16:8, 18:6, 20:4 and OMAD: what actually changes?

What do the numbers in 16:8 and 20:4 mean?

The first number is hours without food and the second is hours in which you eat, across a 24-hour day. OMAD, one meal a day, is not a pair: its window is however long that one meal takes.

Nothing in the notation says what the meals contain or how large they are. Two people on 16:8 can eat very different amounts. A fasting window is a schedule, not a diet.

What actually differs between the schedules?

The schedules differ in how many meals fit, how large each has to be, how easy nutrients are to cover, how protein is spread and how the window fits your life.

What happens in the body during a fast?

During a fast the body draws down liver glycogen and relies more on fatty acids and ketones. In the hours after a meal it mostly uses and stores the energy it just took in. Reviewing the shift in Obesity (2018), Anton and colleagues write that the switch “typically occurs between 12 to 36 hours after cessation of food consumption depending on the liver glycogen content at the beginning of the fast, and on the amount of the individual’s energy expenditure/exercise during the fast.” There is no single hour at which it happens.

Research on what fasting does in the body beyond that is active, and popular summaries often run ahead of it. The National Institute on Aging notes that most research so far has focused on weight loss, primarily in people with obesity. Only a few small clinical trials have been conducted. The institute says there is not enough evidence to recommend any such eating regimen to the public.

Does the timing matter if you eat the same amount?

Two randomized trials found that the window did not add significant weight loss on its own. The question separates a schedule from a diet, and both trials tested it directly.

• 1Eating in a window, without countingRandomized trial

  JAMA Internal Medicine (2020) assigned 116 adults with overweight or obesity to eat between noon and 8 pm or to eat three structured meals a day, for 12 weeks. The authors concluded that time-restricted eating, in the absence of other interventions, is not more effective for weight loss than eating throughout the day.

• 2The same calories, with or without a windowRandomized trial

  The New England Journal of Medicine (2022) put 139 patients with obesity on the same daily calorie restriction for 12 months, with one group also eating only between 8 am and 4 pm. Mean weight loss was 8.0 kg with the window and 6.3 kg without it. The difference between the groups was not statistically significant.

Put together, much of what a fasting window does for weight comes through intake. Closing hours in the day can make it easier to eat less without counting. When intake was held equal, these trials did not find that the clock added a significant amount on top.

Who should not fast?

Fasting is not appropriate in pregnancy or breastfeeding, with a current or past eating disorder, or with insulin or a sulfonylurea unless a clinician supervises it.

Meal timing interacts directly with some common medicines. Reviewing food-drug interactions in the Oman Medical Journal (2011), Bushra and colleagues note that lovastatin should be taken with food to improve absorption. Tetracycline should be taken one hour before or two hours after meals. Taking azithromycin with food reduces its bioavailability by 43%. A pharmacist can say how your own medicines fit a schedule.

Is a longer fast better?

Nothing in the trials above suggests that moving from 16:8 toward OMAD, or beyond it, is a path to better results. Fasts measured in days are a different practice from the daily schedules above, with different risks, and they are outside this guide.

What fasting trials found