Intermittent Fasting (IF) is defined as a “periodic eating pattern with little or no energy intake for a while following normal eating.” While this term may be unfamiliar, all of us fast every day. We eat dinner and then fast until our first meal the next day, hence the term “break fast”. IF is compelling because studies have shown that it supports weight loss, can be utilized with all dietary preferences, and costs nothing.
There are several mechanisms to indicate that IF may be an effective weight loss tool. A key mechanism for IF in weight loss has to do with the low levels of insulin in the blood when food is not being consumed. Elevated levels of insulin prevent fat burning. When we eat food, insulin is released to shuttle glucose into the cells to be used for energy production. Excess glucose is stored in the liver and muscles as glycogen until 34 full, and then stored as fat, resulting in overall weight gain. In his book The Complete Guide to Fasting, Dr. Jason Fung equates this storage to a two-compartment model. He states that glycogen (stored glucose in the muscles and liver) is like a refrigerator, designed for short-term storage. This refrigerator allows easy access to food (energy) but only has a small storage space. Body fat, he says, is like a basement freezer, designed for long-term storage. It is more difficult to access but has a much higher storage capacity. Insulin acts to block access to the basement freezer.
However, when we are not eating, insulin levels are low, allowing full access to the fat freezer and the body 35 can use the stored fat as energy. Studies show that IF decreases circulating insulin, increases insulin sensitivity, and is effective in reducing obesity.32,36,37
A second mechanism of intermittent fasting is that through the regular practice of IF, the body will more easily switch from using glucose for energy to using fat stores, putting the body into a state of ketosis. Nutritional ketosis has been shown to improve metabolic and inflammatory markers, insulin and glucose levels, and aid in weight management. This metabolic flexibility to switch between obtaining energy from glycolysis (the breakdown of glucose) and ketosis, increases fat oxidation during periods of fasting.39
A third mechanism of IF is its potential to remodel the gut microbiome. There is evidence that IF can transform intestinal microbiota and reduce the risk of obesity.40 One study states that IF promotes significant reshaping of the gut microbiome.41 In addition, studies show that the positive effects of fasting on the gut microbiome reduce gut permeability and systemic inflammation, both of which support healthy weight management.
A fourth mechanism for IF is the potential for increased alignment of food consumption with the body’s circadian clock. The effectiveness of this mechanism presumes the feeding window begins early and ends before dark to align with our natural circadian rhythms.43 Food timing that is out of sync with light and dark cues could lead to higher caloric intake due to impaired satiety signaling.42,44
Several studies support a fasting schedule of 16 hours fasting with a feeding window of 8 hours (16:8) for weight loss, which seems to induce many health benefits compared to eating the usual pattern of three meals a day plus snacks over an extendedtime period.42 A study in the journal Cell Metabolism reported that the 16:8 method of daily fasting resulted in weight loss and reduced abdominal fat.Further, fasting daily, compared to other forms of fasting, has the added benefit of matching food intake timing more closely to our circadian clock.46 Last, daily fasting may be easier to follow because it follows a daily routine and the majority of the fasting time occurs during sleep. Although more research needs to be done comparing the various types of fasting, the 16:8 schedule has shown evidence of being a good choice for weight loss.
Due to the flexibility of intermittent fasting, and the zero cost, it is a worthy weight loss tool. A feeding window that can be kept relatively the same each day helps to build a daily, sustainable habit.47 There are people who should not fast, including those who have an eating disorder.Although fasting is something everyone does daily after their last meal until breakfast, being intentional about the timing of one’s fasting and feeding windows, and avoiding snacking, will maximize the weight loss benefits of intermittent fasting.
