Registered dietitian Nicole O’Neill said intermittent fasting works for some people but often leads others to overeat, feel fatigued, sleepy or mentally foggy during long fasting windows.
Common versions include 16:8, 14:10 and 5:2 schedules, which can cut calories and support weight loss when they fit a person’s routine and energy needs.
Skipping breakfast until noon can leave some people so hungry that they consume more calories during the eating window than they would with regular meals.
Type 1 diabetes, pregnancy, breastfeeding and age under 18 are among cases where O’Neill said intermittent fasting is not recommended, and she advised starting only with medical guidance.
O’Neill said newer GLP-1 weight-loss drugs have already replaced intermittent fasting for some patients, underscoring a broader shift toward more individualized treatment.