prediabetes. Cell Metab. 2018;27(6):1212–1221.e3.
- 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.
- 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.
- 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.
- Carter S, Clifton PM, Keogh JB. Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with type 2 diabetes: a randomized noninferiority trial. JAMA Netw Open. 2018;1(3):e180756.
- de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541–2551.
This article is for general education and is not a substitute for advice from your own doctor or healthcare team.