Intermittent fasting 16/8: where to start

🥗 Radish · 8 min

16/8 intermittent fasting is one of the simplest methods: you eat within an 8-hour window and fast the other 16 (sleep included). No complicated diet, no forbidden food. Here's how to start without getting it wrong — and what the research actually says about how well it works.

How it actually works

Classic example: you eat between noon and 8 PM, then fast until noon the next day. Water, unsweetened tea and coffee are fine during the fast. The point isn't to eat anything over 8 hours, but to shrink the window — which often, mechanically, lowers total intake. That's also the framing used by France's national biomedical research institute, Inserm, in its Canal Détox fact-check, which describes 16/8 as fasting "for 14 to 16 hours and eating within the remaining 8 to 10 hours": these diets can help some people lose weight, with no demonstrated advantage over other approaches.

The fasting phases

After the last meal: digestion (0-4h), then the body draws more on reserves (4-16h) — the phase many seek. The longer the fast, the more cellular repair processes kick in. 16/8 stays gentle and sustainable day to day.

Other intermittent fasting protocols

16/8 isn't the only format. Per the same Inserm review, the 5:2 diet means eating normally 5 days a week, then sharply cutting calories the other 2 days — roughly 500 kcal/day for women and 600 kcal/day for men. Alternate-day fasting pushes this further, with a day of severe restriction followed by a day of normal eating. 16/8 mostly restructures the day rather than imposing strict calorie limits on fasting days — which is what makes it the easiest protocol to stick with, and the one Radish supports with a dedicated timer.

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Does 16/8 actually beat other diets for weight loss?

The honest answer, without oversell: according to Inserm's review, one 12-month study of 332 people with obesity (146 completed it) compared three diets — continuous calorie restriction, alternate-week restriction, and classic 5:2 intermittent restriction — with no untreated control arm: weight loss was moderate and similar across all three groups — 6.6 kg, 5.1 kg and 5 kg on average. A second study, following 109 people for 24 months, found comparable weight loss in the first year regardless of diet type. Inserm's conclusion is clear: intermittent fasting shows no proven advantage over classic calorie restriction, and continuous restriction may even improve overall diet quality more over time. What actually makes the difference is long-term adherence — many people abandon a program before seeing any lasting benefit.

Beginner mistakes

Bingeing at the break, neglecting protein, under-drinking, or aiming for 16h on day one. Start at 12h, then 14h, then 16h. And keep balanced meals: fasting doesn't erase a poor diet. Another common trap: expecting a perfectly rigid schedule every single day. Your eating window can shift day to day depending on your plans (dinner with friends, a weekend brunch) without undermining the practice — what matters is returning to your usual window the next day, not flawless execution.

Hydration during a fast

Water stays essential during the fasting window. EFSA's scientific opinion on dietary reference values for water sets a reference total water intake of 2.5 L/day for an adult man and 2 L/day for an adult woman, including water from food. France's food safety agency, ANSES, recommends drinking a minimum of 1.5 to 2 L of water per day, without waiting to feel thirsty. There's nothing wrong with spreading that intake across both the fasting and eating windows — staying well hydrated even helps curb hunger.

Who it's not for

Fasting isn't recommended during pregnancy, with a history of eating disorders, treated diabetes, or for teenagers, without medical advice. When in doubt, ask a health professional. The scientific evidence base for therapeutic use also remains thin: Inserm's dedicated 2014 report found only four randomized controlled trials worldwide evaluating therapeutic fasting, and only one met the expected methodological standards (a 1991 trial of 53 patients with rheumatoid arthritis). The report judges medically supervised fasting to be "generally low-risk," while acknowledging that real risks exist depending on context — a reason not to extrapolate unproven benefits.

FAQ

Does coffee break a fast?

Black coffee with no sugar or milk has almost no calories: it's generally considered fasting-compatible.

Does 16/8 cause weight loss?

It mainly helps by shrinking the window and often total intake. Without an overall calorie deficit, weight won't move — and per Inserm, it doesn't outperform continuous calorie restriction.

Is 16/8 more effective than the 5:2 diet?

No: according to Inserm, no intermittent fasting protocol has shown superiority over continuous calorie restriction for weight loss. The right choice mostly depends on what you can stick with long-term.

Do I need to drink a lot during a fast?

Yes: aim for at least 1.5 to 2 L of water per day (ANSES reference), spread across the day, fasting or not.

How do I track my fast easily?

Radish has a built-in fasting timer and shows your phases live, next to your calories — counting without a connection and free.

16/8 is a simple entry point: 8-hour window, gentle progression, balanced meals. To track it hassle-free, Radish gives you the timer + your calories in one place. nutri-track.fr

General information, not medical advice.

🔎 Methodology & sources. Every nutrition figure in this article is cross-checked against official sources (ANSES, EFSA, WHO, CIQUAL/USDA, French Public Health agency), and health topics are reviewed before publication against our internal reference framework. See our full methodology →