Intermittent fasting has been popularized with claims that it’s superior for weight loss and other health measures. A recent study supports this notion, but methodological issues may limit the results.
What did they test? Researchers compared different eating windows between diets and changes in weight loss, body composition, and various other cardiometabolic health markers and psychometrics.
What did they find? Researchers found a significantly greater weight loss in timed eating. However, some important details in the methods may reduce the confidence in these results.
What does it mean for you? Many forms of intermittent fasting exist with many valid rationales, including restricting calories based on preferences and habits. However, there is insufficient evidence to suggest they are superior to other types of calorie restriction.
What’s the Problem?
Fasting has generally been defined as periods ranging from hours to weeks of minimal or no intake of calories from food or beverages 2. For centuries fasting has been a common religious practice for spiritual and physical purposes 3. While this holds today, forms of fasting, like intermittent fasting, have become mainstream diet fads. Celebrities and some medical doctors have fictionalized the benefits of intermittent fasting, driving its popularity among the public. Despite the lack of controlled human trials, this area of research is quickly evolving.
Intermittent fasting refers to alternating feeding periods with periods of no food for longer than the typical overnight fast 4. The many different forms of intermittent fasting is constantly growing, including alternate-day fasting, whole-day fasting, and time-restricted eating (TRE). While many forms exist, they all include little to no food intake alternated with eating windows. The idea behind this is that limiting when you can eat will limit the amount you eat each day. With the rise in fad diet culture, intermittent fasting research has also increased interest from researchers. However, there still isn’t a ton of studies conducted on humans with tightly controlled conditions. Recently there's been a fair share of literature reviews, systematic reviews, and even meta-analyses on intermittent fasting protocols and their influence on various outcomes.
Time-restricted eating (TRE) is one of the many forms of intermittent fasting. TRE involves alternating a period of fasting (no calorie intake) with a period of feeding. The most common iteration of TRE is known as the "Lean Gains Diet," which includes a 16-hour fasting window alternated with an 8-hour feeding window 5. Another common type is known as the "Warrior Diet," which involves a 20-hour fasting period alternated with a 4-hour feeding window 5. There really isn't a lot of research available on time-restricted eating specifically. Still, some of the purported benefits of intermittent fasting include positive changes in body composition and blood biomarkers. A systematic review and meta-analysis on overweight individuals indicated that intermittent fasting is better than no treatment for weight loss but like continuous dieting strategies, not superior 6. However, the included studies investigated various forms of intermittent fasting and not TRE specifically. Some studies show TRE can reduce body weight and improve various blood markers, but none of these studies included a comparative dieting group 7 8 9 10. When looking at studies that control for calorie intake and compare TRE to another dieting group with no feeding window restrictions, the results are inconclusive. Moro et al. (2016) reported a greater decrease in fat mass (FM) and improvements in blood markers compared to continuous dieting 11. Two similar studies reported no difference in body composition between the TRE and continuous dieting groups 12 13. Lastly, Stote et al. (2007) compared one to three meals daily (no control over calorie intake). They reported mixed results for cardiovascular blood markers and no differences between groups in terms of weight loss 14. Together these studies indicate that TRE can reduce body weight and improve various blood biomarkers. However, there is insufficient evidence to suggest TRE is superior when compared to a continuous dieting approach.
Like all forms of fasting protocols, the period of fasting can reduce total daily calorie intake. Obviously, this assumes calories aren't increased during the feeding window to compensate for the decrease during fasting. Many questions whether the benefits of fasting come from the calorie reduction that leads to weight loss during fasting periods or if there's an inherent benefit to periods of food abstinence. The study by Stote et al. (2007) showed that the one-meal group effectively reduced caloric intake by 65 kcal/day, but this didn't lead to weight loss benefits 14. The study by Moro et al. (2016) controlled for calorie intake and feeding windows and found beneficial changes in body composition and blood markers by using a TRE approach 11. This is supported by a recent meta-analysis that shows a slight weight loss advantage from intermittent fasting 15. Wait, what?! Is there a real advantage? Don’t get too excited. Many of these intermittent fasting studies use a control group that is simply 'eating regularly. Meaning they aren't following a calorie deficit, just eating as they please. Often these control groups eat more than the intermittent fasting groups. So, it may not be specifically the intermittent fasting protocol that is driving weight loss. Rather, the calorie deficit! The current study we review supports eating within a specified time window to enhance fat loss. Before the findings are misinterpreted, let’s break down this study step by step or ‘rep by rep’ ba dump tist!