Intermittent fasting involves alternating between short periods of fasting, where little to no food is consumed, and periods of regular eating. It aims to help with weight loss and improve health markers, including blood pressure and cholesterol levels. The most common ways to fast include:

Alternating between days of no food restriction and days with one meal that provides 25% of your daily calorie needs. For example, one can fast on Mondays, Wednesdays, and Fridays while having no food restrictions on alternate days.
Fasting for one or two whole days per week or consuming up to 25% of your daily calorie needs. For example, the 5:2 diet, where there are no food restrictions for five days of the week and a 400–500 calorie diet is followed for the other two days.
Following a meal plan with a specific time frame for fasting. For example, consuming meals between 8 a.m. and 3 p.m. and fasting during the remaining hours of the day.

This type of eating pattern may not be suitable for individuals who eat small meals or snacks throughout the day. Going without food for long periods or restricting food intake can lead to overeating when food is finally consumed and may contribute to unhealthy behaviors related to food. People with the following conditions should avoid intermittent fasting:
Diabetes
Eating disorders such as anorexia or bulimia nervosa
Taking medications that require food consumption
Adolescents in the active growth stage
Pregnant or breastfeeding individuals.

Calorie restriction has been proven to increase lifespan and enhance tolerance to metabolic stresses in animals. While animal studies strongly support this, human studies have not provided convincing evidence. Supporters of caloric restriction argue that intermittent fasting triggers an immune response that repairs cells and leads to positive metabolic changes. One concern is that individuals may overeat on non-fasting days to make up for lost calories, but studies have not supported this claim when compared to other weight loss methods.

A review of 40 studies discovered that intermittent fasting was effective for weight loss, resulting in an average loss of 7–11 pounds over a period of 10 weeks. Studies also show that the dropout rates ranged from 0-65%. When comparing the dropout rates between the fasting and continuous calorie restriction groups, no significant differences were found. Overall, the review did not find that intermittent fasting had a lower dropout rate, indicating that it may not necessarily be easier to follow compared to other weight loss approaches. Furthermore, when comparing the fasting group to the continuous calorie restriction group in 12 clinical trials, there were no significant differences in the amount of weight loss or changes in body composition. In 10 trials that examined changes in appetite, there was no overall increase in appetite in the intermittent fasting groups, despite significant weight loss and decreases in leptin hormone levels (a hormone that suppresses appetite).

In addition, a year-long study found that there were no significant differences in weight loss, weight gain, body composition, blood pressure, heart rate, fasting glucose, or fasting insulin. However, at 12 months, the alternate-day fasting group had significantly increased LDL cholesterol levels, while there were no differences in total cholesterol or triglycerides. The dropout rate was higher in the alternate-day fasting group, and interestingly, participants in that group ate less food than prescribed on non-fasting days but more food than prescribed on fasting days.

Sources:
https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/intermittent-fasting/