Intermittent Fasting

Intermittent fasting (IF) restricts calories in time rather than every day. The best-studied variant, alternate-day fasting (ADF), alternates a near-zero-calorie day with a normal or near-normal eating day. Stekovic and colleagues ran a controlled ADF trial published in Cell Metabolism and found it safe over more than six months in healthy, non-obese adults, with metabolic and inflammatory benefits comparable to continuous caloric restriction but reportedly better tolerated by participants [1].

A smaller trial in obese women tested a modified ADF over eight weeks and found significant reductions in body weight, waist circumference, and blood pressure [2]. At the cardiovascular level, a 2025 crossover RCT by Kjaerulff and colleagues put 16 overweight adults through three weeks of ADF and measured direct cardiac effects: myocardial oxygen consumption fell about 10%, left ventricular energy use fell about 19%, and myocardial flow reserve rose 34% [3]. These are mechanistic cardiac improvements, not mortality data, but they are objectively measured, not self-reported.

What the evidence does not show is that ADF outperforms simple continuous calorie cutting for the outcomes that matter most, weight loss and long-term cardiometabolic risk. Head-to-head trials generally find IF protocols roughly equivalent to matched continuous CR when total calories are held constant, and a review situating CALERIE-2's exploratory outcomes alongside the IF literature makes the same point: the mechanism doing the work in most trials looks like calorie deficit, not fasting-specific biology [4]. The 6-hour cardiac effects from Kjaerulff's crossover design come from a three-week protocol; nothing in that dataset says the myocardial benefit persists or scales with years of practice.

Who this does not work for, or where evidence is weakest: ADF trials recruit healthy or overweight adults without diabetes on insulin or sulfonylureas, so people on glucose-lowering medication that can cause hypoglycemia on a near-zero-calorie day are a real safety concern outside the tested population. People with a history of binge eating are typically excluded from these trials and are a poor fit clinically, since fasting-refeeding cycles can trigger binge patterns. Pregnant women, adolescents, and underweight adults have almost no ADF trial data. The Stekovic trial's six-month safety window is also the longest well-controlled human ADF data available, so claims about multi-year ADF safety and benefit are extrapolated, not directly tested [1].

The proposed mechanism, beyond simple calorie deficit, is a metabolic switch during the fasted state from glucose to ketone-body and fatty-acid metabolism, which some researchers link to autophagy induction, though direct human evidence of autophagy activation from a single ADF cycle is thinner than the metabolic-marker evidence [4]. Self-experimenters, including some Blueprint-adjacent practitioners, use IF as a lower-effort substitute for continuous CR, on the theory that fewer decision points per day improve adherence; the trial evidence supports the adherence argument better than it supports any fasting-specific longevity mechanism.

Plain takeaway: alternate-day fasting is a well-tolerated way to achieve a calorie deficit over months, with real short-term cardiometabolic and cardiac-function benefits in controlled trials, but it has not been shown to beat matched continuous caloric restriction for the same calorie deficit, and safety data beyond about six months in humans is limited.