What type of fast is right for you?

You’re a woman.

The female body is really sensitive to periods of low energy availability, explains Ryan Andrews, RD, CSCS, a Norwalk, Connecticut-based dietitian and author of A Guide to Plant-Based Eating.

Mainly, fasting can decrease thyroid hormone and estrogen production, he says. In the long term, those changes can lead to missed periods and declines in bone density, which ups your risk of fractures. Practicing IF may also impact the body's ability to manage blood sugar levels, which increases your risk of diabetes, notes Kollias. Scientists aren’t sure why, but the effect hasn’t been observed in men.

Fast this way:

A 12-hour overnight fast is a safe place to start, he says. If you notice signs of hormonal dysfunction, like moodiness, trouble sleeping, or major decreases in energy and sex drive, lengthen your eating window and shorten your fast. 

You’re pregnant, or trying to be.

All the concerns noted above are especially pressing for pregnant women. People tend to eat fewer calories overall when they practice IF, Andrews notes. If you subconsciously under-eat due to extended fasts, your body essentially hits pause on nonessential functions. One effect: Estrogen production may decrease, which slows or even prevents ovulation and thereby, pregnancy.

Fast this way:

Don’t. The rule is simple: When you feel hungry, eat until you’re satisfied, Andrews says. Your body needs fuel when you're pregnant or trying to conceive, so don’t impose schedule restrictions on your diet.

You’re in a period of especially high stress.

Like exercise, fasting is technically stress on the body. You can only handle so much. If you’re having trouble sleeping or you’re experiencing feelings of burnout or anxiety, it’s not the right time to adopt an extreme protocol like alternate-day.

Fast this way:

In this case, you should only shorten your eating periods for one reason, Kollias says: It makes your life easier. “If finding a healthy breakfast would add stress and you don’t have much appetite first thing in the morning anyway, waiting to eat until you’re settled at work could offer relief,” she says.

You train hard for an hour or more on a daily basis.

Research on fasting and intense fitness performance shows a negative correlation between the two, Kollias notes.

If you log an hour of moderate- or high-intensity training at least six days a week, alternate-day fasting could cause stress-induced hormonal responses, Kollias says. Men should be wary of drops in testosterone levels, which doctors can test and monitor. Women should be conscious of missed periods and other irregular menstrual symptoms.

For IF to compliment your routine, you need to log low-intensity exercise, like yoga or easy miles, on three non-fasting days per week, Andrews says.

Fast this way:

Fast for up to 16 hours as often as four times per week and plan your workouts so you can eat within a few hours before and after. Consuming carbs and protein pre- and post-training primes your body for optimal performance and recovery, Kollias notes.

And record your workouts, she adds. If your one-rep max has plateaued or declined or you’re fatigued during cardio sessions that were once a breeze, shorten your fasting periods.

Your priority is to build muscle.

Gaining mass requires ample protein and a very specific approach to training. It can be a lofty goal for many people even when they eat whenever they want, Andrews says. Make your fueling window more narrow and your chances of hypertrophy drop further.

Fast this way:

Try a time-restricted strategy, such as the 12-hour overnight fast. Again, make sure you always eat within a few hours before and after your training sessions, Kollias says. Track your calorie and protein intake to make sure you’re getting what you need.