What’s causing that belly bloat?

Underlying causes, plus food and sensory responses, can lead to distention and discomfort.

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You just finished a meal, but the meal isn't quite finished with you. It's causing an uncomfortable bloated feeling and the perception that you suddenly have a big belly that lasts for hours. And it's not just your imagination. "Very quickly it can make you look like you have a bit of a pooch. There's an enlargement of the abdomen and a sensation of distention," explains Dr. Kyle Staller, a gastroenterologist at Harvard-affiliated Massachusetts General Hospital.

But the cause of belly bloat is often hard to pin down.

A symptom of conditions

In some cases, bloating is a symptom of an underlying condition. For example, any of these disorders can cause bloating:

Irritable bowel syndrome, a condition characterized by a combination of symptoms (bloating, cramping, abdominal pain, diarrhea, or constipation) that last for three or more months.

Inflammatory bowel disease, an inflammation of the lining of the gastrointestinal tract, including Crohn's disease and ulcerative colitis.

Celiac disease, an autoimmune disease in which the immune system attacks the small intestine. It's triggered by a protein called gluten that's found in wheat, barley, and rye.

Constipation, a condition defined by fewer than three bowel movements per week, hard or dry stools, the need to strain to move the bowels, and a sense of an incomplete evacuation. "If you're not emptying your gut, there's no room in your abdomen, and you'll have excess bloating," Dr. Staller says.

Gastroparesis, a sluggish emptying of food from the stomach into the small intestine.

Cancer. Colon, ovarian, stomach, and pancreatic cancer are among the cancers that can have bloating as a symptom.

A reaction to diet

Sometimes bloating results when your body has a hard time digesting sugars in certain foods. The key culprits are in a group known as FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols). Examples include wheat, rye, onions, garlic, legumes (chickpeas, lentils, beans), honey, pistachios, cashews, asparagus, and artichokes. Foods or drinks with fructose or artificial sweeteners are also on the FODMAP list.

"We all have an increased amount of gas in the body after eating them, but some of us react to them more severely than others," Dr. Staller says. If that's the case, it helps to identify which of these foods you react to and avoid them.

Milk and dairy foods are also considered FODMAPs. They contain a type of sugar called lactose, which is hard to digest if you don't have enough of the enzyme that breaks it down (lactase). The result: gastrointestinal distress that includes bloating.

An overgrowth of bacteria

One possible cause of bloating is controversial. It's called small intestinal bacterial overgrowth (SIBO). The idea is that bad bacteria in the small intestine are growing out of control, eating your food, and producing additional gas. "The theory is that by wiping out those bacteria you can sometimes treat those people's bloating. But often the SIBO comes roaring back, and if you're treated with multiple courses of antibiotics, that might be dangerous," Dr. Staller says.

Why is it controversial? "Because we don't have a lot of evidence about it, we still have a lot to learn about it, and testing [a breath test] isn't reliable," says Dr. Staller. Antibiotics do resolve bloating in some people, he notes, but he is hesitant to make a diagnosis of SIBO.

A misperception of gas

Dr. Staller says most bloaters are actually struggling with a more abstract cause: the body's perception of gas and overreaction to it. This is a newer approach to coping with belly bloat. "Increasing evidence suggests that the majority of people with bloating have an abnormal response to a normal amount of gas," Dr. Staller says. The response is triggered by an extremely sensitive nervous system, a problem called visceral hypersensitivity.

When the nerves in the gut overreact to gas, your level of discomfort increases. "Normally if you eat something that causes gas, the abdominal wall tightens, and the diaphragm — which divides the chest and abdomen — rises into the chest so the contents of the gut can spread out more," Dr. Staller explains. "But the data suggest that bloaters do the opposite: their diaphragm pushes down, and the abdomen bulges out."

Visceral hypersensitivity is also linked to other conditions, including irritable bowel syndrome.

A warning about sudden bloating Sometimes bloating can come on suddenly when you get older. "Most people who have bloating start experiencing it at a young age. But if someone is suddenly having bloating in older age, that's sometimes a red flag that tells me something has changed and needs to be investigated," says Dr. Kyle Staller, a gastroenterologist at Harvard-affiliated Massachusetts General Hospital. What causes sudden bloating? It could be an inflammatory bowel condition, constipation, lactose intolerance, celiac disease, or (in rare cases) cancer. If sudden bloating lasts more than a few days, report the symptom to your primary care physician or gastroenterologist.

Banishing bloat

It can take time to figure out what's causing bloating. Dr. Staller says the first step is to investigate the more tangible causes. "If we suspect bloating is caused by constipation or irritable bowel syndrome, for example, we'll treat those. If there's no underlying cause, we'll look at the diet," Dr. Staller says. If lactose intolerance is a possibility, he'll suggest a dairy-free diet. If FODMAPs seem to be the trouble, he'll put patients on a low-FODMAP diet and then reintroduce certain foods to see if they cause bloating.

"For sure there are some people whose bloating goes away when you fix the underlying condition or diet. But in many cases, the bloating will remain. That's when we think of bloating as a sensory issue," Dr. Staller notes. He says low doses of medications called neuromodulators, which change the way nerves react, can help reduce bloating. Examples include tricyclic antidepressants like amitriptyline (Elavil).

And sometimes treating belly bloat requires a multifaceted approach. "It's often a case of using traditional approaches in combination with the newer approach of thinking of bloating as a sensory issue," Dr. Staller says.

Your primary care physician or a gastroenterologist can help you resolve belly bloat. It may require some trial and error, but Dr. Staller says in time, you'll be able to finish a meal without being uncomfortable.

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