Do you ever find yourself on Monday wondering if you went a little overboard with the chips and dip over the weekend? And maybe even with those brownies at the party the weekend before? Oh yeah, and that “fourth meal” burrito from El Farolito Saturday night?

If you generally use weekends as an excuse to indulge a little, you are not alone. And according to a study published last month in Obesity, there is a good chance that weekend splurges are the reason you are gaining weight (or at least one reason it is so difficult to lose it).

Researchers at Washington University in St. Louis wanted to understand how weekends affect weight fluctuations in free-living individuals under different weight loss plans. To address this, they split participants into three groups and followed them for one year. One group was put on a calorie-restricted diet and another was given an exercise regimen. Both the calorie restricted and exercise group interventions were designed to create similar calorie deficits – 16-20 percent of total baseline energy expenditure for each individual – so that the effects of weight loss induced by each method could be compared. The third group was not given any intervention.

Because one goal of the study was to observe free-living individuals (i.e., people at home and not in a clinic), researchers utilized a comprehensive battery of measurements to determine energy intake, physical activity, energy expenditure, body composition and body weight. For instance, participants were provided with a special scientific scale that was collected at the end of each recording period. Body weights were then double checked on a calibrated scale in a clinic on several of the weight recording days. Duel-energy X-ray absorptiometry was used to measure body composition (lean mass versus fat mass) on multiple occasions throughout the study. Food diaries were kept by the participants to record daily food intake, and they received detailed instructions on how to weigh, measure and record everything consumed. Physical activity was measured with both triaxial accelerometers and urine samples using the doubly labeled water method.

Detailed documentation of variables and outcome measures is critical in experiments where people’s activity is not controlled in a laboratory, because self-reporting—particularly when weight loss and eating are involved – is notoriously unreliable. Though the methods used here do not guarantee accurate measurements, the double and triple validation that the scientists performed throughout the study certainly minimizes sampling biases.

The first interesting observations were found when scientists took baseline measurements during the 2-4 weeks before the experimental interventions began. During this period body weight at baseline increased significantly on weekends and was followed by a non-significant trend of weight loss on weekdays. Unfortunately this resulted in an increase of 0.077 kilograms (0.17 pounds) per week.

This increase may not sound like much, but it amounts to about 4.0 kilograms (9 pounds) gained per year. The authors of the study suggest that this finding may be an artifact of the anticipation individuals feel when beginning a weight loss regimen, because on average people gain less than 1 kilogram per year. Regardless, the finding that weight gain occurs on weekends and not on weekdays is significant.

Also during baseline measurements the researchers found that calorie intake was highest on Saturdays, averaging about 200 calories more than weekdays. Physical activity during this time was lowest on Sundays and highest on Saturdays.

During the experimental interventions, the weekend affect on body weight was still evident. As expected, both the calorie restricted and exercise groups lost weight over the course of an average week. On weekends, however, the calorie-restricted group stopped losing weight and the exercise group actually gained weight. The control group did not have significant weight changes on weekdays compared to weekends.

Interestingly, during the experiment all three trial groups consumed significantly more calories on weekends than on weekdays. Those on calorie-restricted diets ate more on Saturdays, while those on the exercise regimen consumed more on both Saturdays and Sundays. The control group ate more only on Sundays.

In contrast, physical activity did not decrease on weekends during the experiment. In the exercise group physical activity remained constant relative to weekdays. The calorie-restricted group actually increased activity on both Saturday and Sunday. In the control group there was a trend toward more physical activity on weekends.

These findings suggest that weekend behavior has a tremendous impact on weight gain in average people and makes weight loss more difficult for those undergoing a calorie restricted or exercise program. Furthermore, the weekend calorie imbalance is primarily caused by increased eating rather than decreased physical activity.

This research is consistent with that of the National Weight Control Registry report in 2004, which suggests that people whose diets are more consistent from weekdays to weekends are more likely to be successful in losing weight and keeping it off. It also agrees with two studies published in the International Journal of Obesity last month showing that human energy expenditure has not changed in America or Europe in the past 20 years and is still comparable to the amount spent by indigenous populations and even animals in the wild.

The bottom line is that while increased physical activity can aid in weight loss, excessive calorie consumption is the primary reason for weight gain in our time.

This story can also be found at Synapse.