In 2016, the World Health Organization (WHO) warned that globally, 39% of adults were overweight and 13% were obese, i.e. 1.9 billion overweight adults, including 650 million obese worldwide. What's more, in 2019, 38 million children under the age of 5 were overweight or obese... Scientific literature has recognized for many years that obesity is a major risk factor in many chronic non-transmissible diseases, such as cardiovascular disease, cancer, metabolic syndrome and type 2 diabetes. To combat this, recognized guidelines for obesity management have been introduced, focusing on lifestyle interventions such as regular exercise and restriction of dietary calorie intake. These measures are primarily aimed at reducing body mass, using body mass index (BMI) as a benchmark, based on the WHO-defined thresholds for overweight (BMI greater than or equal to 25) and obesity (BMI greater than or equal to 30).
However, while BMI correlates with body fat percentage when adjusted for variables such as age, sex, and ethnicity, it does not fully represent the risk of cardiometabolic disease. In fact, visceral fat, i.e. the fat surrounding our internal organs, presents a much higher cardiometabolic risk than subcutaneous fat. Although BMI is a common measure, scientific literature shows that visceral fat should be considered an independent risk factor for cardiovascular and metabolic diseases. This suggests that for effective obesity management, the focus should be on reducing visceral fat rather than BMI.
Several studies have attempted to compare the effects of exercise and calorie restriction on visceral fat reduction. Some studies have suggested that exercise leads to a preferential reduction in visceral fat compared with calorie restriction. However, physiological and metabolic adaptations to exercise and caloric restriction are fundamentally different. These differences may also reflect distinct responses to visceral fat reduction. Yet the independent effects of exercise and caloric restriction on visceral fat, relative to eucaloric conditions (calories ingested equal calories expended) and taking into account the weekly energy deficit, remain unknown. So how do exercise and caloric restriction affect visceral fat reduction in overweight and obese people?
To answer this question, a team of Hong Kong researchers carried out a meta-analysis to determine and compare the dose-response effects of exercise and calorie restriction on visceral adipose tissue in overweight and obese adults, while controlling for the weekly energy deficit induced by the interventions.
To do this, the researchers analyzed the results of 36 randomized controlled studies involving 2190 people and comparing exercise (n=983) or calorie restriction (n=394) with eucaloric control groups (n=813) in overweight or obese adults. The majority of studies were conducted in the USA (n=15), Asia (n=11) and Europe (n=9). Eight studies included people with co-morbidities such as type 2 diabetes, metabolic syndrome, dyslipidemia and non-alcoholic fatty liver. Exercise interventions ranged from 4 weeks to 2 years, while calorie restriction interventions ranged from 12 weeks to 1 year. Half the studies measured visceral fat using MRI and the other half using CT.
The main results of this study show that both exercise and caloric restriction can effectively reduce visceral fat in individuals, but only exercise showed a dose-dependent relationship with its reduction. This means that the more you exercise, the greater the reduction in visceral fat. On the other hand, both interventions had dose-response effects on waist circumference reduction.
Although some studies showed that exercise could lead to greater reductions in visceral fat than calorie restriction, others found no significant difference between the two. Factors such as inaccurate reporting of caloric intake or offsetting the energy expenditure created by exercise with additional food intake could have influenced the results of these studies. Overall, exercise appears to have a more pronounced dose-dependent effect on visceral fat reduction than calorie restriction, although further research is needed to confirm these findings.
Yet this study also revealed a dose-dependent effect on waist circumference for both exercise and calorie restriction. Interestingly, although waist circumference is closely related to visceral fat, the main analyses, taking into account direct MRI or CT measurements, did not show a similar dose-response effect of calorie restriction on visceral fat itself. Several studies have noted that, while there is obviously a link between visceral fat and waist circumference, it is not possible to accurately estimate visceral fat on the basis of waist circumference alone. The correlation between visceral fat and a measure such as waist circumference can vary considerably from one individual to another, particularly between different age and sex groups.
Exercise and caloric restriction promote weight loss by creating a negative energy balance, either by increasing energy expenditure or reducing caloric intake, respectively. Some studies have shown that low-calorie diets can be superior to exercise for weight loss. However, it's important to bear in mind that during calorie restriction, both fat mass and muscle mass are impacted, and this is not always taken into account in these studies. Exercise, on the other hand, can stimulate fat loss while helping to maintain muscle mass.
Metabolic adaptations to a hypocaloric diet may vary from one individual to another, despite similar increases in fat oxidation rates. Some of these metabolic changes may reduce the impact of calorie restriction on visceral fat. Knowing the role of muscle mass in regulating resting energy expenditure, this could explain the results of this study, which suggest that exercise may be more appropriate than caloric restriction for visceral fat loss in overweight and obese people. All that being said, keep in mind that the combination of physical exercise and a diet adapted in quality and quantity is still essential for effective long-term body recomposition.
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