Potatoes and Human Health, Part II

Glycoalkaloids in Commonly Eaten Potatoes

Like many edible plants, potatoes contain substances designed to protect them from marauding creatures. The main two substances we're concerned with are alpha-solanine and alpha-chaconine, because they are the most toxic and abundant. Here is a graph of the combined concentration of these two glycoalkaloids in common potato varieties (1):

We can immediately determine three things from this graph:

• Different varieties contain different amounts of glycoalkaloids.
• Common commercial varieties such as russet and white potatoes are low in glycoalkaloids. This is no accident. The glycoalkaloid content of potatoes is monitored in the US.
  
• Most of the glycoalkaloid content is in the skin (within 1 mm of the surface). That way, predators have to eat through poison to get to the flesh. Fortunately, humans have peelers.

I'll jump the gun and tell you that the generally accepted safe level of potato glycoalkaloids is 200 mcg/g fresh weight (1). You can see that all but one variety are well below this level when peeled. Personally, I've never seen the Snowden variety in the store or at the farmer's market. It appears to be used mostly for potato chips.

Glycoalkaloid Toxicity in Animals

Potato glycoalkaloids are undoubtedly toxic at high doses. They have caused many harmful effects in animals and humans, including (1, 2):

• Death (humans and animals)
  
• Weight loss, diarrhea (humans and animals)
  
• Anemia (rabbits)
• Liver damage (rats)
  
• Lower birth weight (mice)
  
• Birth defects (in animals injected with glycoalkaloids)
  
• Increased intestinal permeability (mice)

However, it's important to remember the old saying "the dose makes the poison". The human body is designed to handle a certain amount of plant toxins with no ill effects. Virtually every plant food, and a few animal foods, contains some kind of toxic substance. We're constantly bombarded by gamma rays, ultra violet rays, bacterial toxins, free radicals, and many other potentially harmful substances. In excess, they can be deadly, but we are adapted to dealing with small amounts of them, and the right dose can even be beneficial in some cases.

All of the studies I mentioned above, except one, involved doses of glycoalkaloids that exceed what one could get from eating typical potatoes. They used green or blemished potatoes, isolated potato skins, potato sprouts or isolated glycoalkaloids (more on this later). The single exception is the last study, showing that normal doses of glycoalkaloids can aggravate inflammatory bowel disease in transgenic mice that are genetically predisposed to it (3)*.

What happens when you feed normal animals normal potatoes? Not much. Many studies have shown that they suffer no ill effects whatsoever, even at high intakes (1, 2). This has been shown in primates as well (4, 5, 6). In fact, potato-based diets appear to be generally superior to grain-based diets in animal feed. As early as 1938, Dr. Edward Mellanby showed that grains, but not potatoes, aggravate vitamin A deficiency in rats and dogs (7). This followed his research showing that whole grains, but not potatoes, aggravate vitamin D deficiency due to their high phytic acid content (Mellanby. Nutrition and Disease. 1934). Potatoes were also a prominent part of Mellanby's highly effective tooth decay reversal studies in humans, published in the British Medical Journal in 1932 (8, 9).

Potatoes partially protect rats against the harmful effects of excessive cholesterol feeding, when compared to wheat starch-based feed (10). Potato feeding leads to a better lipid profile and intestinal short-chain fatty acid production than wheat starch or sugar in rats (11). I wasn't able to find a single study showing any adverse effect of normal potato feeding in any normal animal. That's despite reading two long review articles on potato glycoalkaloids and specifically searching PubMed for studies showing a harmful effect. If you know of one, please post it in the comments section.

In the next post, I'll write about the effects of potatoes in the human diet, including data on the health of traditional potato-eating cultures... and a curious experiment by the Washington State Potato Commission that will begin on October 1.


*Interleukin-10 knockout mice. IL-10 is a cytokine involved in the resolution of inflammation and these mice develop inflammatory bowel disease (regardless of diet) due to a reduced capacity to resolve inflammation.

Potatoes and Human Health, Part I

Potatoes: an Introduction

Over 10,000 years ago, on the shores of lake Titicaca in what is now Peru, a culture began to cultivate a species of wild potato, Solanum tuberosum. They gradually transformed it into a plant that efficiently produces roundish starchy tubers, in a variety of strains that suited the climactic and gastronomic needs of various populations. These early farmers could not have understood at the time that the plant they were selecting would become the most productive crop in the world*, and eventually feed billions of people around the globe.

Wild potatoes, which were likely consumed by hunter-gatherers before domestication, are higher in toxic glycoalkaloids. These are defensive compounds that protect against insects, infections and... hungry animals. Early farmers selected varieties that are low in bitter glycoalkaloids, which are the ancestors of most modern potatoes, however they didn't abandon the high-glycoalkaloid varieties. These were hardier and more tolerant of high altitudes, cold temperatures and pests. Cultures living high in the Andes developed a method to take advantage of these hardy but toxic potatoes, as well as their own harsh climate: they invented chuños. These are made by leaving potatoes out in the open, where they are frozen at night, stomped underfoot and dried in the sun for many days**. What results is a dried potato with a low glycoalkaloid content that can be stored for a year or more.

Nutritional Qualities

From a nutritional standpoint, potatoes are a mixed bag. On one hand, if I had to pick a single food to eat exclusively for a while, potatoes would be high on the list. One reason is that they contain an adequate amount of complete protein, meaning they don't have to be mixed with another protein source as with grains and legumes. Another reason is that a number of cultures throughout history have successfully relied on the potato as their principal source of calories, and several continue to do so. A third reason is that they're eaten in an unrefined, fresh state.

Potatoes contain an adequate amount of many essential minerals, and due to their low phytic acid content (1), the minerals they contain are well absorbed. They're rich in magnesium and copper, two minerals that are important for insulin sensitivity and cardiovascular health (2, 3). They're also high in potassium and vitamin C. Overall, they have a micronutrient content that compares favorably with other starchy root vegetables such as taro and cassava (4, 5, 6). Due to their very low fat content, potatoes contain virtually no omega-6, and thus do not contribute to an excess of these essential fatty acids.

On the other hand, I don't have to eat potatoes exclusively, so what do they have to offer a mixed diet? They have a high glycemic index, which means they raise blood sugar more than an equivalent serving of most carbohydrate foods, although I'm not convinced that's a problem in people with good blood sugar control (7, 8). They're low-ish in fiber, which could hypothetically lead to a reduction in the number and diversity of gut bacteria in the absence of other fiber sources. Sweet potatoes, an unrelated species, contain more micronutrients and fiber, and have been a central food source for healthy cultures (9). However, the main reasons temperate-climate cultures throughout the world eat potatoes is they yield well, they're easily digested, they fill you up and they taste good.

In the next post, I'll delve into the biology and toxicology of potato glycoalkaloids, and review some animal data. In further posts, I'll address the most important question of all: what happens when a person eats mostly potatoes... for months, years, and generations?


* In terms of calories produced per acre.

** A simplified description. The process can actually be rather involved, with several different drying, stomping and leaching steps.

Speaking at Wise Traditions 2010

I'm happy to announce that I'll be presenting at the Weston A. Price foundation's 2010 Wise Traditions conference. The conference will be held in King of Prussia, Pennsylvania, Nov 12-14. The theme is the politics of food.

Sally Fallon Morell has invited me to give a talk on the diet and health of Pacific islanders. The talk will be titled "Kakana Dina: Diet and Health in the Pacific Islands", and it will take place on Sunday, November 14th from 4:00 to 5:20 pm. In preparation for the talk, I've read eight books and countless journal articles. Although some of the material will be familiar to people who follow the blog, I will not be rehashing what I've already published. I have nearly an hour and a half to talk, so I'll be going into some depth on the natural history and traditional food habits of Pacific island populations. Not just macronutrient breakdowns... specific foods and traditional preparation methods.

Learn about the health of traditional Pacific island populations, and what has changed since Western contact. Learn about traditional cooking and fermentation techniques. See unpublished photos from the Kitava study, courtesy of Dr. Staffan Lindeberg. Learn about the nutritional and ceremonial role of mammals including pork... and the most gruesome food of all.

I hope to see you there!


Kitava photo courtesy of Dr. Staffan Lindeberg

Dogen Zenji on Nutritionism

Dogen Zenji was the man who brought the Soto lineage of Zen Buddhism to Japan. He was a prolific writer, and many of his texts are respected both inside and outside the Soto Zen community. Last week, my Zen group was discussing the Genjo Koan, one of his works that is frequently used as a chant. Here's an excerpt. It may seem cryptic but bear with me:

...when you sail out in a boat to the middle of an ocean where no land is in sight, and view the four directions, the ocean looks circular, and does not look any other way. But the ocean is neither round or square; its features are infinite in variety... It only look circular as far as you can see at that time. All things are like this.

Though there are many features in the dusty world and the world beyond conditions, you see and understand only what your eye of practice can reach. In order to learn the nature of the myriad things, you must know that although they may look round or square, the other features of oceans and mountains are infinite in variety; whole worlds are there. It is so not only around you, but also directly beneath your feet, or in a drop of water.

What Dogen meant, among other things, is that the world is much more complex than what our conscious minds can perceive or understand. It was true in the 13th century, and it's still true today, despite our greatly expanded understanding of the natural world.

We can apply this principle to nutrition. For example, what is red palm oil? Two hundred years ago, perhaps we only knew a few basic facts about it. It's a fat, it's red, it comes from an African palm fruit and it has a particular melting point and flavor. Then we learned about vitamins, so we knew it contained vitamin E, carotenes (provitamin A), and vitamin K. Then fatty acid composition, so we found out it's mostly palmitic and oleic acids. Now we know red palm oil contains an array of polyphenols, sterols, coenzyme Q10 and many other non-essential constituents. We don't know much about the biological effects of most of these substances, particularly in combination with one another.

Add to that the fact that every batch of red palm oil is different, due to strain, terroir, processing, storage, et cetera. We know what the concept "red palm oil" means, roughly, but the details are infinitely complex. Now feed it to a human, who is not only incredibly complex himself, but genetically and epigenetically unique. How can we possibly guess the outcome of this encounter based on the chemical composition of red palm oil? That's essentially what nutritionism attempts to do.

To be fair, nutritionism does work sometimes. For example, we can pretty well guess that a handful of wild almonds containing a lot of cyanide won't be healthy to eat, due at least in part to the cyanide. But outside extreme examples like this, we're in a gray zone that needs to be informed by empirical observation. In other words, what happens when the person in question actually eats the red palm oil? What happened when a large group of people in West Africa ate red palm oil for thousands of years? Those questions are the reason why I'm so interested in understanding the lives of healthy non-industrial cultures.

I'm not criticizing reductionist science or controlled experiments (which I perform myself); I just think they need to be kept in context. I believe they should be interpreted within the framework of more basic empirical observations*.

One of the most important aspects of scientific maturity is learning to accept and manage uncertainty and your own ignorance. Some things are more certain than others, but most aren't set in stone. I think Dogen would tell us to be wary of nutritionism, and other forms of overconfidence.

* Wikipedia's definition of empirical: "information gained by means of observation, experience, or experiment." As opposed to inferences made from experiments not directly related to the question at hand.

The China Study on Wheat

Denise Minger has just put up another great China Study post that's worth reading if you haven't already. Denise has been busy applying her statistics skills to the mountain of data the study collected. She noted in a previous post that wheat intake was strongly associated with coronary heart disease (CHD), the quintessential modern cardiovascular disease. I, and several other people, requested that she work her mathmagic to see if the association could be due to some other factor. For example, wheat is eaten mostly in the Northern regions of China, and CHD rates are generally higher at higher latitudes (vitamin D insufficiency?). This is true in Europe as well, and may be partly responsible for the purported benefits of the Mediterranean diet. You can mathematically determine if the association between wheat and CHD is simply due to the fact that wheat eaters live further North.

To make a long story short, nothing could explain the association except wheat itself, even latitude. Furthermore, she found a strong association between wheat intake and body mass index, typically a predictor of fat mass although we can't say that for sure. That finding echos a previous study in China where wheat eaters were more likely to be overweight than rice eaters (1, 2). Head over to Denise's post for the full story.

The China Study has major limitations built into its basic design, due to the fact that it was observational and pooled the blood samples of many individuals. Therefore, its findings can never prove anything, they can only suggest or be consistent with hypotheses. However, the study also has some unique advantages, such as a diversity of diets and regions, and the fact that people had presumably been eating a similar diet for a long time. I feel that Denise's efforts are really teasing out some useful information from the study that have been de-emphasized by other investigators.

There has been so little serious investigation into the health effects of wheat in the general population that I have to rely mostly on indirect evidence, such as the observation that the diseases of civilization tend to closely follow the introduction of white flour around the globe. Researchers studying celiac disease and other forms of gluten allergy, and the efforts of the paleolithic diet community in spreading that information (for example, Loren Cordain and Pedro Bastos), have been major contributors to understanding the health effects of wheat. Denise's analysis is one of the strongest pieces of evidence I've come by so far. One of these days, I'll post all of my references incriminating wheat. There are quite a few, although none of them is the smoking gun. I think there's enough indirect evidence that investigators should begin taking the idea seriously that wheat, particularly in the form of industrial flour products, may contribute to chronic disease in more than just a small subset of the population.