IF refers to extended periods of time, typically 12 to 48 hours, in which individuals consume little to no energy followed by periods of normal food intake on a reoccurring basis (1). Supporters of this practice boast that IF can help significantly help improve health status by regulating insulin levels and by improving functional outcomes for several disease states including diabetes, heart disease, cancers and neurodegenerative diseases.
Many contend that we are built to practice IF due to our history of hunting and gathering. Many years ago, we would have to thrive off of the food we hunted, forcing individuals to go several hours, or sometimes days, without food. While this is not the case for most humans today, energy intake has swung to the opposite extreme, with obesity, diabetes and heart disease constantly on the rise. Thus, IF could be helpful to aid individuals in regulating glucose and insulin levels by providing a regimen that allows for blood sugar regulation to occur.
IF is also present in ancient Chinese, Greek and Roman literature and scientific studies have been cited using fasting as early as 1914 to treat patients with type I and II diabetes (2), suggesting that although “intermittent fasting” is a current buzzword, the practices of fasting are far from infancy. Fasting practices are also still commonly practiced within many religions including those who are Muslim, Christian and Jewish.
The science behind why IF helps to improve health status involves the switch from utilizing glucose to utilizing fatty acids/ketones for energy. Once the glucose is depleted from the liver, fatty acids are taken from the adipose tissue in the body, resulting in fat loss. This shift typically occurs after 12-36 hours without food, varying for each individual. It is important to note that lean muscle mass preservation is still possible with IF, as the fatty acids are being utilized from fat cells, preserving muscle tissue. IF can still be practiced by athletes or those who engage in daily physical activity, as long as one feels that they have the energy to exercise. If you are someone who needs to eat before working out, it may be better to exercise in the evenings or during your time window in which eating occurs.
Typically, those who consume three or more meals each day will unlikely switch over to using fatty acids, keeping their levels of ketones consistently low. In addition, as people gain extra weight and/or develop insulin resistance, it becomes harder to achieve a level of ketosis if and when diet modifications are made. Simply put- the benefits of IF are more difficult to achieve with the more weight that one has to lose.
Current research indicates that, generally speaking, IF can be beneficial for improving cardio-metabolic risk factors, reducing weight, and improving fatty acid/cholesterol levels in the blood (2). IF can also improve cognition through increasing neurotrophic factors and mitochondrial biogenesis, and it can reduce inflammation in the intestines, blood and other surrounding organs by reducing pro-inflammatory cytokines and other markers of oxidative stress (2). In essence, the practice of IF helps to promote a sense of calm for the body, reducing levels of stress.
Research also indicates that fasting practices have the ability to slow aging and the development of disease, as fasting helps to prevent oxidative damage to proteins, DNA and lipids, reduces inflammation, starves dysfunctional proteins, and lowers glucose and insulin levels. By reducing circulating levels of insulin-like growth factor 1 (IGF-1), the process of aging is delayed. This is been shown to be more sustainable than other caloric restriction practices for many individuals, as the benefits of fasting can be obtained with a relatively minimal decrease in overall caloric intake given the leniency of the refeeding period (3). Thus, IF helps to slow the processes that commonly lead to aging, promoting overall longevity.
Some of the strongest support for IF lies in its ability to reduce inflammation. Research shows that for patients with inflammatory diseases, such as rheumatoid arthritis (RA), fasting from one to three weeks significantly reduced inflammation and pain. Once patients resumed a normal diet, inflammation and pain returned unless patients continued to follow a vegetarian diet, in which noticeable improvements were seen over the course of two years (3). Thus, by reducing inflammation via diet, quality of life measures improved.
IF and other fasting practices may not be for everyone and it is important to consult your local physician and/or dietitian before beginning a new dietary practice. Fasting is likely not recommended for those under the age of eighteen, pregnant women, those with certain medical conditions and/or the elderly.