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An unhealthy diet and inactive lifestyle can definitely expand your waistline. But what if you’re eating your veggies and hitting the gym, and your belly is staying the same size while the rest of you is slimming down? That’s so frustrating! Yet it becomes less perplexing once you realize that other things can also contribute to belly fat, even when the rest of your body is relatively lean.

Why Size Matters

There’s more at stake than being able to button your jeans. Having a big belly is associated with serious health risks, even if your body mass index (BMI) — an estimate of overall body fat — stays within the normal range. In particular, women with waistlines larger than 35 inches and men with waistlines larger than 40 inches have an increased risk of developing heart disease and type 2 diabetes.

A study led by a Mayo Clinic researcher pooled data from 650,000 adults around the world. The study showed that men and women with large waist measurements died younger, on average, than those with trim waists. For every two inches added to their waistlines, the risk of dying early went up about 9% in women and 7% in men.

Eating too much of the wrong foods and getting too little physical activity are major causes of obesity. But in recent years, American waistlines have grown faster than BMI levels — a sign that eating and exercise habits are only part of the story. So researchers are now looking for other factors that add specifically to the risk of putting on a pot belly. Below are four culprits you might not suspect.

Cigarette Smoke

Many people say that they smoke to stay thinner, but that’s counterproductive. For one thing, the health benefits of a lower weight are canceled out by the deadly effects of tobacco smoke. Plus, there’s mounting evidence that smoking may actually lead to a buildup of belly fat that’s independent of overall obesity.

One study included nearly 300 visitors to university hospitals in South Korea. Smoking was associated with an increase specifically in abdominal fat — especially in visceral fat, which wraps around organs deep inside the torso and bodes ill for a person’s health. This held true even though researchers controlled for age, exercise and alcohol consumption.

Diet Soft Drinks

Another weight control strategy that may backfire is drinking diet soda. Research from the University of Texas Health Science Center in San Antonio looked at diet soda intake in more than 700 adults ages 65 and up — an age group at increased risk for cardiovascular and metabolic problems.

The more diet soda these older adults drank, the larger they grew around the middle. Over a nine-year period, the average increase in waist size was four times greater for diet soda drinkers (3.2 inches), compared to those who didn’t drank diet soda (0.8 inches).

Noise Pollution

Cortisol, a stress hormone, is thought to promote the deposition of fat around the middle of the body. Any form of ongoing stress might have this effect, but one that’s often forgotten is the stress of living in a noisy environment.

A recent study from the Karolinska Institute in Stockholm looked at the effects of road, train and aircraft noise on midlife adults. Exposure to such noise was linked to having a midriff bulge, and the more sources of traffic noise people were exposed to, the bigger their waistlines tended to be. The findings weren’t explained by socioeconomic factors or exposure to air pollution from nearby roads.

Sleep Loss

Sleep is the ultimate sedentary activity, so it might not seem like an obvious waist whittler. Yet there’s good evidence that lack of sleep increases the risk of becoming obese. One possible reason: Sleeping too little disrupts hormones that control appetite, so people may feel hungrier than they do when well-rested.

Along with making people fatter overall, lack of sleep may lead to having a paunch. In a large, nationally representative sample of U.S. adults, those who slept less than six hours per night had bigger waists, on average, than those who slept seven to nine hours. The association was particularly strong for those in their twenties and thirties. Similar findings have also been reported for adolescents.

For managing your waistline, watching what you eat and being more physically active are the best places to start. But if you’re doing those things and still aren’t seeing the results you had hoped for, ask yourself whether one of these factors might be involved.

Linda Wasmer Andrews writes about health, psychology and the intersection of the two. Connect with her on Twitter and Facebook. Read more from her blog:
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