Is Fasting Good for You? Benefits, Risks & Safety

Intermittent fasting structures when you eat, not necessarily what you eat. The idea is that by limiting your eating window, you naturally eat fewer calories and give your body longer stretches between meals. The most common approaches:

• Time-restricted eating (TRE): You eat within a daily window, often 8 to 10 hours, and fast the rest of the day. The popular "16:8" method means fasting for 16 hours and eating within 8.
• 5:2 diet: You eat normally five days a week and cut calories sharply on two days. In one 12-month trial described in a 2022 review in Nutrients, the two low days were 500 to 600 calories.
• Alternate-day fasting (ADF): You alternate low-calorie "fast" days with regular eating days. The modified version allows a small meal on fast days.

What matters most is not the protocol. It is whether you can stick with it while still eating nutritious food. Results depend on your overall food choices, protein and fiber intake, sleep, stress and physical activity. The clock alone will not change your health.

For a deeper look at how to structure meals during your eating window, see our guide to Intermittent Fasting and Meal Planning: What You Need to Know.

The short answer: it can, but it does not beat other approaches. Here is what the research shows.

About the same as calorie counting. Most trials comparing intermittent fasting with ordinary daily calorie restriction find similar weight loss and metabolic benefits, according to a 2021 review in Experimental Gerontology, although a few suggest fasting may do slightly better and the protocols vary widely. A 2022 review in Nutrients concluded that fasting regimens lead to weight loss and have been linked to better cholesterol and blood pressure, and called for more research on longer-term outcomes.

The clock adds little. In a 12-month trial in the New England Journal of Medicine, 139 adults with obesity all followed the same calorie-restricted diet, and half also ate only between 8 a.m. and 4 p.m. The time-restricted group lost 8.0 kg and the calorie-only group 6.3 kg. The 1.8 kg difference was not statistically significant, and body fat, blood pressure and metabolic risk factors told the same story. When calories were matched, the clock did not add much.

Long-term questions remain. A 2024 analysis presented at an American Heart Association meeting of about 20,000 U.S. adults found that people who ate all their food within less than 8 hours a day had a 91% higher risk of dying from cardiovascular disease over a median of 8 years, compared with people eating across 12 to 16 hours. The same was seen in people already living with heart disease or cancer.

Size that finding carefully. It was observational, presented as a preliminary abstract rather than a peer-reviewed paper, and each person's eating window was classified from just two days of food records. People who choose very short windows may differ in ways the researchers could not measure. It does not show that time-restricted eating causes harm. The practical takeaway: moderation matters, and if you have heart disease or cancer, talk with your doctor before any fasting plan.

So on weight loss, fasting works for some people because it simplifies eating decisions and reduces calories. If counting calories feels tedious but skipping a late snack doesn't bother you, it may be a helpful tool. It is one approach among many, not a metabolic shortcut.

Simplicity. Fewer eating occasions can cut down on mindless snacking and decision fatigue. Some people find "don't eat after 7 p.m." easier to follow than tracking every calorie.

Modest improvements in heart risk factors. A 2025 network meta-analysis of 56 studies compared fasting methods with a usual diet. Modified alternate-day fasting came out ahead for weight (about 5.2 kg lower), waist size and blood pressure (about 7 mmHg lower systolic), and time-restricted eating for waist size, diastolic blood pressure and fasting blood sugar (about 3.7 mg/dL lower). The authors call both promising and ask for longer randomized trials. Note the comparison was a usual diet, not ordinary calorie cutting.

Behavioral structure. Clear eating and non-eating times can help some people build more mindful habits and break a pattern of late-night snacking.

The downsides are just as real.

• Hunger and low energy. Especially at the start, fasting can cause irritability, trouble concentrating and fatigue. These often ease as you adjust, but they are real obstacles.
• Overeating in the window. If you are ravenous when the window opens, you may eat more than you would have otherwise, which cancels the benefit. Planning balanced meals ahead helps.
• Sleep disruption. Eating too close to bedtime, or going to bed hungry, can interfere with sleep. Adjust the window to fit your sleep schedule.
• Low blood sugar. Hypoglycemia is the most troublesome side effect of fasting, according to the 2022 Nutrients review, and people on insulin or sulfonylureas are at particular risk. Dizziness and weakness are also commonly reported.
• Muscle loss. Fasting without enough protein is a well-known cause of muscle wasting, the same review notes.

Intermittent fasting is not for everyone. Avoid fasting, or get medical guidance first, if any of these apply:

• You are pregnant or breastfeeding. Your body needs consistent nutrition for fetal development and milk production.
• You have type 1 diabetes, or type 2 diabetes treated with insulin or other medicines that can cause low blood sugar. Fasting can cause dangerous drops. Any change to your eating pattern needs to be coordinated with your care team.
• You have a history of an eating disorder or disordered eating. The restriction built into fasting can trigger or worsen an unhealthy relationship with food.
• You take medicines that must be taken with food, or have a chronic condition. Cardiovascular disease, gastrointestinal disorders, cancer and other chronic conditions may be aggravated by long stretches without eating.
• You are an older adult, a child, or immunocompromised. The 2022 Nutrients review lists young children, adults of advanced age and people with immune deficiencies among those fasting is not recommended for.

Evidence that fasting helps prevent chronic disease is still limited and not conclusive for many conditions. A 2021 review laid out the gaps, which is one more reason fasting with an existing condition needs individual medical guidance. When in doubt, ask your doctor. A short conversation can tell you whether it is appropriate for you.

Here is what a well-structured day can look like with a noon-to-8 p.m. eating window. Protein numbers are estimates from USDA FoodData Central for the foods named, and from our Nutrition Info page for our meals.

Our meals range from 20 to 55 grams of protein, with a median of 31.5 grams on the current menu, so check the one you pick on our Nutrition Info page if protein is the point of that meal.

If mornings are your best training time, shift your window earlier, say 8 a.m. to 4 p.m., the window used in the NEJM trial. The best eating window is the one you can stick with while still sleeping well.

• Starting with the longest window. The problem: an 18-hour fast on day one is miserable and rarely lasts. The fix: start at 12 hours and lengthen slowly.
• Treating the window as a free-for-all. The problem: the deficit disappears if the window turns into a feast. The fix: plan the meals inside it.
• Skimping on protein. The problem: two small meals rarely reach 25 to 30 grams each. The fix: build every meal around a protein first.
• Fasting through hard training. The problem: intense or long sessions on empty can feel awful and hurt performance. The fix: move the window so you eat before or after.
• Ignoring warning signs. The problem: dizziness and faintness are not something to push through. The fix: eat, rest, and talk with your doctor if they come back.

The hard part of fasting is rarely the fasting. It is the moment the window opens, you are hungry, and the fastest food in reach is the least helpful. A meal that is already portioned and heats in a few minutes changes that moment.

Build A Meal Plan lets you pick meals one by one and filter by protein, with 20 to 30 gram, 31 to 40 gram and 41 gram and up tiers, so you can hit your protein target even with fewer meals. Our meal plans are chosen for you, and the Weight Loss Meal Plan is portion-controlled and calorie-controlled, from $89.95 for 10 meals, so the window does not become a free-for-all.

If you break your fast in the morning, our High Protein Breakfast Burritos are $4.99 each, and the Sausage, Egg & Cheese is 290 calories with 22 grams of protein. There is no subscription, and meal plan orders ship free to the contiguous 48 states.

Intermittent fasting is a tool. For some people, it simplifies eating decisions and makes a calorie deficit easier to keep. For others, it triggers overeating, disrupts sleep or simply does not fit their life. Neither outcome makes you a success or a failure. It means fasting is or is not the right approach for you.

If you try it, start gently, prioritize protein and fiber in your eating window, and pay attention to how your body responds. If you have a health condition or take medication, talk with your doctor first.

Want to go deeper on making fasting work with real meals? Read our guide to Intermittent Fasting and Meal Planning: What You Need to Know. For weight-loss-friendly foods that fit any eating pattern, explore our Best Foods for Weight Loss Complete Guide.