Millions of people fast for religious reasons every year. Muslims observing Ramadan go without food or water during daylight hours for 28 to 30 days. Christians, Jews, Hindus, and others have their own fasting traditions. For most healthy people, these practices are spiritually powerful and don’t cause lasting harm.
But fasting isn’t risk-free — and for some people, it can go sideways fast.
Research focused on Ramadan fasting found that serious complications can develop, including dehydration, hypoglycemia (blood sugar crashing too low), hyperglycemia (blood sugar spiking too high), and diabetic ketoacidosis — a dangerous condition where ketones build up to toxic levels in your blood as your body breaks down fat for fuel. These aren’t minor inconveniences. Some of them require emergency care.
If you have diabetes, this is the big one to know about. Both going without food AND the timing of any medication you take can throw your blood sugar completely out of range. That’s not a small deal.
Dehydration is another real risk, especially during long fasts that restrict water. In hot climates or during physical activity, the effects can hit harder and faster than people expect.
Studies have also looked at how fasting affects metabolic health more broadly — things like cholesterol levels and blood glucose over time. The effects vary a lot depending on the person, the type of fast, and underlying health conditions. That’s exactly why one-size-fits-all advice doesn’t work here.
The good news is that most religious traditions include exemptions — or offer alternatives — for people whose health makes fasting dangerous. You’re not failing your faith by getting medical guidance. You’re taking care of the body you have.
If you’re young and healthy with no underlying conditions, a short or modified fast likely doesn’t require a doctor visit. But if you have diabetes, heart disease, kidney issues, a history of eating disorders, or you’re pregnant, that’s a different conversation — and it needs to happen before you fast, not during a crisis.