Fasting seems something far away from today habits. Nowadays, when proposing simple healthy diets often becomes difficult, recommending a period of total abstinence from food seems completely anachronistic.
Nevertheless, short or long periods of abstinence from food are still part of religious rituals, practiced by millions of faithful around the world.
Catholics follow restrictions during the Easter period, Buddhists refrain from solid foods on full moon days and other holidays. The long daytime fasting of Ramadan is a mandatory practice in Islam, but it is also found on some days of the year in the Jewish calendar, as in Yom Kippur, the day of atonement.
These archaic religious eating habits tend to be less practiced, but at the same time there are increasingly evidences of the positive effects of fasting on health, which can slow or reverse aging and reduce the onset of different diseases.
There are three main models of intermittent fasting: alternate-day fasting, 5:2 intermittent fasting, and daily time-restricted feeding.
In the first, days of fasting, with a calorie intake corresponding to 25% of the daily requirement, are alternate to a day with a normal diet.
Intermittent fasting is based on a consecutive fasting period of more than 24 hours. An example of this method is the 5:2 diet with two fasting days per week. However, on fasting days a calorie consumption corresponding to 25% of the daily requirement is allowed.
Finally, in the daily time-restricted feeding meals are allowed only during a restricted period. Examples are the 16:8 diet, where 16 hours of fasting are followed by 8 hours of non-fasting.
Some studies indicate that intermittent fasting may be effective for decreasing weight, reducing insulin resistance and improving cardiovascular and metabolic health. However, there are some discrepancies in the studies that evaluated efficacy and safety of these food regimes, in particular regarding the long-term effects.
The New England Journal of Medicine has recently published an interesting review on intermittent fasting, written by Rafael de Cabo and Mark P. Mattson from the Translational Gerontology Branch, and from the Laboratory of Neurosciences, of Johns Hopkins University in Baltimore.
If in the first studies the benefits of intermittent fasting were attributed to a reduction of free radicals, this new article describes a more complex underlying mechanism, based on a change in cellular metabolism, where the availability of glucose of hepatic origin is replaced by ketones derived from fat cells.
According to the authors, intermittent fasting elicits archaic adaptive cellular responses that tend to protect the cell itself. Thanks to these responses, glucose regulation improves, resistance to oxidative stress is increased, damaged molecules are repaired, and inflammatory processes are reduced.
In the first part of the review researchers retrace the main metabolic pathways used to manage cellular energy. They remember how glucose is used for energy after meals, while fats are stored in adipose tissue in the form of triglycerides.
On the contrary, in fasting periods triglycerides are broken into fatty acids and glycerol, to be used as energy sources.
The increase in free fatty acids released by adipocytes has many positive effects on different organs and tissues, such as muscle, brain and heart. These include a greater expression of antioxidative defense mechanisms, activation of DNA repair, mitochondrial biogenesis and autophagy.
The review presents the most important studies on intermittent fasting conducted in animals and humans. In these last ones, research seems to indicate that intermittent fasting improves obesity, insulin resistance, dyslipidemia and hypertension. As already mentioned, it also down-regulates inflammation.
In these studies, a significant reduction in fasting insulin, and an increase in insulin sensitivity was also evidenced.
Other favorable effects of intermittent fasting, highlighted in the review, concern physical capacity and mental performance. The different research carried out in this field has shown an improvement in verbal memory, executive function and cognition, as well as important influences on brain aging and neurodegenerative diseases.
The article also considers studies conducted on animals and humans with specific diseases, such as obesity, diabetes, cancer, asthma, cardiovascular and neurodegenerative diseases.
To conclude, the authors provide brief practical information on how to prescribe intermittent fasting.
The proposed schemes include the gradual reduction of the period during which food is allowed, up to a fasting period of 16-18 hours.
An alternative scheme may consist of a 5:2 diet, with a gradual reduction in the calories consumed, from 1,000 to 500, for 1 and then for 2 days a week.
On the other hand, the authors observe that most doctors are not trained to prescribe intermittent fasting and that a dietitian or nutritionist should be consulted. They conclude by specifying that it has yet to be clarified whether people are able to maintain an intermittent fasting for a long time and hope for the future the development of pharmacologic interventions able of inducing the favorable effects of this diet without changing eating habits.
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