Intermittent fasting, what is it exactly? Intermittent fasting is eating within a specified time window, and not eating the rest of the time. The hours of sleep at night count towards the hours of fasting. So let's say I'm doing a 16-8 fasting. That means I'm fasting for 16 hours and eating in an 8-hour window of time. Let's say I eat dinner a 6 PM, and then I don't eat anything until the next day at 10AM. That is a 16/8 fast. There are many combinations of the hours you can fast. You can do a 20/4 where you fast for 20 hours and eat in a four hour window and so on.
Intermittent fasting has been shown in animal and human studies to be a powerful tool in reversing insulin resistance.
Hormonal changes that occur during fasting are completely different from simple caloric restriction, and insulin is one of those hormones that is largely affected. Recall we talked about insulin resistance, which depends on persistence of high insulin levels. Intermittent fasting assists in the prevention of insulin resistance from developing.
Many isocaloric studies have directly compared daily caloric restriction with intermittent fasting. Isocaloric, meaning the total calories were the same between the two groups. In a study where constant caloric restriction was compared to same diet with intermittent fasting two days per week. In over six months there were important hormonal differences between the two groups. Insulin, the key driver of insulin resistance in obesity, was initially reduced in the caloric restriction group, but soon plateaued. However, in the intermittent fasting group, insulin levels continued to drop sharply. This means insulin sensitivity improves with fasting only. Because insulin resistance is involved in many brain diseases, as discussed before, intermittent fasting will succeed where caloric restriction will not. The intermittency of the diet is what makes it effective. In addition to its effects on insulin, during fasting Norepinephrine increases, which keeps metabolism high. Growth hormone also increases, which maintains muscle mass.
Why does this happen? This actually makes sense from an evolutionary standpoint. If our bodies went into starvation mode every time food was scarce, the human species would be extinct. Instead, intermittent fasting mimics the way our ancestors ate with periods of feasts separated by periods of famine.
That was at the peripheral level, outside of the brain. What happens during fasting from a cognitive standpoint? In an experiment done on animals, they subjected three groups to different interventions. One was subjected to a ketogenic diet, one to intermittent fasting, and one control remained on a regular diet. They then evaluated their memory by evaluating how fast they learned some tasks such as getting to a target box. Intermittent fasting showed significant improvement in memory compared to the regular diet animals, and even to the animals fed a ketogenic diet. There was almost a dose response. Animals fed a normal diet did the worst in terms of tasks of learning. Ketogenic diet animals improved. And intermittent fasting animals did the best.
What happens in humans with intermittent fasting? One study shows improvement in memory scores after intervention. Memory scores were much higher in the intermittent fasting group. The lower the insulin levels, the better the memory. Same thing with inflammatory markers.