A 24-hour fast is exactly what it sounds like: a fasting method that involves completely abstaining from calorie intake for a 24-hour period.
Also known as whole-day fasting, 24-hour fasting is considered a type of periodic fasting (PF), which refers to fasting methods that don’t involve fasting every day or every other day.
Unlike other types of fasting which are usually practiced every day or most days of the week, like 18:6 fasting, people using the 24-hour fasting method may only fast once a week or a couple of times per month.
When using the 24-hour fasting method, you’ll only drink water or other zero-calorie beverages like herbal tea during the fasting window. Although there are variations to fasting where people take in a small number of calories—commonly known as “dirty fasting”—experts suggest that completely abstaining from calorie intake is the only way to ensure you’re reaping the full benefits of fasting.
“Ideally, zero calories should be consumed during fasting periods,” Grant Tinsley, Ph.D., a professor at Texas Tech University and intermittent fasting researcher, tells mindbodygreen.
“While a splash of milk in your coffee may not ‘break’ your fast as much as eating a full meal, the only way to ensure that you are not breaking your fast at all is to not consume any calories,” he says.
There’s not a lot of research investigating the effects of dietary supplements on fasting, but it’s likely that vitamin, mineral, and antioxidant supplements are okay to take as long as they don’t impact blood sugar levels. This means that gummy vitamins and calorie-containing vitamin powders are a no-go during fasting.