Fasting is one of the biggest weight-loss trends to arise in recent years. Endorsed by A-list celebrities and the subject of a spate of best-selling books, it was the eighth most-Googled diet in America in 2018.

But fasting shouldn’t be dismissed as just another fad. At the Charité University Hospital in Berlin, I’ve employed what’s called intermittent fasting, or time-restricted eating, to help patients with an array of chronic conditions. These include diabetes, high blood pressure, rheumatism and bowel diseases, as well...