Casazza, K., et al. 2013. Myths, presumptions, and facts about obesity. The New England Journal of Medicine, 368 (5), 446–54.

Exercise professionals are not alone in holding strong opinions about managing and preventing obesity. Just about anyone who has tried to lose weight has a few ideas about what works, but how much of this conventional wisdom has scientific backing?

Casazza et al. (2013) tackled the issue, researching several popular views on obesity and shedding light on scientific findings vis-à-vis various pertinent topics. The scientists note that some traditional viewpoints on obesity management may be unsafe and clinically obstructive for a client. The authors outline strong scientific data that contradict several obesity myths, and they address many presumptions—accepted beliefs that lack supporting science.

The Myths

Small Lifestyle Changes Are Enough

While small changes are a great start, Casazza and colleagues point to evidence backing what most of us have observed: Changes in body composition caused by diet and exercise vary widely among individuals. Furthermore, the body often compensates for changes in energy intake and output in ways that inhibit continual weight loss. Personal trainers need to tell their clients that these physiological changes may require them to work even harder and take more calories out of their diets to keep making progress.

Setting Realistic Goals Is the Best Approach

Although genuine goal setting has always been a steadfast rule in behavior-change models, Casazza et al. say the research does not show negative outcomes from clients setting (and achieving) more ambitious weight-loss goals. In fact, the authors note that several studies say se ting more robust weight-loss goals may result in better-than-expected outcomes.

Rapid Weight Loss Is Always Bad

Casazza et al. say some studies show that rapid initial weight loss (via
low-energy diets) may actually produce favorable long-term outcomes in some overweight people. One fear with this approach has always been that if people lose weight too fast, they will gain it back just as quickly.

The authors note that many over- weight people tend to have meaningful early weight-loss success that they are ultimately able to sustain. However, a major concern with rapid weight-loss strategies is that the person is eating a nutritionally inadequate diet that is very low in calories which will definitely have problematic health consequences in the long term.

People Must Be Ready to Change

Several behavior-change models sug- gest that a person’s readiness to change a behavior is crucial to making the necessary lifelong weight-loss adjustments in his or her lifestyle. Casazza and colleagues cite five rather large trials—with approximately 3,910 participants—that indicate people “willing” to begin a weight-loss program are going to lose some weight, regardless of how ready they are to change their behavior. The authors say readiness to change does not always predict how much weight people lose or how well they stick to a program.

Breast-Feeding Helps Prevent Obesity

Many exercise professionals are familiar with this century-old conjecture suggesting that breast-fed infants are less likely to be obese later in life. Casazza and colleagues argue that better controlled studies in which children were followed for more than 6 years do not show persuasive evidence of an anti-obesity effect from breast-feeding. The authors note that even though breast-feeding may not have a shielding effect on childhood obesity, it should still be encouraged, as it has multiple health benefits for infants.

The Presumptions

Regularly Eating Breakfast Guards Against Obesity

It’s commonly presumed that people who skip breakfast will overeat later in the day. Casazza et al. note that established eating habits are a major determinant on whether this presumption proves true. Thus, eating after a night’s sleep has a profound effect on energy and nutrient replenishment, but one study found that the effect on weight loss of being assigned to eat or skip breakfast was largely dependent on baseline breakfast habits.

Early-Childhood Exercise and Eating Habits Last a Lifetime

It is presumed that early childhood is when people learn eating and exercise habits that last their entire lives. Casazza and colleagues note that no randomized, controlled trials have been completed to test this presumption. However, they suggest that eating behaviors may be a function more of genetics than of learning.

Eating More Fruits and Vegetables Results in More Weight Loss and Less Weight Gain

Clearly, eating fruits and vegetables has credible scientific merit for improved health and disease prevention. However, there is no research to support or reject the presumption that eating more fruits and vegetables causes people to eat less of other foods and reduce their total calorie consumption.

Yo-Yo Dieting (Weight Cycling) Is Associated With Increased Mortality

Observational studies have found that people whose weight is stable have lower mortality rates than those whose weight is unstable (i.e., whose weight keeps cycling), but more investigation is needed to resolve this question authoritatively.

Snacking Triggers Weight Gain and Obesity

It is alleged that eating snacks leads to weight gain and contributes to obesity. Casazza and colleagues report that randomized, controlled studies and observational research have not shown this to be true.

Rethinking Traditional Viewpoints

From this inclusive review of the literature, Casazza and colleagues clearly offer resounding evidence-based statements that challenge many beliefs about obesity and weight loss. As an industry, we must always conscientiously evaluate unproven strategies and interventions to offer our students and clients the best knowledge and guidance possible.

The Facts on Obesity

The following obesity facts presented by Casazza et al. are well supported by scientific evidence and offer practical guidance for personal trainers.

  • Meal-replacement products promote greater weight loss. It appears the structure and calorie intake regimen these products provide enhance weight-loss interventions.
  • Heredity is not destiny. While genetic factors play a role in weight gain, consistent changes in diet and exercise remain central to reducing body weight and preventing obesity.
  • Regardless of body weight, improving fitness has encouraging health benefits. Consistent exercise can reduce many of the damaging health effects of obesity.
  • Encouraging clients to go on a diet does not work well in the long term. Diets effectively reduce weight, but their lasting effects are limited (Donnelly et al. 2009). An overall sustainable lifestyle change that cuts calories and increases physical activity will produce supportable weight-loss goals and prevent weight regain much better than a diet.
  • Exercise in sufficient doses is a critical component of a successful weight-loss program. People need substantial exercise to lose weight and avoid gaining it back. ACSM recommends accumulating 250-300 minutes per week (~2,000 kilocalories per week) of moderate-intensity exercise for weight loss and prevention of weight gain (Donnelly et al. 2009).
  • Continuing the conditions that promote weight loss also promotes weight maintenance. Weight-regain prevention is an ongoing challenge that requires sustainable efforts of energy output and portion control.
  • With overweight children, programs that involve parental support at home have the best results. This does not devalue school, agency and organization efforts to promote weight loss in youth; it merely shows the importance of the home setting for sustain- able weight loss in this population.
  • A few pharmaceutical agents can help over- weight clients lose weight. Moderately effective medical treatments can be prescribed and administered by clinical professionals.
  • For severely obese people, bariatric surgery leads to successful weight loss and a reduction in the incidence of diabetes and mortality.


Donnelly, J.E., et al. 2009. American College of Sports Medicine Position Stand. Appropriate physical activity intervention strategies for weight loss and prevention of weight regain for adults. Medicine & Science in Sports & Exercise, 41 (2), 459-71.

Len Kravitz, PhD

Len Kravitz, PhD is a professor and program coordinator of exercise science at the University of New Mexico where he recently received the Presidential Award of Distinction and the Outstanding Teacher of the Year award. In addition to being a 2016 inductee into the National Fitness Hall of Fame, Dr. Kravitz was awarded the Fitness Educator of the Year by the American Council on Exercise. Just recently, ACSM honored him with writing the 'Paper of the Year' for the ACSM Health and Fitness Journal.

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