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Obesity: Exercising, not dieting, may be key to avoiding health risks

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New research suggests that fitness may be more important than weight loss for staving off the health risks associated with obesity. Alexander Köpke/EyeEm/Getty Images
  • Guidelines for obesity management tend to focus on weight loss through calorie restriction and increased physical activity.
  • Weight loss is often difficult to sustain, and repeated weight loss attempts are associated with adverse health outcomes.
  • A recent review shows that increased physical activity and improvements in fitness levels can reduce the risk of obesity-related health conditions and mortality, even in the absence of weight loss.
  • An obesity management approach that primarily focuses on improving fitness rather than weight loss may be at least as effective as weight loss in reducing the adverse health consequences of obesity.

According to the Centers for Disease Control and Prevention (CDC), 42.4% of adults in the United States were obese in 2017–2018, a marked increase from 30.5% in 1999–2000.

An increase in adults attempting to lose weight has mirrored this surge in obesity rates. However, the restriction in calorie intake that is necessary for weight loss can be difficult to sustain over a prolonged period.

Moreover, many individuals are unable to achieve their target weight, while those who manage to often struggle to maintain it.

Both of these situations can lead to frustration and reduced adherence to the weight loss program, eventually resulting in a cycle of gaining and losing weight. This fluctuation in weight is known as weight cycling, and it is associated with adverse health outcomes.

The rapid rise in obesity rates despite the increased focus on reducing weight reflects the limits of this weight loss-centric approach to obesity management. Yet, guidelines for obesity management continue to advocate the practices of limiting calorie intake and increasing physical activity levels.

Over the past 2 decades, some scientists have argued that an approach focused on weight loss may be misplaced when it comes to obesity management. Instead, they suggest that a “fat-but-fit” approach based on increasing physical activity levels and improving cardiorespiratory fitness should be the primary focus for those looking to lose excess body weight.

Advocates of the fat-but-fit approach to obesity treatment argue that improving fitness, even in the absence of weight loss, can help reduce the risk of cardiovascular disease and mortality.

While physical activity refers to any movement that results in energy expenditure, cardiorespiratory fitness is a measure of overall physical fitness. Cardiorespiratory fitness measures the ability of the cardiovascular and respiratory systems to sustain physical activity over an extended period.

Now, a recent review that features in the journal iScience looks at how to reduce the risk of obesity-related health conditions and mortality. It compares the effectiveness of physical activity and cardiorespiratory fitness with that of weight loss.

The evidence that the review presents suggests that the fat-but-fit approach may be at least as effective as weight loss in reducing the risk of cardiovascular disease and mortality due to obesity. Furthermore, this fitness-focused strategy avoids the pitfalls of the weight loss approach.

Doctors classify individuals as having overweight or obesity based on their body mass index (BMI), which is the ratio of a person’s weight to the square of their height.

According to the standard classifications, people with a BMI greater than 25 kilograms per square meter (kg/m2) but less than 30 kg/m2 are overweight, whereas those with a BMI of 30 kg/m2 or more have obesity.

The authors of the current study note that although some studies show that individuals with a BMI of more than 25 kg/m2 are at increased risk of mortality, others do not support these results.

Some of the aforementioned studies suggest that individuals with a BMI in the overweight range may have a lower mortality risk than individuals with a weight within the healthy range — a BMI between 18.5 and 24.9 kg/m2. Furthermore, one of the studies showed that individuals with obesity do not have an increased mortality risk compared with those with a ‘healthy’ weight.

While some studies show that intentional weight loss through calorie restriction and physical exercise can reduce mortality risk, others show a lack of association between weight loss and mortality risk.

In other words, the research has not consistently shown that weight loss reduces mortality risk.

Advice for weight loss generally involves restricting calorie intake alongside increasing physical activity levels. Thus, in studies reporting a reduction in mortality risk associated with weight loss, an increase in physical activity may account for this finding rather than weight loss itself.

The authors of the new study also note that sustaining weight loss over a prolonged period is often challenging. As more individuals attempt to lose weight by reducing calorie intake, there has also been an increase in the prevalence of weight cycling.

Three recent meta-analyses suggest that weight cycling is associated with increased risk of cardiovascular disease-related and all-cause mortality.

Medical News Today asked the article’s co-author, Dr. Glenn Gaesser, a professor at Arizona State University, whether unhealthy practices such as extreme calorie restriction could explain the limited benefits of weight loss in individuals with obesity. Dr. Gaesser said:

“Unhealthy weight loss practices are much more common among persons with a high BMI who also attempt weight loss more frequently. We contend that it is entirely plausible that [many] of the health risks associated with obesity are due to the adverse effects of weight cycling. Weight cycling is associated with increased mortality risk, and weight cycling is more prevalent among persons with obesity.”

In contrast to weight loss, there is more consistent evidence suggesting that cardiorespiratory fitness can greatly attenuate or even eliminate the mortality risks associated with high BMI.

For example, a meta-analysis showed that cardiorespiratory fitness could significantly reduce all-cause and cardiovascular disease-related mortality risk associated with high BMI. Furthermore, fit individuals with excess body weight had a lower risk of all-cause mortality than unfit individuals with a weight in the healthy range.

Similarly, physical activity can lower all-cause and cardiovascular disease-related mortality risk associated with high BMI. However, the impact of physical activity on mortality risk is less pronounced than that of cardiorespiratory fitness.

Follow-up studies also show that increasing physical activity levels and improving cardiorespiratory fitness can reduce mortality risk associated with BMI in the long term.

Moreover, improvements in cardiorespiratory fitness and physical activity tend to produce a larger reduction in mortality risk than weight loss.

Notably, a number of these studies show that these effects of physical activity and improved cardiorespiratory fitness accompanied either modest or no weight loss. This indicates that it is not possible to attribute the reduced all-cause and cardiovascular disease-related mortality risk associated with improved fitness to weight loss.

Physical activity and cardiorespiratory fitness can also result in improvements in markers for obesity-related conditions, including cardiovascular diseases and type 2 diabetes. These improvements in cardiometabolic markers are comparable to those that weight loss produces.

For example, the authors behind the current study note, both resistance training and aerobic exercise produce a decrease in blood pressure, with the extent of decline being similar to that resulting from weight loss.

Exercise training also improves blood glucose control, blood cholesterol levels, and vascular function to a similar extent as weight loss. These effects of exercise training on the aforementioned cardiometabolic markers were independent of weight loss.

Exercise training is also effective in reducing the amount of fat that the body stores in the liver and visceral adipose tissue. Visceral adipose tissue is the fat that surrounds internal organs, especially in the abdomen.

The fat stores in the liver and visceral adipose tissue are associated with an increased risk of cardiovascular disease and type 2 diabetes.

While the decline in both fat stores tends to correlate with the extent of weight loss, a clinically relevant reduction in the fat stores in the liver and visceral adipose tissue can occur with exercise training, even in the absence of weight loss.

The authors note that while severe calorie restriction is more effective than exercise training in reducing these fat stores, calorie restriction over long periods is generally not sustainable.

Exercise training can also improve the response of fat tissue to insulin, with reduced insulin sensitivity of fat tissue occurring in obesity and type 2 diabetes.

The authors recommend a weight-neutral approach that primarily focuses on improving cardiorespiratory fitness and increasing physical activity for obesity management.

The authors clarify that while healthcare professionals should not discourage weight loss, it should not be the primary focus of obesity management.

Dr. Gaesser said, “Current obesity treatment guidelines do not even mention ‘fitness’ and only encourage physical activity as a means to facilitate weight loss. This approach ignores the major improvements in mortality and disease risk associated with increased physical activity and improved fitness in the absence of weight loss. In fact, as our review shows, improving fitness by increasing physical activity is associated with greater reductions in mortality risk compared to weight loss.“

“So, if the goal of obesity treatment is to improve health and longevity prospects, it seems only logical that improving fitness should be emphasized more than weight loss. We would like to see current physical activity guidelines be an important focus of obesity treatment programs, with cardiorespiratory fitness assessed and monitored over time to document improvements in fitness.”

– Dr. Glenn Gaesser

“Fitness should be included as an essential ‘vital sign’ for assessing a person’s health status. Although we focus on obesity in this review, it is important to highlight the fact that fitness impacts health and longevity prospects for everyone, regardless of body weight,” added Dr. Gaesser.

MNT spoke with Dr. Jennifer Kuk, an associate professor at York University, Canada, about the challenges in understanding the precise role of fitness in obesity-related health conditions.

Dr. Kuk said, “Unlike body weight, which is a very simple measure that is also often done by self-report, fitness requires an exercise test, wherein the gold standard is a maximal exercise test. This is expensive [and] time- and labor-intensive for both the researcher and participant.”

“Physical activity is most often done by self-report, but often associations between activity and health are not as strong as what is observed with fitness. Physical activity and fitness do not always track, as there is a genetic component that will dictate your ability to increase your fitness with activity. Further, there may be recall errors in self-report issues, which would make it more difficult to see the true relationship between activity and health.”

Both the article’s authors and Dr. Kuk noted that it is possible to categorize only a small number of individuals as fat but fit, making it difficult to study the relationship between fitness and obesity-related health conditions. There are also challenges associated with the lack of standardized criteria for adjudicating whether an individual with a specific BMI value and fitness test score (VO2 max) is fit or unfit.

“[Another] issue is that with physical activity and fitness, there may be an effect of reverse causality. Is your fitness or activity level reflective of your health or the reverse (i.e., you are less active and fit because you are sick vs. you get sick because you are less active and fit)?,” said Dr. Kuk.

“This is particularly important given that the largest gains in mortality risk reduction are associated with even modest amounts of physical activity, often far lower than what is recommended in the physical activity guidelines.”

There is widespread agreement about the importance of improving fitness levels for the management of obesity, as advocated by the fat-but-fit approach. However, there are concerns among the scientific and medical community about the degree of emphasis on weight loss and other factors associated with obesity.

Dr. Jennifer Bea, who advocates a nuanced, multidimensional approach to obesity management, told MNT: “Physical activity has been shown to improve health independent of weight loss. One of the ways in which physical activity influences cardiometabolic health is by improving body composition, i.e., increasing skeletal muscle mass and reducing adipose.”

“BMI, which is used to classify overweight and obesity in the U.S. and other places, is not a direct measure of body composition […]. Of course, physical activity also improves cardiovascular fitness, which is also risk-reducing.”

“However, it is important to recognize that obesity is a multidimensional health issue with multifactorial contributors. Thus, being physically active is not a license to maintain a poor diet or to ignore other contributors to obesity. Other contributors to obesity may include issues with sleep, certain medications, the microbiome, and many more.”

– Dr. Jennifer Bea

Dr. Bea added: “We must not forget about individuals that are considered normal weight either. Poor body composition among those with a ‘normal BMI’ has also been associated with poor cardiometabolic biomarkers and health risks.”

“While there is minimal risk in advocating for increased physical activity across weight categories when following the [American College of Sports Medicine (ACSM)] guidelines for physical activity testing and prescription, it would behoove us to move beyond BMI to a more comprehensive evaluation of risk factors to at least include body composition, physical activity, blood pressure, and blood biomarkers.”

Dr. Bea is an associate professor of medicine at the University of Arizona Health Sciences and a member of the ACSM.

Dr. Kuk similarly noted: “In the end, I think there is consensus that fitness and physical activity is beneficial, and obesity is deleterious to health. Focusing solely on one at the detriment of the other factor is likely not a wise approach.”

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