As you age, the proportions of muscle, fat and bone that comprise your body change. Even if you’re naturally thin, your muscle volume decreases as your fat percentage increases. Some of this weight gain occurs in your abdomen, thickening your waist. Hormonal changes also contribute to packing more weight on your waist, while osteoporosis may change your posture and emphasize thickness at your middle.

Natural Weight Gain

A lengthy Berkeley Lab study of almost 5,000 male runners between the ages of 18 and 50 found that men gained an average of 3.3 pounds per decade. That translated to an average per-decade gain of 3/4 of an inch at the waist. These figures stayed consistent across all activity levels; the men who ran more often didn’t stave off this natural weight increase any more than the men who ran infrequently or for shorter distances. Women also gain as they age, peaking at around age 65.

Hormonal Changes

Menopause triggers overall weight gain, but it also specifically causes extra inches at your waistline. Hormones that used to direct your weight gain to your hips and thighs now allocate more of the weight to your belly. As estrogen levels drop, your weight gain patterns change. Men face hormone-related weight fluctuations too, although theirs may not be as pronounced as those that women undergo. Maintaining a good diet and exercise can help offset these hormonal changes.

Osteoporosis

Osteoporosis, a disorder that diminishes bone density and leaves your bones brittle, may contribute to a thickening waist. Microfractures of osteoporotic vertebrae shrink your spine, compressing internal organs and increasing your abdominal diameter. Just as a stretched rubber band is thinner than a compressed one, a fully erect and upright posture makes you look slender, while a stooped posture thickens your waist. Talk to your doctor if you feel you might have diminished bone density or spinal changes that are changing the shape of your body.

Bloating

Not all thickening at your waist is related to fat. Abdominal swelling and bloating could be a consequence of a host of gastrointestinal or cardiopulmonary concerns. If your diet has changed, you may be eating more of foods that cause gas, leading to a puffy abdomen. Irritable bowel syndrome can also cause bloating. More serious health issues such as heart failure cause abdominal swelling due to fluid retention. Only your doctor can diagnose these more serious concerns, so arrange a visit if you think your abdominal swelling could be something other than fat gain.