5:2 fasting
What it is
Five days a week eating without rules, two non-consecutive days at roughly a quarter of usual energy — five or six hundred kilocalories, in the usual version. It belongs to the intermittent-fasting family alongside daily time windows (16:8) and alternate-day fasting, and of the three it is the one with the most reassuring long-term data in the umbrella review.
How it works in practice
Choose the two days, keep them apart, keep them consistent. On a fast day: protein and vegetables, soup, yoghurt, eggs — food that fills for little energy — and plenty of water and tea. On the other five: ordinary eating, which is the whole point and the whole risk. The weekly arithmetic is simple: two days at a quarter of usual intake remove roughly a fifth of the week's energy, about the same as a moderate daily deficit, provided the five free days stay ordinary rather than celebratory.
What the evidence shows
An umbrella review of meta-analyses of randomised trials, graded for certainty, found that modified alternate-day fasting of the 5:2 type produced sustained weight loss at six to twelve months compared with usual eating, with moderate-quality evidence — the strongest long-term signal among the fasting patterns reviewed. Where the comparison was with continuous daily restriction of the same energy, the results were similar: the pattern is a way of arranging a deficit, not a different mechanism. The review's own caveat is that most comparisons are against usual eating or inconsistent controls, so "better than a matched daily deficit" is not established.
Who it tends to suit
People who prefer two hard days to seven moderate ones; people whose week has a natural shape (quiet Mondays and Thursdays, busy weekends); people who dislike counting every day and will accept counting on two.
How it most often fails
On the five days, by compensation: the fast day earns a feast, the feast erases the fast, and the week nets to maintenance. And on fast days that land on the wrong day — the family dinner, the work lunch — and get skipped, then skipped again.
What to watch
Protein across the week, since fast days cannot carry it; the five days, honestly logged; and, for anyone on diabetes medication, a doctor before starting, because fast days change medication needs. Anyone with a history of restriction and bingeing should not use a pattern that alternates the two by design.
In the app
The app plans each day with the same kind of target and does not schedule fast days, so 5:2 is not something it verifies. The pace calculator's weekly share can be arranged this way by hand; the app's diary will simply show two low days and five ordinary ones.