Fat Tuesday (aka, Mardi Gras) comes once a year, but “Heavy Monday” is a weekly occurrence, according to an international study of weight-gain cycles.

Even conscientious dieters tend to eat differently - and exercise less - on weekends, the team of Cornell and Finnish researchers noted. Weighing in a little heavier on Mondays than on Fridays is just part of a weekly cycle.

“We found a clear weekly rhythm to weight variation. Weight was at the highest point after weekends, on Sunday and Monday, and decreased toward the end of the week,” said Cornell’s Brian Wansink, a professor in the Charles S. Dyson School of Applied Economics and Management. “Interestingly, people who were successful at losing weight experienced the greatest ‘Heavy Monday’ effect.”

Researchers’ findings were reported online Jan. 31 in Obesity Facts, the European journal of obesity. Besides Wansink, who is the director of the Cornell Food and Brand Lab, other authors were Mikka Ermes, Erik A. Mattila, Anna-Leena Orsama and Mark van Gils, all of the VTT Technical Research Centre of Finland, and Ilkka Korhonen of Tampere University of Technology, Tampere, Finland.

The American and Finnish researchers compiled and analyzed results from four separate studies that had asked dozens of people to weigh themselves each morning before breakfast for several weeks. The survey populations included subjects who were steadily losing weight, gradually gaining weight or trying to maintain their weight.

The “Heavy Monday” effect held true for the aggregated population of weight losers, gainers and maintainers, but was most pronounced among those successfully losing weight. Then, five weekdays of better eating and exercise behavior tipped the scale in favor of the weight losers. They did experience the “Heavy Monday” effect, but that small weekend gain proved temporary and did not impact long-term weight loss.  

The researchers agreed that “successful weight control is not distracted by slight increases in weight during weekends.” They recommended “allowing more flexibility on weekends and holidays” as a more realistic long-term perspective to avoid dietary boredom.

Humans experience all kinds of periodic changes, authors of the report observe, from circadian rhythms and menstrual cycles to fluctuations in body temperature, blood pressure and glucose metabolism. Without understanding the cause and transitory nature of weekend weight gain, they note with empathy, it’s natural to feel remorseful when the scale edges upward.

Said Wansink: “If you’re giving up fattening foods for Lent (or some other good cause) and generally losing weight, don’t let ‘Heavy Monday’ spoil your week. Keep your eye on the goal: a lifetime of consistently healthful eating habits.”

Funds for the study, “Weight Rhythms: Weight Increases during Weekends and Decreases during Weekdays,” were provided by the Finnish Funding Agency for Technology and Innovation and the VTT Technical Research Centre of Finland.