Why Intermittent Fasting Isn’t Right for Everyone

Intermittent fasting (IF) may sound like a panacea, but it’s not right for everyone. Experts suggest that the following groups avoid the practice:

• Pregnant or breastfeeding women. Pregnancy is a period of growth, and fasting doesn’t support what the body needs during this time. Likewise, a woman who’s breastfeeding has higher nutrient and calorie requirements.
• Children. Fasting isn’t a good match for growing bodies. “Don’t limit nutrients during this time,” says Jason Fung, MD.
• Anyone with a history of disordered eating. “Restricted eating windows can be very triggering” for people with a history of eating disorders, says Sara Gottfried, MD. (
• Underweight people. Most people lose weight during IF, so if you are underweight to begin with, avoid it. “If you have a BMI under 20, don’t do it,” says Fung.
• People suffering from heart problems or high blood pressure. IF may be cardioprotective, but it is critical for anyone with heart problems or high blood pressure to consult their healthcare provider before embarking on a fasting regimen. “Intermittent fasting might lead to electrolyte imbalances or other medical conditions in people suffering from heart problems or high blood pressure,” says Gottfried.
• Those with adrenal issues or blood-sugar imbalances. IF can cause spikes in cortisol, which may further aggravate adrenal issues and make blood-sugar problems worse. “Because cortisol and insulin regulate blood sugar, if you have impairments in either of those systems, you might struggle with IF at first,” says Susan Blum, MD. She asks patients with adrenal fatigue or blood-sugar issues like diabetes or insulin resistance to start with a 12-hour fast and build from there. Gottfried advises patients with reactive hypoglycemia (dangerously low blood-sugar levels after a meal) “to avoid IF or perform it only under careful physician observation.”

A note of caution for people with thyroid issues: While IF can ultimately be useful for people with thyroid conditions, the early stages can be tricky because of the impact on insulin and cortisol. Listen to your body, says Gottfried. “Start slow and ramp up step by step,” she says. “If it doesn’t feel right, you may need to address the root cause first instead of simultaneously.”

*For details on the dos and don’ts of IF, see “The Case for Intermittent Fasting”.

This originally appeared as “The Fasting Track” in the January/February 2020 print issue of Experience Life.