Saturday can be the worst enemy for our waistlines, according to School of Medicine researchers.
They found that study subjects on strict diet and exercise programs tend to lose weight more slowly than expected because they eat more on weekends than during the week. The investigators report their findings in the advance online publication of the journal Obesity.
Past research had confirmed that people tend to gain weight during the holidays, particularly between Thanksgiving and New Year’s. Yet this is the first study to carefully monitor daily body weight, calorie intake and calorie expenditure for several weeks throughout a year and to demonstrate that increased caloric intake isn’t just a problem during the holidays — it also happens on most weekends.
“We thought weekends would present a problem for some people attempting to lose weight, but the consistency of our finding before and during the interventions was surprising,” said first author Susan B. Racette, Ph.D., assistant professor of physical therapy and of medicine. “Subjects in the diet group lost weight during the week, but over the weekend, they stopped losing weight because they were eating more.”
Racette’s team followed 48 adults between the ages of 50 and 60 who took part in the CALERIE (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy) study. Body mass index ranked subjects as overweight or healthy weight when the study began. None was classified as obese.
Following earlier studies demonstrating that mice and rats live longer, healthier lives when on a calorie-restricted diet, the CALERIE study is designed to determine whether taking in fewer calories over a long time period will slow down or reverse some of the common markers of aging and disease.
“But rats don’t have weekends the way people do,” Racette said. “On weekends, human lifestyle patterns can be very different. People have social events, parties to attend, and if they have children who play sports, they might be at games all day long, relying on concession stands for food.”
Study participants were divided into three groups: The first lowered their daily calorie intake by 20 percent, a second increased daily physical activity by 20 percent and a third control group did not change diet or activity levels. All three groups were monitored for one year. They kept food diaries, tracked exercise with accelerometers and were weighed regularly. Racette said people in the study didn’t always realize they were eating significantly more food on weekends.
“It was surprising how consistent the findings were,” she said. “We also were surprised by the dramatic way in which weekends continued to slow weight loss throughout the course of the study.”
Before the interventions began, the researchers established “baselines” for each study participant’s exercise and eating habits. This preintervention data determined that participants consumed the most calories on Saturdays. An average of 36 percent of their total calories came from fat on Saturdays, but less than 35 percent came from fat during the rest of the week. The typical weekend weight gain before the diet and exercise interventions began would have led to an average increase of 9 pounds a year.
When study participants were asked either to cut calories by 20 percent or to increase activity by a like amount, the pattern remained the same. Those in the calorie-restriction group took in more energy on Saturday. Those in the exercise group ate more on both Saturday and Sunday. As a result, people in the calorie-restriction group stopped losing weight on weekends, and those in the exercise group actually gained weight on weekends.
“People on diets often don’t lose as much weight as we would expect, and this finding helps to explain why,” Racette said.