The increasing rate of obesity has become a growing international health concern. Not only are obese individuals are risk of cardiovascular disease (CVD), diabetes, and cancers, among other conditions, they are also vulnerable to other lethal diseases such as COVID-19. Therefore, physical exercise is paramount for them in the process of regaining their health. However, in what seems like an unfair twist, a new study has stated that long-term exercise can reduce the number of calories burned.
According to an international team of researchers, people who engage in regular exercise, burn lesser calories on body maintenance than those who do not perform any vigorous physical activity. This substantially reduces the calorie-burning gains achieved through exercise. Thus, individuals with higher body mass index (BMI)—who have higher fat levels—may begin burning fewer calories per day in response to a long-term increase in activity. They are also likely to gain weight gain as a result of compensation.
"The process might occur within individuals: as we get fatter, our body might compensate more strongly for the calories burned during activity, making losing fat progressively more difficult," wrote the authors. The research was published in the journal Current Biology.
Calories Burned Throughout the Day
The mechanism of energy expenditure by the body is as complex as the human body itself. Total Energy expenditure (TEE) is the total amount of calories burned by the body in a day. It is divided across different activities and functions performed by the body. One of the simplest ways of breaking TEE down is that it is the sum of Basal energy expenditure (BEE) and Activity energy expenditure (AEE).
BEE refers to the number of calories used by activities such as sleeping, breathing, and diet-induced thermogenesis (digestion, absorption, and conversion), among other metabolic functions. It is dependant or adjusted for factors such as age, sex, height, weight, and level of physical activity among others. AEE is the number of calories expended in activities such as exercise, sports, walking, and other regular pursuits that involve physical exertion.
Decrease in Calories Used
For the study, the authors analyzed data from 1,754 adults in the International Atomic Energy Agency's (IAEA) doubly-labeled water database (DLW). It is a collection of daily energy expenditure data acquired using the stable isotope method of DLW analysis (a technique of estimating TEE). Currently, the database consisted of data from over 20 countries and more than 6500 DLW measurements between 1981 and 2017.
The team's evaluation led to the finding that the calories burned by the body for BEE decrease during periods when daily levels of exercise are consistently high. It was gleaned that the reduction was around 28 percent in people with normal BMIs. This meant that only 72 percent of extra calories that are were burned from extra physical activity translated into extra calories used that day.
However, in the case of individuals with the highest BMI (obese), only 54 percent of calories burned through activity or exercise, translated into calories burned on that day. This is particularly significant as exercise is considered essential along with a balanced diet for weight loss.
"This analysis using data from the DLW database shows how individuals are not all the same in the way they budget their energy use. People living with obesity may be particularly efficient at hanging onto their fat stores, making weight loss difficult," noted Dr. John Speakman, -corresponding author of the study, in a statement.
Impeded by Compensatory Mechanisms
In addition to the decrease in BEE, weight gain was also noted, particularly in those with higher BMI. "When enrolled into exercise programs for weight loss, most people lose a little weight. Some individuals lose lots, but a few unlucky individuals actually gain weight," said Dr. Speakman.
The weight gain could be due to what is known as 'compensatory mechanisms' of the body, the authors suggested. It includes overeating on account of stimulated appetite triggered by exercise, or reduction in expenditure on other areas such as resting metabolism, so that exercise is essentially less expensive.
"But we wanted to find out why some people show such compensatory mechanisms and others don't," explained Dr. Lewis Halsey, lead author of the study. Their examination discerned that two factors dictated the extent of compensation—age and obesity.
The bodies of older people compensate more. And in the case of people with obesity, their bodies reduce their resting metabolism while they are engaging in physical activity. As a result, for every calorie they burn during exercise, around half a calorie is saved on resting.
Need for Revision of Guidelines
Several existing weight loss programs employ additive models to calculate the number of calories that must be burned in order to achieve fat loss. Also, several devices that track physical activity emphasize the number of calories spent in a day. However, the new study challenges the long-held belief that calories expended through exercise readily translate into additional calories burned in a day.
"Around the world, national guidelines tend to recommend a 500–600 calorie deficit through exercising and dieting to lose weight. However, they do not take into account the reduction of calories being burned in the most basic of human functions as the body compensates for the calories burned on the exercise. Not only should these guidelines be revised, but there is also a need for greater personalisation of exercise plans depending upon body mass," concluded Dr. Halsey.
