There’s growing evidence that intermittent fasting and conventional calorie restriction, or daily calorie-cutting diets, have similar impacts on weight loss and other metabolic indicators. But this isn’t discouraging news, despite the nature of many recent media headlines. In fact, the news that intermittent calorie restriction has similar weight loss and metabolic health impacts as continuous calorie restriction is good news for many. Many individuals find that intermittent fasting or restricting their calories on a limited number of “fasting days” per week is easier than depriving themselves of 25% of their normal caloric intake on a daily basis.
In a study published this month in the American Journal of Clinical Nutrition by researchers from the German Cancer Research Center, a 5:2 intermittent calorie restriction intervention had equivalent but not superior impacts on weight and metabolic disease indicators as continuous calorie restriction. The results are based on a clinical trial that enrolled 150 participants.
In this randomized trial, a 5:2 intervention did not exert stronger effects on obesity, metabolic disease and inflammation biomarkers than did continuous calorie restriction over an intervention period of 12 weeks. The 5:2 intermittent fasters lost marginally more weight during the intervention phase of the study (12 weeks), but by week 50 had experienced roughly the same amount of weight loss as individuals who restricted their calories on a daily basis.
“All study groups experienced reductions in fasting serum lipids (LDL, HDL, cholesterol, triglycerides), insulin concentrations, HOMA-IR levels [a measure of insulin resistance], adipokines (adiponectin, leptin), liver function parameters (gamma-glutamyl transpeptidase, alanine transaminase, aspartame transaminase), and brain-derived neurotropic factor, and increases in resistin and IGF-1 concentrations, without significant between-group differences.” – Schübel et al., 2018
The good news is that the 5:2 intermittent calorie restriction intervention was well-tolerated and resulted in up to 7% relative change in body weight over the 12-week intervention phase of the study. Participants in the 5:2 diet group were coached on how to restrict their calories such that on “fasting” days, they were eating only a quarter of the calories they should normally eat given their individual energy requirement. In contrast, individuals in the continuous calorie restriction intervention group were coached on how to restrict their calories on a daily basis to 80% of their individual energy requirement.
“For calorie-restricted days, detailed personalized meal plans were created, in which possible choices for meal components arranged by food groups were given; participants had to select 4 food items out of the vegetable group, 2 out of the low-fat dairy product group, and 1 food item out of each of the meat/fish, carbohydrate, and fruits groups, in combination with a minimum intake of 2 L of low-energy drinks.” – Schübel et al., 2018
While this study offers a new window into the impacts of the popular 5:2 diet, it has limitations when it comes to unveiling the potential impacts of intermittent fasting. The researchers did not measure or did not report measurements of ketone levels or the duration of no-calorie fasting intervals for individuals in the intermittent calorie restriction group. While the 5:2 diet is one approach to intermittent fasting, other approaches including time-restricted feeding (such as the 16:8 schedule) and occasional prolonged (e.g. 24 to 36-hour) water fasting may exert different impacts on the body. In particular, other types of intermittent fasting that push the body into the metabolic state of ketosis or prompt metabolic switching could have greater impacts on insulin sensitivity and mTOR / insulin signaling activity.
For example, previous trials conducted by Michelle Harvie and colleagues have resulted in similar weight loss impacts but greater decreases in insulin concentrations, biomarkers of insulin resistance and fat mass for individuals practicing two consecutive days of intermittent calorie restriction as opposed to continuous calorie restriction. In another study, Dr. Krista Varady found that although alternate day fasting produced similar weight loss results as continuous calorie restriction over time, some individuals with pre-diabetes experienced “bigger improvements in insulin resistance, inflammatory factors, and triglyceride levels with alternate-day fasting.” These results could be a function of longer fasting durations, the depletion of glycogen and the increased production of ketones (which have been shown to be beneficial for brain health) with longer fasts, and the impacts of intermittent metabolic switching.
“During fasting, the body uses up glucose and glycogen, then turns to energy reserves stored in fat. This stored energy is released in the form of chemicals called ketones. These chemicals help cells—especially brain cells—keep working at full capacity. Some researchers think that because ketones are a more efficient energy source than glucose, they may protect against aging-related decline in the central nervous system that might cause dementia and other disorders. […] In addition, studies show that ketones may help protect against inflammatory diseases such as arthritis. Ketones also reduce the level of insulin in the blood, which could protect against type 2 diabetes.” – National Institute on Aging
The American Journal of Clinical Nutrition study complements previous findings that 5:2 and alternate day fasting interventions that involve intermittent calorie restriction are not superior to continuous calorie restriction in most respects, including impact on weight. However, much remains to be studied on the impacts of other intermittent fasting schedules, particularly time-restricted eating and extended water fasting. For now, if you find it easier to stick to diets where you restrict your calories to a greater extent for two days a week instead of every day, science says you’ll probably still enjoy similar weight loss benefits. So go ahead!
Learn more about intermittent fasting with our fasting FAQ with Dr. Krista Varady and the LIFE Fasting Tracker app.
