Fasting has become fashionable lately, and interval or intermittent fasting is currently the most talked about form of fasting. The original Latin meaning of “intermittere” is “to take a leave, in the case of fasting a leave from the traditional rhythm of food intake. The promise made by proponents of interval fasting is that it will reduce cardiovascular risk, diabetes risk, improve the regeneration of the body after psychological or physical stress, improve blood parameters and reduce elevated blood pressure values.
Starting with observations in rodents in 1997, scientists noticed multiple beneficial effects of calorie restriction in mammals, including in humans. While the degree of benefit varies depending on who practices what type of caloric restriction, the evidence for an overall beneficial effect is clear.
How are the beneficial effects achieved? Caloric restriction reduces blood glucose levels and blood pressure. This leads to reduced insulin levels and tumor- necrosis factor alpha. Oxygen consumption is reduced, as are free radicals. As the body considers caloric restriction a stressor and tries to find other sources of nutrition, the cells will utilize their internal “leftovers” by the processes of autophagy and apoptosis. In a matter of speaking, the cells will recycle the least functional parts of themselves, thus clearing the system from unwanted “junk”. On the level of the whole body, after circulating carbohydrates, the fastest available nutrient, are depleted, glycogen will be moved out of its storage in the liver. This takes 24- 48 hours, depending on how big the reserves were. Next, the long-term energy storage, fat cells, are used up. Through hormonal changes, the sympathetic nervous system is downregulated, which reduces blood pressure.
The most attractive form of intermittent fasting is the 16/8 type. Simply put, one can eat as usual for 8 hours daily, while during the remaining 16 hours consciously no food is consumed. During the 8 hours period, food intake should be normal, not hypercaloric, and food choices should be made such that the body receives the necessary micro- and macronutrients.
This kind of fasting is recommended for all adults and should be repeated every 3- 6 months for a duration of one week. It can, without any problems, be done long term if one is so inclined. This type of fasting is easy to incorporate in daily practice, and food remains varied and interesting. It is easiest scheduled by leaving out one meal, either breakfast or dinner. If the other meals are then eaten at their usual times, the 16 hour fasting does not cause any problems. The fasting window may also be widened to 18 hours or more- starting the day with a late lunch and ending with an early dinner. The most extreme form of this is OMAD fasting- “One Meal A Day”.
Some people prefer the 5/2 version. Fasting is limited to two days per week, while food intake is normal on the other five days. Ideally, the fasting days should remain the same week after week and not be directly joined. The fasting period thus becomes approximately 36 hours twice weekly, from dinner on the last normal eating day to breakfast on the first following normal eating day. This type of fasting is particularly good for people with well filled glycogen deposits in the liver, as the body metabolizes first circulating carbohydrates, then the glycogen from the liver, and only then switches to other energy sources- the bodies fat cells- when these are emptied. Depending on overall nutritional status, it takes 24- 48 hours before the glycogen deposits are emptied.
An alternative to this is “alternate day eating”. This is done, as the name implies, by eating normally on one day, then eating nothing the next day, and repeating this pattern. Just as in 5/2 fasting, there are repetitive 36 hours fasting periods, only more frequently. These types of fasting are more likely to give the body enough time to deplete glycogen and start attacking the fat cells, thus leading to more weight reduction and reduction of fat deposits.
Intermittent fasting works by the mechanism described above to reduce body weight, smoothen out glucose spikes and thus the need for reactive insulin. Blood glucose levels may reduce by 12 percent, while insulin levels may go down as much as 53%. Over time, this will also impact the blood cholesterol levels, which are linked through metabolic cycles to carbohydrate metabolism. Total cholesterol, LDL and triglycerides will go down in most people (although up to 20% may see at least an interim worsening of their blood fat metabolism, values). The “happiness hormone” serotonin is mainly produced in the first part of the small intestine, but only when the intestine is not busy digesting. Fasting increases the time between meals, so the small intestine is less busy and has more time to produce serotonin. Another hormone that is secreted more is growth hormone. As a result, muscle mass can be built up more effectively while fat is reduced- but this mechanism works only if fasting is combined with strength training. The benefit of intermittent fasting in that aspect is that it is long enough for carbohydrates and fats to be metabolized, but the body receives new nutrients before it needs to attack its own proteins. More muscles mean better and more efficient glucose metabolism with reduced insulin resistance, leading to a beneficial feedback loop. With reduced body weight and insulin levels, blood pressure will indirectly also go down.
The effect of weight loss on blood pressure is significant: 1 kg weight loss typically reduces systolic BP by 1-2 mm Hg. Five to ten kg weight loss are thus equal to the effect of a standard anti-hypertensive medicine!
Reference for the scientifically minded:
de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019 Dec 26;381(26):2541-2551. doi: 10.1056/NEJMra1905136.