Lifestyle+ — 14

There has been considerable interest in whether calorie reduction or time-restricted eating leads to healthy aging. During World War I in Denmark, data from a two-year natural experiment showed dietary restriction associated with a 34 percent reduction in mortality. A standard approach entails a 20 to 50 percent reduction in calories, with preservation of macronutrient ratios. That differs from a ketogenic diet, which markedly reduces carbohydrate intake to less than 50 grams per day and derives 70 to 80 percent of calories from fat. Calorie restriction has been intensively studied for promoting longevity in multiple model organisms, such as yeast, fruit flies, and mice. In rhesus monkeys, however, one study reported marked increase in lifespan, and another showed no difference.

Less stringent, more palatable alternatives to calorie restriction are time-restricted eating and intermittent fasting. Time-restricted eating usually refers to a schedule of eating only for eight hours a day, with no caloric intake the other sixteen hours (known as 16:8). Intermittent fasting varies from alternative day fasting to a 5:2 schedule of fasting two days each week. Compared with regular food intake that is throughout the day and evening every day, these diets have been shown to match the benefits of more extreme calorie restriction, in particular the effects on insulin-signaling and other pathways. The participants in the many small, randomized trials vary considerably from being lean, being overweight or obese, or having type 2 diabetes. Some incorporate intensive coaching and monitoring, while others don’t. Of course, the problem with comparing a standard calorie-restricted diet and a time-restricted eating regimen is that eating only in a circumscribed time window results in some caloric restriction.

Perhaps the most important message here is that our digestive processes seem to work best when fed on a regular routine. But I’ll review a sample of the trials to give you a flavor of their results, some of which are contradictory. A randomized trial of 139 participants with obesity found little difference between calorie-restriction and time-restriction diets with regard to weight loss and metabolic markers after twelve months. Similarly, in a twelve-week trial of 116 participants who were overweight or obese, time-restricted eating had no benefit in weight loss or metabolic parameters (that included insulin, glucose, lipids, activity, energy expenditure, and fat mass) compared with the usual diet of eating throughout the day. However, another randomized trial for weight loss in ninety participants with obesity found a time restriction early in the day (7 a.m. to 3 p.m.) versus the usual diet (continuous eating) was superior for losing weight and fat mass, lowering diastolic blood pressure, and improving mood and sleep. Another randomized trial of time-restricted eating compared with continuous eating in seventy-five people with type 2 diabetes demonstrated weight loss and improved glucose regulation. In a trial of 209 people at risk for type 2 diabetes, twenty-hour fasting periods three nonconsecutive days per week was compared to basic calorie restriction or continuous eating and provided only modest improvement in glucose regulation. Many small trials of intermittent fasting, be it alternate days or two days a week, have shown parallel benefits, as did a systematic analysis published in 2024.