Intermittent fasting, an eating pattern that fluctuates between periods of eating and fasting, has rapidly become one of the most popular health and fitness trends for the many benefits it offers. Recently, researchers from the University of Illinois at Chicago (UIC), after a comprehensive review of various studies, have concluded that following an intermittent fasting diet can produce clinically significant weight loss as well as improve metabolic health in individuals with obesity.
The study published in the Annual Review of Nutrition set out with a goal to dispel some myths regarding intermittent fasting, by reviewing several studies that have been conducted on three particular types of intermittent fasting. They found that following an intermittent fasting diet, does not negatively affect metabolism, nor does it cause disordered eating.
"We noted that intermittent fasting is not better than regular dieting; both produce the same amount of weight loss and similar changes in blood pressure, cholesterol and inflammation," said Prof Krista Varady, at the UIC College of Applied Health Sciences and author of Cardiometabolic Benefits of Intermittent Fasting.
For this review, researchers looked at over 25 studies involving three types of intermittent fasting—alternate day fasting which, typically, involves a feast day alternated with a fast day, where 500 calories are consumed in one meal; a 5:2 diet which is a modified version of alternate day fasting that involves five feast days and two fast days per week; and time-restricted eating which confines eating to a specified number of hours per day, usually 4 to 10 hours, with no calorie restrictions during the eating period.
All forms of fasting that the researchers reviewed, produced mild to moderate weight loss (1%-8% from the baseline weight) which represent results that are similar to that of more traditional, calorie restrictive diets. Intermittent fasting diets may also benefit health by decreasing blood pressure and insulin resistance and, in some cases, cholesterol and triglyceride levels are also lowered. Researchers were also able to demonstrate other health benefits, such as improved appetite regulation and positive changes in the gut micro-biome, through their study.
Among the studies reviewed on alternate day fasting, the results showed a weight loss of 3%-8% of body weight over three to eight weeks, with results peaking at 12 weeks. According to the researchers, individuals who follow alternate day fasting, typically, do not overeat or binge on feast days, which results in mild to moderate weight loss.
Surprisingly, studies on the 5:2 diet showed similar results to alternate day fasting. Followers of the 5:2 diet fast much less frequently than those who follow alternate day fasting, but the results of weight loss are similar. Weight loss in both, alternate day and 5:2 fasting, are comparable to more traditional daily calorie-restrictive diets. Ultimately, both fasting diets showed that individuals were able to maintain an average of 7% weight loss for a year.
Studies done on time-restricted eating, on the other hand, have shown participants with obesity losing an average of 3% of their body weight, regardless of the time of the eating window.
"You're fooling your body into eating a little bit less and that's why people are losing weight. Fasting people are worried about feeling lethargic and not being able to concentrate. Even though you’re not eating, it won’t affect your energy. A lot of people experience a boost of energy on fasting days. Don’t worry, you won’t feel crappy. You may even feel better," said Prof Varady.
For those who may want to try intermittent fasting, the study includes a summary of practical considerations that takes into account factors of adjustment time, exercise, diet during fasting and consumption of alcohol and caffeine.
There can be certain side-effects during the first one or two weeks of fasting, such as headaches, dizziness and constipation. The researchers suggest increased water intake to help alleviate headaches caused by dehydration. Some study participants have reported having more energy on fast days; but researchers recommend moderate to high-intensity endurance or resistance training during fasting periods. They also recommend that those who follow alternate day fasting should eat their fasting day meal after exercise.
While there are no specific recommendations for food consumption during intermittent fasting, eating fruits, vegetables and whole grains can help boost fibre intake and also relieve constipation. For those following an alternate day or 5:2 fasting regimen, alcohol is not recommended on fast days as the limited calories should be used on healthy that provide nutrition.
The researchers also list several groups of people who should not follow an intermittent fasting diet—pregnant or lactating women, children under the age of 12, those with a history of disordered eating, those with a body mass index (BMI) less than 18.5, shift workers and those who need to take medication with food at fixed times.
"People love intermittent fasting because it's easy. People need to find diets that they can stick to long term. It's definitely effective for weight loss and it's gained popularity because there are no special foods or apps necessary. You can also combine it with other diets, like Keto," explained Prof Varady.
Prof Varady has recently been awarded a National Institutes of Health grant to study time-restricted eating for 12 months in order to observe and analyse its effects over a longer period.