Diabetes Diet

Natto is a Low-Carb Food

When I decided in 2007 that it was healthy and safe to eat nothing but low-carb meals, I wasn’t sure if I could do it for a long time. I wondered if my diet would have enough variety to remain interesting.

After all, I had stopped eating more than half the products that all our supermarkets sell. No more bread, bagels, croissants, pasta, pizza, or other products made from wheat. No more rice, corn, potatoes, and other starchy foods.

“I imagined that a very-low-carb diet would feel limiting and boring,” I wrote in “Why I Low-Carb” for Diabetes Self-Management in 2008. “I used to lust after bread and potatoes and rice. But that ‘carb lust’ passed away as I broke my addiction to it.”

Now, many years after going low-carb, I still love the great variety of foods that I find to eat. Yes, my diet really does have a great variety as I learn about different foods all the time.
My newest discovery is a type of fermented soybean that is little known in this country but is popular in its native Japan. It’s called natto.

Other fermented foods from Asia that are better know here are kimchi from Korea, miso, made in Japan by fermenting rice, barley, and/or soybeans, and tempeh, the Indonesian fermentation of soybean cake.

Unlike tempeh, however, the soybeans in natto look like soybeans, not like a cake. For centuries people in Japan have made natto by adding a good bacteria called Bacillus natto to boiled or steamed soybeans.

This probiotic makes sticky strings around the soybeans. Its essential stickiness comes from its beneficial amino acid. This glutamic acid is responsible for the interesting taste of both natto and of many strong cheeses.

While I love the taste of the natto that I eat, other people have written that it is “an acquired taste.” But that’s because until recently only previously frozen natto was available here.

Now, however, a company in northern California called Japan Traditional Foods Inc. offers unfrozen, refrigerated natto. For several months this company has been shipping their “Megumi Natto” to stores around the country, which they list at their “Where to Buy” page. You can also buy it online directly from the company on their “order” page.

Traditionally, people in Japan eat natto with rice, which is too high in carbs for people with diabetes. I sometimes eat mine as a snack with some Dijon mustard and a little soy sauce. But usually I eat it as one of the tasty ingredients in the salad that I have almost every day for lunch.

Natto is healthy, unlike unfermented soy products like soy milk, tofu, soybean oil, and soy burgers, which are made from texturized vegetable protein (TVP) and/or soy protein isolate. Unfermented soybeans have lots of antinutrients, which can cause inflammation, the most likely reason why people following a paleo diet won’t eat them.

One of the special health benefits of natto is the large amount of vitamin K2 that it has. Vitamin K2 assists if the formation of bones and helps prevent osteoporosis. Natto can prevent or dissolve blood clots that contribute to heart disease. Since natto contains high levels of vitamin K, it could conflict with the medical effects of anti-coagulant medications such as warfarin, sold under brand names including Coumadin.

Soybeans generally have a very low-glycemic index, which will increase our blood glucose levels hardly at all. Listed as “soya beans” in the definitive tables of glycemic index and glycemic load that I have permission to include on my website, their glycemic index is 15 to 20 (where white bread is 100).

But at first glance soybeans don’t appear to be low carb. Unfermented soybeans are in fact not low carb. But natto is.

A 3-ounce package of Megumi Natto has 12 grams of total carbohydrate, which is equal to the total amount of carbs I allow myself for lunch or dinner. I follow the recommendations of Dr. Richard K. Bernstein, whose book, Dr. Bernstein’s Diabetes Solution, is the best guide I know for controlling my diabetes and weight.

How could those 12 grams of carbohydrates be called low-carb? Because they mostly aren’t there after fermentation.

The book, Four Corners Diet, explains what happens with the fermented food we all know and love, yogurt. Written by Drs. Jack Goldberg and Karen O’Mara with my friend and colleague Gretchen Becker, this book explains how fermentation coverts milk sugar into lactic acid, which doesn’t raise our blood glucose levels and isn’t carbohydrate.

“For a standard 8-ounce container of plain yogurt, which usually says it has about 12 grams of carbohydrates, you need to county only 4,” they write. “This is not just speculation. Dr. Goldberg has actually measured the carbohydrate content of yogurt in his own laboratory.”

So I asked him if the carb reduction via fermentation was equivalent to that of yogurt and kefir, then would the residual total carb content be about 4 grams (per page 229 of is book). “Does that sound about right to you?”

“Sounds like the right ballpark,” he replied.

Meanwhile, Gretchen discovered two test results reported in chemistry books. KeShun Liu writes in Soybeans: Chemistry, Technology and Utilization that “during natto fermentation…soluble carbohydrate almost completely disappeared.” Another book, however, reported that “during an 18-hr natto fermentation, the reducing sugars decrease by about 15% after the sixth hour.”

Obviously, bacterial strains and conditions could make a difference. Perhaps the difference in the two books is that one is talking about sugars and the other about sugars and starch.

To resolve the question as well as possible I tested what effect eating natto had on my blood glucose. I tested three separate mornings on an otherwise empty stomach. Two hours after the first bite of plain natto, my blood glucose level went from an average of 83 mg/dl to 81 mg/dl. As a result I am convinced that natto has earned a prized place on my very low-carb diet.

Now that natto is an important part of my diet it helps me to answer my original question whether what I eat would be varied enough to be interesting. Natto for one is indeed an interesting, healthy, and tasty food.

This article is based on an earlier version of my article published by HealthCentral.

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  • Dale Neff at

    I have read that natto is especially good if you are taking vitamin d supplements as taking d by itself may lead to an increased risk of heart disease. (Vitamin K2 and the Calcium Paradox: by Kate Rheaume-Bleue). But I also think natto should not be your sole source of k2. I think eating grass fed beef, omega 3 or free range eggs and organ meats are also necessary because k2 from bacteria MK7 is different from that derived from animals Mk4.

  • Don at

    Hello David,

    Do you have any concerns, or information, on the alleged estrogen-mimicking qualities of soy? Thanks much!

    • David Mendosa at

      Dear Don,

      Yes, I do indeed have concerns about soy’s estrogen-mimicking qualties — when the soy is unfermented. Concerns yes, proof no.