In 1956, the effects of alternate-day calorie restriction were monitored for 3 years in a Spanish nursing home. Pensioners aged > 65 years were permitted 1 L of milk and 2-3 pieces of fruit on calorie-restricted days, which amounted to about 900 calories. On alternate days, they could eat 2,300 calories. Study investigators also assessed control participants who were provided a standard institutional diet.
Overall, the investigators found that the fasters spent less time in the infirmary and lived longer compared with the control group. They did not, however, lose weight.
Today, more than 60 years after this study was published, experts are still very interested in the health effects of oscillating patterns of food consumption. Let’s take a closer look at the different types of fasting and their health effects.
Fasting generally falls under one of two eating patterns: continuous caloric restriction and cyclical eating.
An eating pattern centered around caloric restriction generally involves a decrease in daily caloric intake without ensuing malnutrition, and this type of fasting has been shown to reduce body weight and extend longevity.
A cyclical eating pattern, on the other hand, can take many forms:
- Alternate-day fasting (ADF): Entails eating no calories on fasting days, and alternating fasting days with one day of unrestricted food intake (ie, a ‘‘feast’’ day).
- Alternate-day modified fasting (ADMF): Entails eating < 25% of baseline energy needs on ‘‘fasting’’ days, alternated with a “feast” day.
- Intermittent fasting (IF): Entails fasting for varying periods of time, typically for ≥ 12 hours.
- Periodic fasting (PF): Entails fasting only 1-2 days per week, and consuming food unrestricted for the rest of the week.
- Time-restricted feeding (TRF): Entails restricting food intake to specific time periods of the day, usually between 8 and 12 hours each day.
The placement of time restrictions on feeding has been demonstrated to exert a gamut of systemic effects that compare with continuous calorie restriction, including the following:
- Improvement of cardiometabolic risk factors (eg, insulin resistance, dyslipidemia, and inflammatory markers)
- Decrease in fat surrounding the viscera
- Improvements of thrombophlebitis
- Improvements in osteoarthritis
- Healing of bed sores
- Surgery tolerance
Importantly, time restrictions on eating have been shown to result in weight loss comparable with calorie-restriction regimens.
In animal studies of ADF, this practice was found to be just as effective as calorie restriction in curbing diabetes risk. Researchers of human studies, however, have failed to show any such benefit. Instead, these trials have noted greater insulin-mediated glucose uptake with no effect on fasting glucose or insulin levels.
From an evolutionary perspective, humans may have adapted to cyclical fasting and eating patterns depending on food availability—for instance, availability of excess food after a hunt would result in feasting. In fact, indigenous peoples throughout the world who have avoided Western lifestyles and diet have exhibited fewer signs of age-induced diseases, including cancer, neurodegeneration, cardiovascular disease, hypertension, and diabetes.
Fasting enhances health by minimizing age-related risks for hypertension, neurodegeneration, cancer, and heart disease. For instance, in some studies, less than 2 weeks of water-only fasting significantly reduced and controlled blood pressure levels in hypertensive patients who were previously taking antihypertensive medications.
Furthermore, in some mouse models, PF was shown to mitigate the repercussions of age-related neurogenerative diseases, including Alzheimer disease, Parkinson disease, and Huntington disease. Researchers have also shown that fasting may be as effective as chemotherapy for the treatment of certain tumors in mice.
On a final note, the beneficial effects of fasting on systemic conditions may be due to what experts hypothesize is the “flipping” of a metabolic switch. This switch is a shift from the use of glucose derived from glycogenolysis to glucose derived from fatty acids and fatty-acid derived ketones.
This switch is preferential because ketones are the preferred fuel sources for the body and brain during fasting and exercise. In terms of weight loss, this switch represents a shift from lipid synthesis and fat storage to the movement of fat as free fatty acids and fatty-acid derived ketones. These effects have prompted some experts to herald IF as a way to treat obesity, metabolic syndrome, and type 2 diabetes.