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Why Am I Gaining Weight? Science Explains Summer Fat

Just because it's getting warmer out does not mean you're going to get more fit. Here's why it's so common to pack on summer pounds.

Does your beach body seem to disappear as the summer progresses — covered in beer, braut, and sedentary fat? You’re not alone. While weight gain is seen as a winter sport, summer weight gain is much more common than people think. And like a thunderstorm on a sunny day at the beach, it sneaks up on parents and kids alike.

Two recent studies suggest that children lose weight during the school year, and gain weight in the summer due to increased consumption of sugary drinks and decrease structure when it comes to meals, activity, and sleep. It’s possible similar variables cause summer weight gain in adults. There’s evidence that people generally sleep less during the summer, due to increased daylight exposure and just being too hot to sleep comfortably, and sleep deprivation increases the production of cortisol — a stress hormone that drives sugar cravings, swelling, and overall weight gain. Making matters worse, adults tend to diet in preparation for the summer, which really prepares them to pack on the pounds.

“Summer weight gain is often simply the biological response to pre-summer attempts at weight loss,” health coach and fitness specialist Ragen Chastain explains. “That lose-gain cycle can be compressed with more extreme dieting. So that crash diet they sold you to get your summer body will actually be likely to give you your summer weight gain.” 

The lose-gain cycle Chastain is referring to is often seen in crash or yo-yo diets, or regimens designed for rapid weight loss that are rarely sustainable and often backfire. When people try to lose weight a month before vacation instead of gradually throughout the year, it depletes their metabolism in several ways. Calorie deprivation slows thermogenesis, the amount of energy the body spends digesting food, while increasing overall fatigue, which discourages exercise. Extreme dieting also leads to muscle breakdown, which means fewer calories burned both during physical activity and while resting. Likewise, depriving the body of nutrients further disrupts sleep, which takes a toll on a person’s metabolic rate, studies show. Once summer starts and these diets end, or just get cheated on a lot, an uptick of parties, barbecues, and vacations, and the all the high-calorie indulgences that come with them create a perfect storm for a summer belly. That’s why summer is surprisingly the perfect time to get fat.

While warm temperatures encourage physical activities, really hot ones have the opposite effect, and people opt for sedentary summer days blasting the AC. The American College of Sports Medicine recommends canceling outdoor exercise when the heat index is above 90 degrees, for anyone who works out regularly. For those who are not as fit, the ACSM suggests tapping out closer to 86 degrees and calling off any sport competitions or forms of extended activity at 82 degrees. That means in places with the highest heat indexes — which accounts for humidity as well as temperature — like Texas, Louisiana, and Florida, outdoor physical activity may not be a safe option for most of the season. Regardless of where they live, people are more likely to put off exercising outside in the summer than in the winter due to the weather, according to one study. Consumer trends data confirms that many people fill this time with more beer, ice cream, and hot dogs, only making matters worse. 

That’s not to say that winter weight gain isn’t real. Research shows that people tend to pack on the pounds between October and January because of the holiday season, as well as the cold weather, which tends to decrease physical activity increase the urge to indulge. However, exposure to cold weather helps activate brown fat to heat the body, which in turn, burns other fat, and shivering burns calories further, so it’s not all bad.

Still, if people are prone to gaining weight in the summer and winter, is everyone just getting fatter year-round? Not so. Spring, it turns out, is more conducive to healthy eating, exercise, and weight loss than any other season. Research shows that people are more likely to think about weight loss during winter and summer months, but the spring months are when they actually can do it. On average, people consume 86 fewer calories per day in the spring compared to the fall, are more likely to exercise outside, and engage in physical activity for longer periods of time. The best thing parents who don’t want to pack on the summer pounds can do is treat it like spring. Think of it as summer with a little more structure, sleep, and healthy food.