Ask Americans about what they think is the biggest problem with their diets and you'll probably hear a variety of responses...

"Too many calories." There's certainly a strong case for this. Americans consume an estimated 3,800 calories per day, roughly twice what is needed for sedentary adults.

"Processed foods." There's a growing case for this. Processed foods are often designed to be hyper-palatable, leading to massive overeating.

"Too many carbohydrates." There's less of a case for this. Processed carbohydrates and simple sugars are empty calories, but whole grain-based foods are quality additions to any diet.

"Not enough protein." There's absolutely no evidence for this. Ninety-seven percent of Americans eat the recommended amount of protein. Food companies plaster protein claims on their products to ensnare consumers who associate the macronutrient with muscles and thinness, but simply consuming more protein will not reshape one's body for the better.

In actuality, the greatest problem with the American diet will probably sound decidedly boring to most people: Americans don't eat nearly enough fiber. That's right, fiber – the substance associated with bran flakes, old people, and pooping. Just 5% of Americans eat enough of it, and this deficit is contributing to significant health problems.

First off, what is fiber? Most simply, it is a type of carbohydrate that the body cannot digest. There are a few different types, all with varying health benefits. Some soften your stools, while others nourish your gut microbiome. Think of fiber almost as an oil that lubricates an engine and keeps it running smoothly.

For the overwhelming majority of Americans who don't eat enough fiber, they are far more likely to suffer intestinal syndromes resulting from gut bacteria dysfunction. They are also at increased risk of constipation, which is responsible for more than 700,000 visits to emergency rooms each year.

Researchers summed up the benefits of fiber intake in a 2017 review:

The fiber component of foods is associated with reduced risk of cardiovascular disease, coronary heart disease, stroke, hypertension, certain gastrointestinal disorders, obesity, and metabolic dysfunctions, including prediabetes and type 2 diabetes, and colorectal, gastric, and breast cancers. Fiber also is associated with digestive benefits, such as increased stool bulk, decreased transit time, and fermentation by colonic microflora.

A more recent review of 185 studies and 58 clinical trials published in The Lancet and summarized by Julia Belluz at Vox found that if "1,000 people transitioned from a low-fiber diet (under 15 grams per day) to a high-fiber diet (25 to 29 grams per day), 13 deaths and six cases of heart disease would be prevented." Extrapolating those numbers to a million people means 13,000 deaths could be prevented.

Fiber's evidence-based resumé vastly exceeds any touted "superfoods." It's a pity that dietary gurus rarely push fiber with the same zeal that they do unproven fad diets.

Luckily, the recipe for boosting fiber intake is fairly simple. Canned beans are cheap and contain a boatload. Whole fruits and vegetables are also prodigious sources. Seeds, nuts, and whole grains are also excellent. Even popcorn contains a ton!

A half a cup of black beans, one medium potato, an apple, an orange, a small bowl of pasta, and a cup of broccoli the fulfills the recommended daily fiber for almost everyone. Seeing as how all of that comes to just 850 calories, there's still plenty of room to eat some "unhealthy" stuff as well.