Saturday, September 25, 2010

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.


Nathaniel said...

Stephan, thank you for doing what you do. I think we all need to recognize how rare it is for someone so educated and qualified to spend his own free time writing informative blog posts.

Three cheers for potatoes, a whole food! And more delicious than any grain could ever hope to be. I can't wait for Part III.

Unknown said...

When are YOU writing a book? I'm surely not the only one who would buy it. This information is incredibly useful.

zach said...

I don't eat potatoes because they give me Horrible gas. There's an adverse effect. I can eat any other real food without any problem at all. I can even eat industrial food and not have problems in the short term. Just not potatoes.

EL 66K said...

Very interesting article, but what about the skins of the potatoes having the higher concentration of nutrients, and the most protein being close to the skins, is that true?

Also, Stephan, it's off-topic (again), but I wonder if sporadic wheat intake (like once a week pizza) can be physically damaging to someone who is not celiac, but still is sensitive to wheat. I used to have panic attacks before leaving wheat, and I wonder if a small amount could still damage me, even if there's no overt symptoms.

Chris Kresser said...

Great article, Stephan. I agree that potatoes probably aren't harmful for most people. But it's interesting that anecdotally people with gut issues tend to react to them. I see this in my clinical work with patients, and I notice that I do much better with sweet potatoes and yams than potatoes. Nothing conclusive there, but it does make me wonder what might be causing those reactions.

Chris Sturdy said...

Phew - I feel much better about the 2 bags of beautiful new potatoes I just purchased this afternoon at the Farmer's Market!

Aaron Blaisdell said...

@Chris. Remember, it's all about how you look, feel, and perform when including/excluding any particular food from your diet (a la Robb Wolf). I seem to tolerate potatoes just fine; and dairy (to an extent). But I bought a bag of freshly roasted peanuts from the farmer's market a few months ago, and after eating just a handful I became as bloated as a balloon and no fun to be around. I've definitely learned that legumes are my enemy, as are most grains, though I seem to do just fine on white rice.

Unknown said...

@ Zach: After sticking to strict Paleo for a while I had the same problem with potatoes, basically pretty bad stomach discomfort when eating and for a few hours after (no pain just rumbling, gas etc)

Funny thing is I could eat wholewheat bread and other supposedly "hard" to tolerate foods just fine. Basically I built up a tolerance to the potatoes having them on a regular basis for a while....

Now no discomfort or bloating whatsoever. Another thing is if you are boiling them make sure they are thoroughly cooked.

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TheBigThink said...

Hmmm.. very interesting. I cut grains a while ago and experimented with a carb up with sweet potato but instantly started putting on weight. I stopped after a week. Not sure what this says. Don't think I was eating more in total. I look forward to your next post

Krissie said...

Hi, wouldn't gut bacteria account for a person's a ability to tolerate the digestion of certain starches?
So, Zach's gut flora make-up responds to potatoes and Aaron's gut flora reacts to legumes?.

Stephen said...


I look forward to your blog at the end of every week. You do a good job of putting any bias you may in a box.

As to potatoes, I eat them almost all the time, usually roasted or pan fried leaving the skin on (I'm too lazy to peal them), with no negative side effects.


DogwoodTree05 said...

Makes sense that the toxins would be concentrated in the skin, but isn't that also where the nutrients are concentrated? I recall reading that plant parts with edible skins should not be peeled in order to maximize nutrition. I quit eating potatoes, but when they were still on the menu, I'd choose smaller ones and eat them with the skins, so I'm curious to know if the skins do contain more nutrients.

Chris Keller said...

Stephan, I was wondering, aren't most of the nutrients in the skin? At least the minerals? Obviously in a sweet potato the beta-carotene is in the flesh, but I thought skins were the most nutritious part of the food!

Innovator said...

Hi Stephan,

Please allow me to note for readers here, who may not have read your previous posts and my comments there, that a simple explanation for any effects of glykaloids -- or for any effects of any food -- is this:

Modern diets have more fructose in than our "natural limit" (i.e., when there is more 45/55 fructose to glucose, and/or more than 25g of fructose per sitting).

The observable result of excess fructose is that there is an adverse change in the gut bacteria. In turn, adverse changes in gut bacteria correlate to every metabolic disorder from fat gain to cardiovascular disease.

Hypothesis: gut bacteria can adapt to, and protect us from just about everything EXCEPT an excess of fructose. Fructose is absorbed in the small intestine before the gut bacteria ever have a chance to adapt!

Apparently, there is some whole-body endocrinological mechanism at work here that we do not (and may never) understand. If we do not understand the mechanism, why do we suspect this hypothesis to be true? Two reasons:

1. Because we do observe the "natural limit" in (apparently) all functional gut disorders.

2. Because people survive nicely on all manner of arguably toxic diets, including potatoes, UNTIL the consumption of fructose is greater than the "natural limit."

Finally, there appears to be no basis in any literature to dismiss this hypothesis, or even argue against it.

Fructose is a toxic food our gut bacteria cannot adapt to directly, and thus protect us from, because they never see it coming!

Anna said...


If you raise your carb intake, you will also begin to retain some more water, which can account for at least a few pounds of "weight gain". So what you are experiencing might not truly be "fat gain".

Losing those first few pounds with a reduced carb intake are the reverse, due to a reduction in water retention.

Anna said...

Gut bacteria are really interesting and I'm paying more attention to them all the time. I think the condition of one's colonic bacteria population has a lot to do with how much gas one experiences from various plant foods, though perhaps enzyme production is also a significant factor earlier on in the digestive process, too.

I've been eating LC (low starch, little or no grain, very low sugar) continuously for 6+ yrs (initially for weight loss & management, then for BG management). My main issue with potatoes is its glucose-raising effect, so on the rare occasions I eat them, I limit them to only a couple bites.

Over the years I had gotten used to little or no intestinal gas at all, except if I ate a few bites of anything containing wheat. My husband also experienced a profound reduction in intestinal gas (except if he ate bread while dining out - it made for a very musical time once back at home - wives notice these things).

The past year or so I've been experimenting with eating plant foods that are good sources of soluble fiber such as inulin and pectin in order to better tend "my inner garden" of gut bacteria (esp those in the colon).

Gassiness was an immediate and hard-to-ignore side effect of consuming more sources of these soluble fibers than I was accustomed too. Some plants like onions and leeks produced little or no intestinal gassiness. Jerusalem artichokes, on the other hand, were at the other end of the gas-producing spectrum and had the most pronounced effect (they contain a lot of inulin). So I backed off the worst offenders and increased the others slowly to allow the colonic bacteria population a chance to adjust. If I keep the soluble fiber intake fairly "regular" (ha-ha), I'm almost almost back to the point of little or no intestinal gas again, so I assume my gut bacteria have adjusted.

I haven't again tried Jerusalem artichokes though, as I haven't seen them for sale at all - they are tasty, but wow, oh-so-potent. They would be a good test to see if my gut bacteria can handle that much inulin now, though.

So perhaps those who experience gassiness from potatoes might just need to back off and then more slowly reintroduce them to see if it that helps.

Colldén said...

I always eat potatoes with the skin on and haven't noticed any adverse effects. How is glycoalkaloid content affected by cooking? Wouldn't a large part leak out into the boiling water?

Dave said...

Thanks for the potato articles - due mainly to this new information, I've tried swapping some fruits and veg. for potatoes with good results. My goal is low fructose, and potatoes, especially Russett's, are very low in fructose.

My main concern with potatoes at this point is acrylamide - I'd love to see some discussion on that.

Nathaniel said...

Acrylamide formation is temperature dependent, and as far as I know it has mostly been found in deep-fried potato products such as chips and French fries. And, as far as I know, it has not yet been proven to be harmful when consumed.

Acrylamide is one of those advanced glycation end-products which are known to be bad when produced in the body, but are not yet proven to be harmful when present in food.

Obviously further research is necessary but I don't see any reason to worry about it yet, and even if you are, you can still enjoy mashed potatoes.

Innovator said...

Hi Ned,

Looking over your post on "fructose in fruits is good for you." Nice work, lots of good links, and considerable controversy!

In the meanwhile, I want to caution readers that fructose in fruits is probably NOT harmless, UNLESS you are consuming a few certain fruits (those with a fructose to glucose ratio of less than 45/55) AND IF the total fructose consumed is less than 25g in a sitting.

Based on what we know so far, fructose may well be the most toxic food found is super-abundance in our modern diet. Avoiding fructose altogether is quite easy and certainly harmless, until further studies can be done.

Stephan Guyenet said...

Hi Melissa,

Thanks. I decided a while back that I was emotionally committed to writing a book. But since then, I've been buried in an avalanche of work. I also don't want to go off half-cocked. I think there will be one eventually, but probably not for a while.

Hi EL 66K, Dogwood and Chris Keller,

The skins do contain a disproportionate amount of protein, fiber, and micronutrients. But they also contain the toxins. Traditional cultures tend to peel their potatoes, and so do I.

Art Ayers has argued that modern commercial potatoes have so little glycoalkaloid that it doesn't matter. He may be right, I don't know how the new varieties compare to the old. The more potatoes contribute to the diet, the more it matters. It's probably irrelevant if you're only eating a few potatoes a week.

Hi Chris Kresser,

Thanks for your perspective. I'm going to acknowledge in the next post that some people react poorly to potatoes.

Stephan Guyenet said...

Hi Collden,

Cooking has very little impact on glycoalkaloids.

Kimi Harris said...

I've been looking forward to your next post on potatoes. Your posts are always interesting! I find it very intriguing that the skins contain both more nutrients and the toxins. It reminds me a bit of grains and how the bran has the "anti-nutrients" as well as more nutrients. But it certainly makes me feel a bit better about not eating the peel (I've never liked them).

Have you looked into the topic of nightshades and arthritis? I would be curious to see what you had to say about it.

Kimi @ The Nourishing Gourmet

Riceball said...

great work again, Stephan.

I just wanted to say that you might be interested in an article prof. Harriet Friedmann had written for the book "Food and Agrarian Orders in the World- Economy." her article is called "Food Politics: New Dangers, New Possibilities". She is a sociology professor mainly interested in Agriculture.This article partly talked about the history of potato and a few other foods that have become the western staple food; and what is the relationship between these food and the social structure we have in our economies.
It's another perspective looking at food. in her opinion, this food was driven by economic and political powers to become the staple food, rather than through a natural process of people selecting food.

I always think, even this food doesn't harm us as much, when it wasn't usually eaten before, then why do we have to eat it now? I eat potatoes, but pretty rarely.

thanks for your post, it is always a pleasure to read your work. =)

L said...

ja deep

could you explain how you arrived at your fructose limits or post links to studies?


Innovator said...

Hi L.

The proposed limit for fructose comes from a study here.

EL 66K said...

Stephan, If you remove just the skin, and nothing at all below, like you can when you peel boiled potatoes, is that useful?

Also, Stephan, it's off-topic (again), but I wonder if sporadic wheat intake (like once a week pizza) can be physically damaging to someone who is not celiac, but still is sensitive to wheat. I used to have panic attacks before leaving wheat, and I wonder if a small amount could still damage me, even if there's no overt symptoms.

Amy said...

J.A. Deep
I find your discussion on fructose interesting. After reading your comments on Whole Health Source, I went and read everything I could find about fructose malabsorbtion. After a month of testing I can confirm that I was misdiagnosed with intestinal bowel syndrome with constipation. Instead, I have fructose malabsorption.

I'd like to think that because I have fructose malabsorption that I'm going to be healthier, but I think there is still a lot of research to be done on the topic. One of the problems is that even in the fructose malabsorption world people are affected differently. For instance: some people have more issues with eating fructans ( wheat, onion) while other people have more trouble when they have too much fructose in their diet. Some people can have problems eating too many potatoes or too much table sugar. Therefore, telling people to cut out too much fructose is incomplete or to worry about fructans is incomplete. We're all different.

I've tried and tried to make my body digest fermented beans and brown rice, but after 6 years of trying various home remedies available for constipation, I've come to accept that I don't have the digestive system to process very many different types of foods.

Low carb was excellent for weight loss, but bad on my blood sugars (higher than normal) and horrible on my digestion. I was eating things like cabbage, onions, leeks, peppers, kale, and romaine lettuce that I can't digest.

I'm very miffed that I'm completely regular on a diet of meat and potatoes or worse, a diet of sweet tarts (fake glucose candy).

I am concerned that more and more people are going to suffer from fructose malabsorption. For some people it may be genetic, for others it could be environmental reasons. I believe fake sugars, low-fat, mega doses of onion powder and an over abundance of fresh fruit, are making fructose malabsorption more common. I remember when most people I knew who had diabetes were children with type 1. Now, more and more people over the age of 65 have type 2 diabetes.

But, thank you J.A. Deep for pointing out that some people can have their digestive issues cured by eliminating a lot of "healthy foods".

Greg Truthseeker said...

One thing potatoes are high in, however, is oxalic acid. This is the reason I personally can't eat them. I am very sensitive to oxalates, so much so that when I eat a high oxalate food I get burning sores in my mouth.

I'm just sayin'... there are other things in potatoes that can be troublesome...

Stephan Guyenet said...

Hi Greg,

Why do you say potatoes are high in oxalic acid? From what I can tell, they contain very little of it. Are you thinking of sweet potatoes?

Rene Pinilla said...

Great... I didnt know these facts.... I realy liked your post...

Chris Masterjohn said...

Great posts, Stephan.

EL 66K, perhaps you are reacting to the opioids generated by wheat digestion. Wheat gluten and A2 casein found in most modern milk digest into tripeptides that activate opioid receptors and presumably can have psychoactive effects. Presumably people with lower levels of the enzyme DPP-IV, which digests these tripeptides (I don't know too much about their presence in the digestive tract, however); lower secretions in digestive fluid of sugars such as N-acetyl-glucosamine and mannose, which bind harmful wheat proteins, and/or lower secretions of protective IgA antibodies to such proteins are more vulnerable.

There is some evidence that foods containing histamine or chemicals that promote histamine release contribute to panic attacks. Histamine promotes inflammation in peripheral tissues but promotes alertness in the brain, and at high brain levels will cause panic attacks. So an allergy could also be involved.


EL 66K said...

Thank you Chris! Interestingly, I also react to dairy and get "addicted" to it (even goat milk). I wonder if there's a solution to these problems, other than simple avoidance, specially to diary.

Chris Masterjohn said...

Hey EL66K,

Well, the most effective solution in the case of dairy is to find A2 milk. I made a mistake by saying that A2 casein generates opioids when it is A1 casein. The problem is that no one is regularly testing their cows for this so it is probably unrealistic. There is an "A2 Corporation" that does this type of testing but I think they are based out of Austrailia and I don't know how popularly their services are used.

Another thing you could try is supplementing with DPP-IV enzyme, which is supposed to digest the tripeptides.

There may be certain forms of processing such as fermentation that digest the opioids but I have not read any literature on this and I doubt any particular method is 100% effective. Ultimately, you'll probably be forced into trial and error if you want to attempt to find a fix to this apart from avoidance.

Hope that helps,

Robert Andrew Brown said...

Hi J A Deep

I have looked at your fructose refs with interest.

Do you know where the 25 grams figure comes from.

Is that per hour.

I found this
which seems to me to sugest that if human digestion can be related to pigs that it is possible we can absorb more.

Any thoughts would be very welcome.

Robert Andrew Brown said...

^ Ooops just found your ref to paper above,

but would very much appreciate your thoughts on link I posted

Many thanks fro highlighting this important issue.

UPower said...

Your chart on glycoalkaloid content of potato varieties is critical, fascinating! Do you know of any similar info on other nightshades, eg peppers and tomatoes? I'm curious to see relative contents and have searched EVERYWHERE. Your chart is by far the best info so far, although just for potatoes.
Thank you for an insightful article.