A waist size greater than 40 inches indicates a man is at a high risk of type-2 diabetes and heart disease. This inflated, firm belly is due to an overabundance of visceral fat, which lies deep inside your trunk nestled around internal organs. Visceral fat -- unlike the subcutaneous fat that resides just under the skin on the abdomen, hips, thighs and arms -- secretes hormones and compounds that raise inflammation, which contributes to heart disease. Men are genetically more likely to store fat in their gut, rather than the lower body, and certain lifestyle habits exacerbate weight accumulation in the middle.
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Aging and Hormones Make a Fat Belly
While women tend to gain weight in the hips and thighs -- especially during child-bearing years to support pregnancy and breast-feeding -- men are wired to gain fat in their bellies. Hormones drive women's fat storage to the lower body, but why men store fat in their belly is unclear, as it doesn't provide any physiological advantage.
As men age, they're more likely to develop big bellies. After age 40, the natural reduction in testosterone means excess calories are often stored as visceral fat. Aging also makes you naturally lose muscle mass. Muscle keeps your metabolism burning at a solid rate. When you lose this muscle -- about 1 pound per year after age 30 -- your metabolism declines, and it becomes easier to gain fat, which often goes straight to the belly in men.
Dietary Habits and Belly Fat
While eating too much of any food can cause weight gain, some foods are more likely to lead to the accumulation of belly fat than others. If you have a serious soda habit, you're more likely to have a large, hard belly. Sugar-sweetened drinks, including soda, fruit punch and energy drinks, contain numerous calories, encouraging weight gain. Drink water or unsweetened herbal tea instead of soda to help reduce calories and lose weight.
Refined grains, more so than whole grains, expand your waist. In a 2010 issue of the American Journal of Clinical Nutrition, researchers found that increased intake of refined grains correlated with a greater amount of belly fat, while an increased intake of whole grains did not. Eat brown rice, quinoa and 100-percent whole-wheat bread in lieu of white rice and pasta. Include ample watery, fibrous vegetables at meals too -- they'll help fill you up without too many calories.
Too much saturated fat can also make belly fat more likely to develop. Fatty cuts of meat and full-fat dairy contain this type of fat. Consume polyunsaturated fats from nuts, salmon and seeds instead. Opt for leaner meats, such as chicken breast, flank steak and white fish, over ribs, brisket and chicken thighs.
Sedentary Living Equals a Hard, Fat Belly
A physically inactive lifestyle is a primary reason men gain weight -- especially visceral fat weight. Moving more helps you reduce your overall size, specifically a hard, fat belly. Visceral fat is particularly responsive to classic diet and exercise techniques.
You don't have to become a marathoner to see benefits. Work up to 150 minutes per week of moderate-intensity exercise, such as brisk walking, to prevent additional gains; exercise more if you want to lose fat. Strength training is also essential to offsetting the natural loss of muscle mass that occurs with aging and disuse.
Even if you use exercise to lose just 5 to 10 percent of your body weight -- that's 12 to 24 pounds for a 240-pound man -- you'll experience benefits in health markers, including blood sugar, high blood pressure and cholesterol levels.
Smoking presents numerous health risks -- you can count an increase in visceral fat among them.
Sleeping too little, or too much, can also cause you to gain more visceral fat, according to a 2010 study published in Sleep. Researchers found that after five years of consistently getting less than six hours or more than nine hours per night resulted in greater abdominal fat in people younger than 40.
Stress is also a reason men's bellies become hard and fat. Too many bills or work deadlines can cause you to pump out the hormone cortisol, which is associated with visceral fat gain.