Two things I’ve changed my mind about, part II: Phytic acid

Phytic acid is a substance found primarily in whole grains, beans, and nuts that reduces the absorption of specific minerals from food.  I previously wrote that minimizing phytic acid may be an important part of a healthy diet, but new evidence—and a reexamination of old evidence—has convinced me that it probably isn’t as important as I initially thought.  At least in the context of a diverse, omnivorous diet.

Phytic acid is a small molecule found in seeds like grains, beans, and nuts that binds (chelates) certain essential minerals—particularly calcium, iron, magnesium, and zinc—and reduces their absorption from food.  What this means is that the nutritional value of these foods isn’t as high as you might expect if you looked them up in a nutritional database.  Many traditionally-living cultures with grain-heavy diets used techniques such as soaking, grinding, and fermentation that reduce phytic acid levels and increase mineral availability (1).  Let me explain why I think phytic acid is less of a nutritional concern than I used to.

The human gut adapts somewhat to phytic acid-rich food

The small intestine of certain animals produces phytase, an enzyme that breaks down phytic acid and releases the minerals it binds.  Humans are among these animals.  However, until recently it was thought that the production of phytase by the human small intestine is too low to have much effect (2).  A 2015 study suggests that in fact, our intestine can ramp up phytase production in response to a diet rich in phytic acid, and this eventually helps us absorb more minerals from seeds like whole grains (3). [update 5/30: commenter Carl pointed out that the mechanism isn’t necessarily an increase in phytase production.  Iron absorption increased over time, but the study didn’t directly demonstrate that phytase was responsible.]

That doesn’t mean phytic acid has no effect on mineral absorption in the long term—it still does (4).  But our intestines are able to adapt over time and mitigate this effect, meaning that phytic acid eventually becomes less important.

Most people have bigger problems than phytic acid

The more I learn about health, the more I tend to focus my attention on a few simple factors that I think account for most of the health benefits of diet and lifestyle.  Don’t smoke cigarettes or use alcohol excessively.  Get regular physical activity.  Eat whole foods.  Get restorative sleep.  Manage stress.  And don’t eat too many calories.  These guidelines may not be very exciting, but they deliver a lot more value than the details that often distract us.

The US diet, and the diets of most other affluent nations, are a nutritional disaster in some ways.  Although we’ve largely solved the problems of frank nutrient deficiency and starvation, we grow fat and undermine our health by eating calorie-dense processed and refined foods.  The number one source of calories in the diet of Americans of all ages is “grain-based desserts” like cake, cookies, donuts, and pastries (5).  Pizza, soda, and alcohol are also in the top six.  The diets of most other affluent nations aren’t much better– nearly all of them are high in refined starch, sugar, added fats, and convenience foods.

Considering this context, avoiding phytic acid is simply not high on my priority list anymore.  Replacing refined and processed foods with unrefined foods is more important, and if that means a higher intake of phytic acid, then so be it.

A diverse omnivorous diet is robust to modest changes in mineral availability

Diets that are overly reliant on whole grains can get people into nutritional trouble—and that is at least in part due to phytic acid.  Populations with a high intake of unfermented whole grains sometimes suffer from nutritional problems including deficiency of calcium and zinc leading to rickets, osteomalacia, and stunting (6, 7).  And this doesn’t just happen to poor rural farmers—it also happens to affluent people following the macrobiotic diet, which is an extreme type of vegan diet based largely on brown rice, vegetables, and soy (8).

But there’s a simple solution to this nutritional problem: eat a diverse diet.  Meat, eggs, vegetables, potatoes, and particularly dairy contain easily absorbed minerals that complement the shortcomings of whole grains.

We often think of food in terms of the essential nutrients it provides, and we tend to intuitively assume that more is better.  But despite a large volume of research, there is hardly any evidence that exceeding an adequate intake of essential vitamins and minerals is beneficial.  If it were, multivitamin pills would be the bee’s knees—but the evidence overall suggests that they add little or no value to the average affluent diet* (9).  Given these findings, I’m even more skeptical that they add value to a whole-food-based diet for most people.

The point is that unless your diet is based primarily on unfermented whole grains, if you eat a nutrient-dense diet you can probably get away with a lower absorption rate of certain minerals without any ill effects.  And few people in affluent nations eat a diet that’s based primarily on unfermented whole grains.

Phytic acid remains a nutritional concern among people whose diets are based primarily on unfermented whole grains and who eat little or no animal foods.  This includes people from parts of India or Pakistan who rely heavily on unleavened whole wheat chappatis, as well as people following a vegan diet, particularly a macrobiotic vegan diet.  This follows a general nutritional principle: the more restricted your diet, the more carefully you have to compose it to achieve adequate nutrition.

Conclusion

Phytic acid is a substantial nutritional concern in people with a high intake of whole grains and a low intake of animal foods, but it’s probably much less important in the context of a diverse, omnivorous diet.  In countries like the US, most of us have more pressing things to worry about than how much phytic acid we’re eating, and in my opinion, focusing on whole foods is a higher priority.  I now often eat regular rolled oats for breakfast, but I tend to eat it with plain yogurt to make up for its high content of phytic acid.  I don’t worry about the phytic acid in beans and nuts.

 

*There are a few exceptions.  Folic acid supplementation decreases the risk of neural tube defects in infants.  But these are fairly rare to begin with (less than 1 in 1,000 live births in the US; 10).  Vitamin D supplementation can prevent rickets and perhaps reduce the risk of respiratory tract infections (11).

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