From the beginning of history, humans used to hunt or collect their food, meaning that there were hours or even days that they didn’t eat anything at all. Since then, the human brain has developed significantly and we are now in a position to eat anytime we choose because there is available food. Contrary to our ancestors, we usually stop eating only when we sleep. Therefore, we spend more than half of the day in a feeding state which contributes to the obesity epidemic. Nowadays, several diets suggest intermittent fasting for a few hours up to a day or more. Additionally, there are cultures and religions around the world who use to fast for hours during some periods of the year.
What is fasting?
As mentioned previously, fasting is not a scientific finding, but a necessity for our ancestors who used to fast intermittently for hours until they find some food. Intermittent fasting means ingesting no food or caloric beverages for a period of 1-3 days/ week. Intermittent fasting allows for free food consumption during the rest of the days. It is important to clarify that intermittent fasting is different than caloric restriction in which you reduce the caloric intake by 20–40% but you are still eating regularly. (1)
Mechanism of fasting
During intermittent fasting after your body burns the energy stored in cells by the carbohydrates eaten before, it starts to break down fatty acids to get the energy needed for its normal functions. This results in ketone bodies production, which can be used as fuels. Intermittent fasting promotes changes in metabolic pathways and cellular processes such as stress resistance and lipolysis. In other words, the body uses fats for energy during intermittent fasting, resulting to reduction of adipose tissue. (2)
Specifically, intermittent fasting influences metabolic regulation by effecting the circadian biology (organisms evolved to restrict their activity to the night or day by developing an endogenous circadian clock to ensure that physiological processes are performed at the optimal times), the gut bacteria and lifestyle behaviours. (3)
Fasting and health benefits
A review of animal studies on alternate-day fasting, reported that it was as effective as a calorie restriction diet in reducing fasting glucose and insulin levels. Additionally, the alternate-day fasting resulted to reduced triglyceride and total cholesterol levels. The same study reported that in humans the benefits were limited to improvement in triglycerides and an increase in HDL-cholesterol (good cholesterol) levels. (4)
In a study published in 2013, researches tried to evaluate the impact of fasting on long and short-term health of healthy volunteers. According to the results of the study, one day fasting intervention resulted to increased human growth hormone, decreased triglycerides, an increase in HDL-cholesterol (good cholesterol), and decreased weight compared to a day of usual eating. (5)
A review article (2016) compared the efficacy of alternate-day-fasting and very-low-calorie dieting. Researchers concluded that alternate-day-fasting is an effective weight-loss method, and may be superior to a very-low-calorie diet. This can be explained by the fact that fasting is easier to adopt and results to greater fat-mass loss and preservation of fat-free mass. (6)
Another recent study (2016) compared the effects of alternate-day fasting to a standard weight loss diet. In their results, scientists concluded that alternate-day fasting is a safe and tolerable way to lose weight. Moreover, fasting resulted to similar changes in body composition and did not increase the risk for weight regain during the 6- month follow-up period after the intervention. (7)
A 2016 review summarizes some of the health benefits caused by intermittent fasting: reduced inflammation, increased resistance of the heart and brain to stress, increased resistance to diabetes, reduced body fat and blood pressure. Additionally, it presents that intermittent fasting is promising and may potentially delay the onset and progression of diseases such as Alzheimer and Parkinson. (8)
Drawbacks of intermittent fasting
Intermittent fasting may cause some “side effects” like dehydration, headaches, weakness, fainting, nervousness, cravings and hunger pangs. Excessive fasting, more than 2 days/week could lead to malnutrition, nutrient deficiencies, eating disorders, weakening of the immune system and potential damage to organs. (9) In healthy, overweight or obese individuals there is little evidence showing that intermittent fasting results to physical or mental disorders. (3)
Intermittent fasting is not for diabetics because they might experience hypoglycemia. Moreover, the production of ketones is a high- risk process for diabetics which might deteriorate their condition. Additionally, fasting is not allowed for a pregnant or a breastfeeding woman because they have high energy daily demands. Furthermore, children have high metabolic rate and increased energy demands, meaning that fasting would affect their normal development.
On the top of the above, fasting regimens have not been performed in children, elders or underweight individuals, and it is possible that it might be harmful to these populations. Intermittent fasting periods lasting longer than 24 hours and especially those lasting over 3 days should be avoided and done only under the supervision of a physician and preferably in a clinic. (2)
To sum up, fasting has no direct financial costs and it reduces food expenses. Although intermittent fasting has its roots in the ancient civilizations and cultures, it was recently that fasting became a trend and attracted scientific interest. Intermittent fasting requires much of dedication and discipline. Even though, research on intermittent fasting is not that developed, recent findings are very promising. The more solid data about intermittent fasting and health benefits, are related to the cardiovascular health and weight-loss. Specifically, intermittent fasting has been shown to help individuals reduce the risk of the metabolic syndrome, reduce their body weight and fat mass significantly, lower their blood pressure, lower triglycerides, total cholesterol and bad cholesterol levels. There are some groups of people who should avoid intermittent fasting, but healthy individuals or overweight-obese people can try intermittent fasting as an efficient method to lose weight. (10) Besides the body weight reduction, the rest of the benefits caused by intermittent fasting are very important and if current findings will be supported by further research, it is sure that intermittent fasting will attract even more attention in the future.
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