Intermittent Fasting: What the Big Studies Actually Say

• 16:8 (time-restricted eating): a daily 8-hour eating window, 16 hours off — the everyday variant, usually by dropping breakfast or evening snacks.
• 5:2: five normal days, two days at sharply reduced calories (~500–600 kcal).
• Alternate-day fasting (ADF): fasting or sharply restricting every other day — the toughest and best-studied intensive form.

Common to all: they structure WHEN you eat — in the hope that this automatically means eating less. That hope is exactly where success is decided.

2017, JAMA Internal Medicine: one year of alternate-day fasting versus classic calorie restriction — no difference in weight loss, but the highest dropout rate in the fasting group. The first big sobering result.

2022, New England Journal of Medicine: the cleanest separation of variables — both groups ate calorie-restricted, one additionally only within an 8-hour window. Result after one year: no additional benefit of the window for weight, belly fat or metabolic markers. The deficit works, not the clock.

2023, randomised trial with 209 participants: a narrow 8-hour eating window edged out classic calorie restriction for weight loss after six months — but by 18 months the difference had disappeared, with both groups level. A textbook example of short-term fasting advantages fading over the long run — patience pays off when judging diet trials.

2025, BMJ network meta-analysis across 99 randomised trials: all fasting forms beat "just keep eating", but against continuous calorie restriction only alternate-day fasting retains a mini lead of roughly one kilo — below the authors' own pre-defined relevance threshold, in mostly short trials of moderate evidence certainty. A direct 5-arm comparison trial from 2025 adds a head-to-head nuance: both a ketogenic diet and moderate alternate-day fasting outperformed time-restricted eating (TRE) for weight loss there — a sign that not all fasting variants are equally effective.

The 2019 NEJM review that fuelled the mechanism excitement (ketone bodies, cellular repair, metabolic switches) remains worth reading — but its promises largely stem from animal models. The human trials to test them have since arrived, and they say: solid, not magical.

Well suited if: you snack in the evening out of boredom (the window cuts exactly that off), you follow clock rules more easily than quantity rules, breakfast means nothing to you anyway, and your life allows stable meal times. In that case 16:8 is the most sensible entry variant — combined with the fundamentals from the deficit article.

Rather unsuitable if: your family or work life has fixed shared meals that fall into the "forbidden" window; you tend toward ravenous rebound (starving at noon is not a character flaw but predictable physiology); or you train hard and struggle to fit your protein into 8 hours (why distribution matters).

Not without medical supervision, or not at all: with medicated diabetes (hypoglycaemia risk — align fasting plans with your practice), with a history of disordered eating (rigid forbidden windows are a known trigger), in pregnancy and breastfeeding, for children and teenagers, and with underweight.

• The window is no free pass: "I fasted, so I've earned it" is the classic self-deception mechanism — the total balance still decides.
• Plan protein first: two to three solid protein portions must fit into the window — especially combined with strength training (guidelines in the protein guide).
• An honest 4-week audit: weight trend, energy, training, social life — if two of those suffer, intermittent fasting is the wrong tool for you. That is not failure but a test result: the best method is the one you sustain.