Intermittent Fasting and Weight Loss

Does Intermittent Fasting Work

Intermittent fasting produces weight loss by reducing total food intake across set eating windows. A randomized 12-month trial by Liu and colleagues in the New England Journal of Medicine evaluated adults with obesity. The trial compared time-restricted eating combined with calorie restriction against calorie restriction alone. Participants in the time-restricted group ate only between 8:00 a.m. and 4:00 p.m. each day. The investigators tracked weight changes across the entire 12-month study period. The 12-month NEJM trial found weight loss similar to calorie restriction alone. The NEJM trial found no added weight loss from an eight-hour eating window once calories were held equal. Most fasting trials are short. Because trials remain brief, long-term health effects are unknown.

According to the Mayo Clinic, intermittent fasting may help with weight loss. Early side effects can include hunger, tiredness, and irritability. Weight reduction occurs because limiting eating hours leads people to consume fewer calories.

Common Intermittent Fasting Schedules

Common intermittent fasting schedules range from daily time-restricted eating windows to multi-day calorie reductions. Time-restricted eating designates specific daily hours for fasting and eating. The 16:8 schedule represents one of the common forms of time-restricted eating. The 16:8 schedule involves fasting for 16 hours and eating within an 8-hour window. This schedule avoids full days of very low calories.

Under the 5:2 eating pattern, individuals consume their regular diet for five days each week. On the remaining two non-consecutive days, individuals sharply cut calories. This schedule allows normal meals on most days of the week. Alternate-day fasting alternates between days of normal eating and days of fasting or very low calories. Calorie restriction occurs every other day under this protocol.

One meal a day puts all daily eating into one meal. This is an extreme form of time-restricted eating. Learn more about this approach in our guide on OMAD. Both the Mayo Clinic and a 2019 review by de Cabo and Mattson identify these routines as common fasting patterns.

How Intermittent Fasting Works in the Body

Intermittent fasting triggers a metabolic switch in the body from glucose fuel to fat-derived ketones. In a review in the New England Journal of Medicine, Rafael de Cabo and Mark Mattson outline this transition between the fed state and the fasted state. When a person fasts, circulating insulin levels fall. Lower insulin lets fat cells release fatty acids, which the liver turns into ketones. This process marks the metabolic switch from glucose reliance to ketone production.

A 2019 NEJM review describes improvements in blood pressure, resting heart rate and insulin resistance in some studies, and calls for larger long-term trials. Fasting reduces circulating insulin during the fasting window. Readers interested in insulin regulation can read our guide on how to reverse insulin resistance. Clinical testing can also assess your fasting insulin.

Who Should Not Fast

People taking diabetes medicines, people who are pregnant or breastfeeding, children and teens under 18, and anyone with a history of eating disorders should not try intermittent fasting without medical advice (Mayo Clinic). The Mayo Clinic lists who should get medical advice first. If a person takes insulin or other diabetes medicines, fasting can cause low blood sugar. Adjusting medicine for blood sugar control requires direct guidance from a clinician.

Women have their own questions about fasting, covered in the guide on intermittent fasting for women.

Early side effects can include hunger, tiredness, and irritability. Review our guide on intermittent fasting side effects to learn more. Consult our directory of doctors before changing your eating pattern if you take prescription medications.

What the Trials Do Not Tell You

Existing clinical trials on intermittent fasting do not prove that scheduled eating extends human lifespan or outperforms standard calorie restriction. The 2022 New England Journal of Medicine (NEJM) trial by Liu and colleagues followed adults with obesity for 12 months. It evaluated an eight-hour eating window from 8 a.m. to 4 p.m. paired with calorie restriction against calorie restriction alone. Weight loss was similar between the two groups. Intermittent fasting may help primarily because individuals end up eating less. Because that study monitored one specific daily window, it provides no findings on 5:2 schedules, alternate-day fasting, or one meal a day (OMAD).

Earlier literature reviews also point to gaps in human evidence. A 2019 NEJM review by de Cabo and Mattson synthesized animal and human data. The authors stated that large, long-term human trials remain necessary. Neither paper claims that fasting lengthens human life.

Some groups should check first. People taking diabetes medicines, pregnant or breastfeeding individuals, anyone under 18, and anyone with a history of eating disorders should seek medical advice first. Read about intermittent fasting side effects before altering your daily meal schedule.

Getting Started and What to Track

Starting intermittent fasting requires choosing an eating schedule you can sustain without compromising basic nutrition. Fasting does not suspend the basics of daily nutrition. Adequate protein, dietary fiber, and sufficient fluid intake remain necessary components of your daily meals. During eating windows, food quality still decides overall nutrient intake.

Read our guide on practical intermittent fasting tips to learn what fluids are fasting-safe and how to break a fast.

Pick an eating window you can keep, and log your daily protein and fluid intake to see how intermittent fasting fits your routine.