Who the fasting evidence does not cover
This is not a list of people who are forbidden to fast. It is a list of people the studies did not include — which means that for them, the honest answer to “is this safe for me” is that these papers cannot say, and a doctor can.
What the trials actually studied
The 12-week trial most often cited for 16:8 enrolled adults aged 18 to 64 with a BMI between 27 and 43. The 12-month trial that tested a window against calorie counting enrolled 139 patients with obesity. A review of the wider field — alternate-day fasting, 5:2, and time-restricted eating — reports that intermittent fasting is generally safe, producing 1–8% weight loss without energy-level disturbances or increased disordered eating in the trials it reviewed.
That last phrase is the whole point of this page. “Safe in these trials” is a statement about the people in them.
Who was outside that
If any of these describes you, the evidence above is not evidence about you, and this is a question for a doctor rather than for a blog:
- You are pregnant or breastfeeding. Not studied. Requirements are higher and different.
- You are under 18. Below the enrolment age of the trials above.
- You have diabetes and take insulin or a sulfonylurea. Long gaps without food interact directly with medication that lowers blood sugar. This needs a prescriber, not a schedule from the internet.
- You have or have had an eating disorder. Rules about when eating is permitted are the same shape as the illness. The review reports no increase in disordered eating among the people studied — who were screened, and would not have included this group.
- You are underweight, or losing weight without trying. Weight loss is the intended effect. It is the wrong intended effect here.
- You take medication that must be taken with food, or timed through the day. The window can silently break the schedule.
- You are an older adult losing weight. No trial here followed that group, and losing lean mass matters more with age.
Two things worth knowing even if none of that applies
Fasting without resistance training costs lean mass. In the 12-week trial, a meaningful share of the weight lost was lean mass rather than fat. That is true of weight loss generally, but it is not nothing, and the answer is training rather than a different window.
Hormones moved in one trial. Over eight weeks of 16:8 in resistance-trained men, testosterone and IGF-1 both fell significantly. Strength and muscle area did not. Nobody has followed that out over years.
The honest summary
For healthy adults with overweight, in trials lasting weeks to a year, this way of eating looks safe and produces modest weight loss, mostly by reducing how much you eat. Outside that description, the studies are silent, and a page like this one should be too.
If you are unsure which side of that line you are on, that uncertainty is itself the reason to ask a doctor before starting — not a reason to try it and see.