Almost everyone can benefit from intermittent fasting (IF), which—as its name implies—means alternating between periods of eating and not eating for various amounts of time. "Fasting is the oldest dietary intervention in the world," writes Dr. Jason Fung in The Complete Guide to Fasting. "Because it differs from conventional dieting in so many important ways, fasting carries many distinct advantages."
Among them, fasting is simple, free, convenient, effective, and allows you to enjoy life’s occasional indulgences. You can do IF on almost any plan, whether you’re vegan or paleo, and it gives your overworked digestive system a much-needed break.
IF challenges conventional health theories: Many practitioners skip breakfast, don’t count calories, and go long hours—sometimes days—without eating. They think when you eat may be just as important as what you eat. And it gets results. Intermittent fasting’s numerous benefits include weight loss and chronic disease management.
At the same time, no one plan works for everyone, and that includes IF. Even Fung notes certain people shouldn’t do IF, including pregnant or breastfeeding women, underage people, and malnourished folks. He advises others like people with type 1 diabetes to proceed cautiously with fasting. While you can usually work with a credentialed health care expert and modify fasting to your condition, I’ve found people with these five issues should think twice about, and—in one case—absolutely avoid intermittent fasting:
1. You have gallstone disease.
When you fast, your gallbladder doesn’t release bile. As your liver continues to deliver bile, it becomes concentrated. Breaking your fast means your gallbladder could forcefully release sludge or small stones from that buildup that could get stuck in the bile duct. If you have gallbladder issues, proceed cautiously with IF. One study showed that among people with gallstone disease, a long overnight fast increased hospitalization risk.
2. You have an eating disorder.
Here’s my one big thumbs-down for fasting. A systematic review of IF’s potential harms mentioned eating disorders, and writer Emily T. Troscianko asked whether IF is "A Fast Route to Disordered Eating?" in an intriguing Psychology Today essay. If you have bulimia or otherwise struggle with other psychological eating disorders, IF could exacerbate those problems. An eating disorder is one condition with which you absolutely should not do fasting. Rather, always work with your doctor when struggling with any sort of disorder.
3. You have adrenal fatigue.
Fasting can keep your stress hormone cortisol ramped up, stressing your already-overworked adrenals. One study with 16 young, healthy female volunteers who fasted for 48 hours had elevated cortisol levels, suggesting fasting could create additional stress. You’re probably not going to fast that long, but beware if you have adrenal fatigue or your adrenals are already overworked from chronic stress—fasting could make your condition worse.
4. Your thyroid is shot.
Your thyroid performs many functions, including balancing energy, body temperature, and emotions. When this tiny gland isn’t working correctly, numerous problems can result. Triiodothyronine (T3) is your active thyroid hormone. Studies show fasting decreases T3, so if you have mild to moderate hypothyroidism, you might want to reconsider intermittent fasting.
5. You're sick.
Your body needs a steady supply of nutrients if you’re ill, and if you’re not eating you can’t get them. IF could also create physiological or metabolic stress: the last things you want to create when you’ve got a cold, flu, or other virus. At the very least, I recommend bone broth and a nutrient-dense protein shake or green drink to meet those nutrient requirements.
If you don’t fall into those conditions and want to try IF, start out slowly. Have a big dinner, close up the kitchen for the night, and then have breakfast as late as possible the following morning. That creates about a 12-hour or longer fasting window—most of it while you’re sleeping—that helps your body dip into those fat stores. Gradually increase that fasting time, but if you feel nauseous or otherwise unwell, please eat something. Don’t risk your health!