Therapeutic Fasting and Intermittent Fasting Explained Simply

1. What It Is About

During fasting, a person eats nothing or significantly less than usual over a set period. Therapeutic fasting involves consuming little or no food for several days to weeks, often with medical supervision. Intermittent fasting means eating only during specific times, with the rest of the day being a pause. A common pattern is 16:8: a 16-hour pause followed by an 8-hour eating window. Another is 5:2: five regular days and two days with significantly reduced food intake. Both approaches stem from older and newer nutritional-medical strategies for regulating metabolism. During the pauses, the body taps into stored energy. Additionally, a form of cellular cleansing called autophagy is triggered; the cell breaks down and recycles its own damaged components. Studies primarily measure changes in weight, blood sugar, blood lipids, and liver fat. The fasting methods differ significantly in the duration of the pauses and whether they are carried out at home or in a clinical setting.

2. What Studies Show

For evaluation, 299 works were available, including 119 controlled human studies, 67 observational studies, and 26 review articles. 'Controlled' means: one group fasts, another does not or fasts differently, and both are compared. Most of these studies lasted between 8 and 12 weeks. The observations on weight and blood sugar in people with overweight, obesity, or type 2 diabetes are best supported. In one study with 120 overweight adults with type 2 diabetes, participants ate only between 8 am and 6 pm for 12 weeks. Afterward, their long-term blood sugar levels had decreased, and they had lost around three kilograms 2. A study with 88 adults who had obesity and fatty liver disease lasted 12 months. One group ate only between 8 am and 4 pm, while the other reduced daily calories. The result for liver fat was similar in both groups — the eating window did not perform better than calorie reduction 4. In a 14-week study with 90 adults with obesity, the eating window was early in the day, from 7 am to 3 pm. Blood pressure and insulin sensitivity improved more strongly in this group than in participants with a longer eating window; the study was not blinded 1. Another study with 209 adults at increased risk for type 2 diabetes compared fasting days plus an early eating window with daily calorie reduction. After 6 months, blood sugar processing was better in the fasting group, but this difference was no longer detectable after 18 months 5. Short-term studies with healthy adults, overweight individuals, and people with type 2 diabetes found intermittent fasting to be well tolerated.

3. What Remains Unclear

It remains unclear what happens after one year. Whether the observed changes persist beyond 12 months or disappear has not been investigated. Potential risks are also unresolved: whether prolonged use leads to muscle loss or malnutrition has not been systematically examined in the short-term studies — particularly not in people who are already at risk. There are indications from animal studies regarding inflammation; human studies on this topic are still pending. In cancer, a small study with breast cancer patients showed that fasting before and after chemotherapy reduced the burden on blood values and improved quality of life 3; however, the authors note that fasting during cancer therapy should not be used outside of studies due to potential risks. Results on attention and memory are inconsistent. The question that would clarify much of this is a longer comparative study directly comparing fasting with medication-based treatment for diabetes.

4. What This Means in Everyday Life

The studies show: when fasting helped, it was usually accompanied by less eating or a change in diet. How much the eating window alone contributes remains unclear. The observed changes were also often moderate and not always greater than simple calorie reduction. Some people respond strongly to fasting, others not at all; why this is the case remains open. Those wishing to interpret the studies can consider three factors: how long the study lasted, whether a comparison group was used, and whether eating behaviour was objectively recorded or self-reported. Self-reports are considered a weak point. What remains to be clarified is whether a fasting plan can be found that reliably suits specific groups — such as people with diabetes or cancer — and how it interacts with medication-based therapy.

All details with study numbers and sources: detailed page on therapeutic fasting and intermittent fasting. For children: Therapeutic fasting and intermittent fasting for children