What carbs are safe on a low-carb diet?

Are there any carbs that, unlike, grains and sugars, are actually healthy? Well, yes and no. Yes: there are sources of carbohydrates that are absolutely essential for human health but no: overdoing them yields undesirable effects.

To understand this issue, we need to understand a quirk of nutritional definitions. All fibers, regardless of type—insoluble, soluble, prebiotic, cellulose, arabinoxylan, amylose, fructooligosaccharide, isomaltooligosaccharide, galactooligosaccharide, etc.—are all chains, or polymers, of sugars. Most people don’t think of fibers as sugars but, biochemically speaking, fibers are indeed sugars. And virtually all fibers are sourced from plants, not from animal products.

But humans lack the digestive enzymes to break fibers down into their component sugars. Fibers thereby have no blood sugar-raising nor insulin-provoking effects unlike glucose, fructose, sucrose and other sugars. Some fibers, such as cellulose, are inert, passing from mouth to toilet untouched, undigested, unchanged. Other fibers such as fructooligosaccharide or inulin are not broken down by human enzymes but are broken down by bacterial enzymes and converted to various metabolites, such as the fatty acid, butyrate, that yields numerous health benefits.

There is no intrinsic human need for sugar (contrary to silly and outdated nutritional advice). But there is most definitely a human need for fiber, especially of the prebiotic variety, i.e., fibers that are metabolized by bowel flora. Take a low-carb diet too far and take in too little prebiotic fiber and, over time, peculiar things happen. In particular, the bowel flora species Akkermansia muciniphila and several other species, ordinarily beneficial, become overpopulated. It is believed that the ideal proportion of Akkermansia is around 3.5-5.0% of the total microbiome population; lack of prebiotic fibers can send it up towards 8-9%. This is because Akkermansia is unique in that, deprived of prebiotic fibers, it can thrive on human intestinal mucous and survive while most other species cannot. But this means that the intestinal mucous lining will be degraded by excessive Akkermansia populations, setting you up for intestinal inflammation, increased intestinal permeability, and metabolic endotoxemia, the process that leads to inflammation elsewhere in the body (and resulting in fibromyalgia, autoimmune diseases, neurodegenerative diseases, etc.). There is also a loss of species diversity when sufficient prebiotic fibers are not taken in. Recall that health is marked by wide species diversity.

We therefore require the carbohydrates that are prebiotic fibers. Ideally, you take in 20 grams or more per day, the level at which maximum health benefits develop. (Here’s a list of sources and prebiotic fiber content.) Problem: While there are some low-carb sources of prebiotic fibers such as a raw white potato (zero net carbs), green unripe banana (zero net carbs), inulin (0.2 grams net carbs per teaspoon), and dandelion greens (3.2 grams net carbs per cup), many other sources can be rich in net carbs. For example, one cup of sliced turnips contains 17 grams net carbs, while one cup of kidney beans contains 64 grams net carbs. Recall that, in the Wheat Belly lifestyle, we try not to exceed 15 grams net carbs per meal to avoid provocation of insulin, insulin resistance, formation of small LDL particles, liver de novo lipogenesis that causes fatty liver and increased blood triglycerides.

The key with carb-rich sources of prebiotic fibers is therefore portion size, as well as gravitating towards lower carb choices. In the world of legumes, rich sources of the galactooligosaccharide prebiotic fiber, for instance, white beans are the richest source of prebiotic fibers with around 3 grams prebiotic fibers per 1/4 cup and 8.4 grams net carbs.

The carbs that are therefore absolutely essential for health are those from prebiotic fibers. We don’t think of fibers as carbohydrates, but indeed they are. And, without them, adverse bowel flora and health changes develop.