What is Intermittent Fasting? 

Intermittent Fasting Schedules

Adapted from Patterson, 2015.

How Does Intermittent Fasting Work for Weight Loss?

Typically, when a person wants to lose weight, they will reduce the number of calories below their normal daily intake over time. For example, a rule of thumb is to reduce your daily calorie intake by 500 to lose one pound in a week, since one pound is equal to ~4500 calories.  This is called continuous caloric restriction (CCR) and is the ‘traditional’ weight loss diet. Studies have shown that CCR regimes not only reduce body weight they may also help prevent obesity-related diseases [ NIA 2018, Schübel 2018].  It is important to note that CCR does not cause malnutrition or deprivation of essential nutrients [NIA 2018].   A fasting diet or intermittent caloric restriction (ICR) is an eating regimen where a person does not eat at all or severely limits intake during a certain time frame (e.g, day, week, month) [NIA 2018].  Over time on a fasting diet, a person may consume fewer calories thereby leading to weight loss.

Decreased calorie intake, whether it’s done through CCR or ICR, leads to weight loss. This might seem like a no-brainer, but research is starting to show that not only do fasting regimes lead to weight loss, but they may have greater metabolic benefits including lower insulin levels and decreased fat mass compared to traditional dieting [Harvie 2011, 2013].

Intermittent Fasting Benefits

A recent systematic review of four intermittent fasting studies reported a decrease in fat mass and reductions in both low-density lipoprotein (LDL) and triglyceride levels, suggesting improvements in cardiovascular health [Ganesan 2018].  Additional studies are also suggesting that these fasting regimens help lead to weight loss with potential effects on glucose metabolism and inflammation. These are also benefits shown by traditional weight loss diets, suggesting intermittent fasting may have similar benefits. As more studies on intermittent fasting are conducted, systematic reviews continue to support these cardiometabolic benefits

Like other intermittent fasting regimens, time restricted eating is also associated with weight loss and may be important for optimal metabolic function [Patterson 2015].  Why researchers are observing these metabolic benefits with ICR is still unknown.  More research will help to identify the mechanisms behind the benefits.

Fasting is a major component of Ramadan, and a review of studies focusing on the effects of fasting during Ramadan reported weight loss and lower levels of low-density lipoprotein cholesterol and fasting blood glucose in both men and women when compared to the beginning of Ramadan [Kul 2013]. Other studies have also reported that Ramadan fasts are associated with lower levels of inflammatory molecules in the blood stream (e.g. C-reactive protein, interleukin-6, tumor necrosis factor-alpha; Aksungar 2007, Faris 2012).  However, these types of molecules are produced normally by our bodies as part of metabolizing the food we eat, so it may make sense that restricting food would lead to lower measurements. The more important question, and what is still unknown, is whether these changes are permanent or if they go away when people resume their normal eating habits.

Which one is better, continuous calorie restriction (CCR) or intermittent fasting?

To date, most systematic reviews of intermittent fasting studies have suggested that intermittent fasting is equal to standard CCR regimens when it comes to weight loss [Patterson 2015]. More recently in 2018, Shubel et al. showed that the 5:2 diet is equivalent, but not superior, to traditional CCR diets for weight loss and the prevention of metabolic diseases. A systematic review from 2021 comparing intermittent fasting diets versus CCR diets found that intermittent fasting diets were associated with higher weight loss in four out of 18 studies. At this point, it appears that both dietary methods of weight loss have similar effects.

How does fasting help improve health?

While health improvements from fasting might sound counter-intuitive, science is starting to show there might be something to it.  The exact mechanisms of action are still unknown, but to date a few hypotheses have been proposed to explain how intermittent fasting can benefit overall health [Patterson 2015]. Recent studies have suggested there is a possible link between the microbiome and these cardiometabolic intermittent fasting benefits.

These hypotheses include:

1. Improvement in the gene expression associated with the circadian rhythm to favor energy metabolism and body weight regulation. In other words, fasting regimens may impose a daytime rhythm in food intake, leading to improved energy metabolism and body weight regulation.
2. Enhancement of beneficial colonies of gastrointestinal microbiota which may be more favorable to weight loss.
3. Modification of lifestyle behaviors (e.g. energy intake, energy expenditure, sleep) may accompany fasting behaviors.

Should you fast or not?

So, does intermittent fasting work? The short answer is yes, it is at least equally effective as traditional weight loss methods. Every year more studies are reporting benefits beyond weight loss with intermittent fasting regimens.  It is still too early to say that one fasting regimen is better than the other, but studies are showing at the very least these eating patterns lead to weight loss and could even improve overall health.  The questions that remains to be answered is whether or not these regimens are sustainable over a lifetime in adults as well as whether these regimens help reduce risk of diseases.  So, if you are considering trying an intermittent fast, be sure you speak with a health professional to ensure that you are getting adequate macro- and micro-nutrients in your diet.