Diet vs. Exercise: Which Is Superior for Weight Loss?

November 9, 2018

By Kurtis Carlock, MD

Peer Reviewed

Obesity is a problem that affects over one-third of adults in the US and increases the risk of numerous health problems in these individuals.[1] The question of how best to combat obesity is a problem that has interested physicians and scientists since it was first recognized as a major health issue decades ago. Presently, there are two main aspects of patients’ lives that are targeted to effect weight loss: dietary caloric intake and caloric expenditure via exercise. But is either of these strategies superior in actually helping patients lose weight?

There are health benefits to both dietary restriction and increased exercise. Dietary changes motivated by a desire to lose weight often lead to reduced intake of simple carbohydrates and saturated fats, foods with established adverse health effects. As for exercise, increasing one’s daily activity is associated with many positive cardiovascular, musculoskeletal, and mental health benefits. With regard to weight loss, however, the effectiveness of the two strategies may not be equivalent.

To lose weight, a person’s energy intake from food must be less than their total energy expenditure (TEE). As described by Carneiro and colleagues, TEE is composed of the resting energy expenditure (the basal metabolic rate or BMR), the activity energy expenditure (calories burned by movement and exercise), and diet-induced thermogenesis (energy consumed as part of food processing).2 In most people, diet-induced thermogenesis accounts for 8-15% of TEE, while resting energy expenditure is more variable and accounts for the majority (50-75%) of TEE. Activity energy expenditure accounts for the remainder of TEE, and is the target of exercise-based weight loss strategies.[2,3]

The theory behind diet or exercise as a tool for weight loss is simple: either decrease the energy intake or increase the activity energy expenditure to a point where total energy expenditure exceeds energy intake. Multiple studies have shown, however, that a phenomenon termed metabolic compensation complicates this process. In a study by Byrne and colleagues, obese patients underwent a diet-plus-exercise intervention for weight loss in which both energy intake and TEE were closely monitored.[4] Even with meticulous supervision of these subjects’ energy balance, weight loss was disappointing: only 33% of what was originally predicted. This discrepancy was caused by unexpected decreases in the subjects’ BMR. In response to the mismatch in energy balance created by the diet-plus-exercise intervention (energy expenditure >intake), the subjects reduced their BMR to partially correct the mismatch. Metabolic compensation may be an evolutionary mechanism to defend against weight loss.

While metabolic compensation complicates both diet- and exercise-based weight loss plans, the phenomenon is more pronounced in patients who use exercise-based methods. A meta-analysis of 28 weight-loss studies by Duhurandhar and colleagues demonstrated that dietary restriction and exercise techniques result in 12-44% and 55-64% less weight loss than expected, respectively.[5] Patients employing exercise-based weight loss techniques have a greater drop in their BMR than patients utilizing dietary restriction. Lower BMR explains why people training for a marathon sometimes gain weight. Exercise-based patients will need to burn more calories through exercise than diet-based patients will need to cut from their diet to achieve the same amount of weight loss.

To provide a clearer picture of the metabolic changes associated with exercise, Myers and colleagues are currently performing a randomized controlled trial (E-MECHANIC) studying the compensatory changes in energy intake, total energy expenditure, and BMR that occur in response to increased exercise.[6] Once complete, this study will yield important information about the underlying mechanisms behind metabolic compensation, and may provide a clearer explanation of the discrepancies described above.

In conclusion, the current data suggest that dietary restriction has significant advantages over exercise as a weight loss tool. A strategy employing both techniques is the most effective for weight loss, and will have the greatest benefit to patients’ overall health. This discussion has focused solely on the metabolic aspects of weight loss strategies, and other factors such as behavioral compensation and patient compliance play a critical role as well. Further research is needed to clarify the utility of various weight loss strategies and, until more clear recommendations are established, weight loss plans should continue to be tailored to each individual patient.

Commentary by Dr. Sapana Shah

With two thirds of Americans currently overweight, the best strategy to shed excess weight is still hotly contested. While the food industry would like to have us believe that the obesity epidemic has to do with a sedentary lifestyle, the added weight can easily be explained by the increase in daily calorie consumption over the decades.[7] Studies have shown that diet trumps exercise when it comes to weight loss strategies and this review elegantly explains the mechanism behind why that is the case.[8] That metabolic compensation thwarts both dietary restriction and exercise strategies but appears to affect exercise more suggests that we may get more bang for our buck discussing dietary choices before addressing exercise strategies.

Dr. Kurtis Carlock is a 4th year medical student at NYU School of Medicine

Peer reviewed by Sapana Shah, MD, MPH Assistant Professor, Dept. of Medicine, NYU School of Medicine

Image courtesy of Wikimedia Commons


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