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What is intermittent fasting?
Intermittent fasting has gained popularity among celebrities, executives, and even politicians like UK Prime Minister Rishi Sunak, who reportedly begins each week with a 36-hour fast. The approach involves extending the gap between the last meal of one day and the first of the next, typically fasting for 16 hours and eating within an eight-hour window. While some studies suggest fasting may support cellular repair and longevity, dietitians caution that it may not be the most effective strategy for sustainable weight loss.
How does intermittent fasting work?
Unlike traditional calorie-restrictive diets, intermittent fasting focuses on when you eat rather than what you consume. Proponents claim it can trigger autophagy-a process where cells remove damaged components and recycle them into new structures. This mechanism is linked to potential benefits like improved immune function, cancer suppression, and extended lifespan, though most evidence comes from animal studies, such as experiments with nematode worms and mice.
Autophagy typically occurs during sleep, exercise, or prolonged fasting. After eating, the body enters a "fed state," using glucose for energy. Once glucose reserves deplete-usually three hours post-meal-the body shifts to breaking down glycogen stored in the liver and muscles. When glycogen is exhausted, it transitions to ketosis, burning fat-derived ketones for fuel. This metabolic switch is believed to activate autophagy.
Weight loss vs. health benefits
Despite its popularity as a weight-loss tool, experts warn that intermittent fasting may not lead to long-term results. Rachel Clarkson, founder of The DNA Dietitian, argues that fasting reduces calorie intake without teaching healthy eating habits. "If you feel starved and restricted, you might overeat later," she says. "Without learning what a balanced diet looks like, people often regain weight after stopping the fast."
Clarkson advises focusing on fasting's potential health benefits rather than fat loss. However, an unpublished 2024 study presented at an American Heart Association conference found that adults who ate within an eight-hour window had a higher risk of cardiovascular death compared to those who spread meals over 12-16 hours. The findings, based on data from 20,000 U.S. adults tracked for up to 17 years, have not yet undergone peer review.
The science of hunger and ketosis
Hunger is regulated by hormones like ghrelin, which signals the brain to seek food, and leptin, which suppresses appetite when fat stores are sufficient. Clarkson explains that fasting requires overriding short-term hunger cues, often by drinking water to ease discomfort during the adjustment period. "The first couple of weeks are tough, but your body adapts," she notes.
Ketosis and autophagy typically begin 12-24 hours after the last meal. For example, finishing dinner by 8:30 p.m. could trigger ketosis by 8:30 a.m. the next day-if no late-night snacks or sugary drinks extend the fed state. Clarkson suggests starting with small changes, like eating dinner an hour earlier or delaying breakfast, to gradually increase fasting windows.
Who should avoid intermittent fasting?
While fasting may offer cellular benefits, it's not suitable for everyone. Pregnant individuals, people with diabetes, or those with a history of eating disorders should consult a healthcare professional before attempting it. Clarkson emphasizes that fasting is not a substitute for a balanced diet. "Autophagy declines with age, so boosting it later in life might help," she says. "But no diet replaces nutritious eating."
Updated 20 March 2024 to include findings from an unpublished study linking time-restricted eating to increased cardiovascular risks.
