Are Nightshade Vegetables Inflammatory?

As a climber, you probably think about inflammation a fair bit more than the average person—it’s hard not to when your fingers feel achy and swollen from the rigors they undergo on the wall. You may also have wondered if there are any dietary interventions you could use to reduce inflammation, or even prevent it in the first place.

Then, you hear from a friend (or a friend of a friend) who’s interested in nutrition that a dastardly group of veggies known as the nightshade vegetables could be exacerbating your joint swelling and pain. So, with a heavy heart you decide to give up tomatoes and chili peppers, potatoes and eggplants, not to mention spices like paprika and red chili flakes, and consign yourself to a life free of Mexican, Italian, and Thai food.

Sure, it’s hard, but at least your joint pain is guaranteed to go away, right?

Right?!

Well… frankly, that would be miraculous. Nightshade veggies are about as inflammatory as any other vegetables—which is to say not inflammatory at all—and the logic behind avoiding them is as strong as the choss you carefully avoid.

Why Do People Think Nightshades Are Inflammatory?

Why did people ever start believing nightshades are inflammatory in the first place? Perhaps it’s easier to begin with why someone might think nightshade veggies are bad at all.

Nightshades belong to the family Solanaceae, and a common trait to this family is the production of “alkaloid” phytochemicals. Different nightshades make different alkaloids with effects as diverse as the plants themselves—some are deadly in small amounts (like those in belladonna AKA “deadly nightshade”), some will get you high (like nicotine from tobacco), and some will make you sick but probably not kill you unless you really try (like those in the leaves, stems, and unripe fruit of tomato plants).

In large enough amounts, all alkaloids are toxic. Thus, in a completely non-contextual way, all nightshades are “toxic” because they all produce at least some alkaloids—but not all parts of all nightshades are poisonous! The parts of the nightshade plants we use—the fruits or roots—by and large contain little-to-no alkaloids and are completely safe.

In fact, solanine poisoning is exceedingly rare the world over (solanine is the most common alkaloid in the nightshades we eat), and it requires eating essentially impossible quantities of nightshade veggies to induce it—think 10 lbs or more of potatoes in a single sitting. Toxicologists estimate that a toxic dose of solanine is between 1 to 2.5 milligrams for every pound you weigh, or somewhere in the range of 100 to 400 milligrams, but Americans average only 12.75 milligrams daily. Suffice to say, you’re not going to suffer solanine poisoning unless you make a salad of tomato leaves.

Could a lesser dose potentially be inflammatory, though, even in the absence of acute symptoms? The answer is still “no”.

The alkaloids in nightshades are toxic in large amounts because they inhibit the enzyme “acetylcholinesterase”, which is heavily involved in neural activity and gut motility (the contractions of our digestive system which move food from throat to stomach to small intestine to colon). As such, all the symptoms of solanine poisoning—headaches, stomach cramps, diarrhea, vomiting, and hallucinations, among others— involve these systems. Acetylcholinesterase inhibitors are not inflammatory, and in fact are being explored as potentially being anti-inflammatory for the very systems they affect (in controlled doses, of course—do not try at home!).

So there’s really no evidence that the alkaloids in nightshade veggies are inflammatory, even if they might be toxic in large amounts—and just because they’re toxic in large amounts doesn’t mean they’re toxic in the small amounts present in our diet. The dose makes the poison, and we simply don’t absorb or retain large enough quantities of nightshade alkaloids to cause either short- or long-term problems, so the “alkaloids are bad” argument is a long road to nowhere.

(Oh, and by the way, the world’s most popular drug [caffeine] also happens to be an alkaloid, and not just any type of alkaloid, but an alkaloid that inhibits acetylcholinesterase. Now there are obviously other differences between the alkaloids in tomato leaves and caffeine—tomato leaves don’t give you energy—but it’s tough to argue that these acetylcholinesterase inhibitors are good while those are bad. If you take enough caffeine, you’ll even experience many of the same symptoms, right down to the hallucinations. So when you’re busy avoiding all the delicious nightshades, like tomatoes and peppers, you should simultaneously be giving up coffee and tea, too. Sounds like fun, right?)

Can Nightshades Increase Food Hypersensitivity?

Okay, so alkaloids as a direct cause of inflammation are out, but is there perhaps an indirect way they could cause inflammation? I’m glad you asked! A second hypothesis on why nightshades are inflammatory posits that those same alkaloids—or occasionally other molecules in nightshade veggies like lectins—increase the permeability of our small intestine and allows undigested food particles to enter the body intact. If food particles did enter the blood intact, our body would respond with an immune reaction, and inflammation would be the tattered wreckage left behind.

Alas, there is little actual evidence that this occurs. All we have are a handful of poorly done studies offering extremely weak evidence—and even then it’s a wild leap from the studies’ modest conclusions to fullblown “leaky gut syndome” for the hapless nightshade nibbler.

In fact, neither of the studies even remotely demonstrate that nightshade alkaloids are capable of increasing the permeability of a human intestine, despite being two of the most frequently cited studies in the defense of this hypothesis. To begin with, neither used humans or even human cells, which is pretty much a necessity if you’re going to demonstrate something is true for humans. Even after this, neither used realistic conditions for their study, which I’m going to tentatively say is also a necessity.

Let me be specific though (if you don’t care about deconstructing the studies, skip the next two bulletpoints):

• In the first study, the researchers used two strains of mice that were both genetically predisposed to inflammatory gut diseases (like ulcerative colitis or Crohn’s) and then fed them only fried potato skins for 20 days. The fried food diet naturally caused significantly greater inflammation than the normal mouse chow, but then we know that a diet high in fried food is generally inflammatory. Potato skins also happen to be the part of the potato that is eaten in the smallest amounts but which is highest in alkaloids, so the researchers essentially fed the rats nothing but the alkaloid-rich part of the potato for three weeks in a row—which in no way resembles a normal diet for any species, let alone humans.
• The second study used intestinal cells from a mouse (not even from a human, for who knows what reason) and then noted that the alkaloids disrupted the cell membrane when pH was lowered to around 6, with the least disruption at a pH of 6.6 to 7—AKA the normal pH of a human small intestine. They also used alkaloids we rarely if ever consume, like tomatine from green tomatoes (it disappears by the time the fruit is ripe), and explicitly state that human intestinal cells are at least three times less susceptible to the alkaloids they used than rodent cells.

It’s grasping at straws to use these studies as any sort of “evidence” that nightshade alkaloids can harm a human gut. Not only are they incredibly weak even for what they are, but no serious nutrition professional would ever base their dietary recommendations off in vitro or animal studies (let alone an in vitro animal study)! The most I can say after reading the studies above is that you probably shouldn’t eat only fried potato skins for three weeks straight—but did you really need a study to tell you that?

Do Nightshades Cause Joint Calcification?

We’re almost done here, but I really want to clear up each and every random myth the internet has propagated for why nightshade veggies could be inflammatory. The final of them is the most laughable of all, but still, it should be dealt with.

A few online “resources” have explained that nightshade veggies contain active forms of vitamin D, a hormone the body uses to increase calcium absorption and retention, and this active vitamin D could cause joint calcification. Joint calcification would be bad news in general, and in severe cases it leads to osteoarthritis. So does this make sense?

Tomato plants may, in fact, have some vitamin D in them, which is honestly kind of cool news for botanists—but only in the leaves, stems, and roots, and not the fruit.  In addition, vitamin D can cause joint calcification when consumed in excess amounts, but you would need to reach a serum level of approximately 500 nmol/L (200 ng/mL) before this occurred.

The core of this myth is that you would need to get so much vitamin D from a nightshade veggie that your serum vitamin D rises to toxic levels—and this hypothesis just doesn’t line up with the facts. According to CDC data, 97.5% of people tested have serum vitamin D levels at or below 108 nmol/L, and 50% are just barely above deficient with levels of 58 nmol/L. Those are nowhere near toxic levels, and they apply to the vast majority of the population.

The truth is that if nightshade veggies were actually a good source of vitamin D, we wouldn’t have a near-epidemic of insufficient levels. Trust me when I say it would be revolutionary to discover you could maintain vitamin D with tomatoes, peppers, and potatoes alone—but since you can’t, you might as well also stop worrying you’ll overdose on vitamin D from them.

Does Anybody Need to Avoid Nightshades?

It should be clear by now that there is absolutely no good argument for how nightshades can cause inflammation, and maybe you’re considering adding them back into your diet (and directing that friend or friend of a friend to this page). But there’s one last thing we should cover: special exceptions.

Are there special exceptions? Some people think so. Not everyone is gung-ho against these beautiful vegetables, and some people are tolerant enough to call them “healthy” except in the case of sensitivities or pre-existing inflammatory conditions like arthritis or an autoimmune disease. Does this subdued recommendation make sense? Not really!

This is just a reduced argument, but it doesn’t matter what you divide zero into—you still get zero. The fact remains that there is absolutely no evidence that nightshades are capable of producing an inflammatory reaction, whether due to alkaloids, gut permeability, vitamin D, or anything else. If nightshades cannot cause an inflammatory reaction, then it makes no sense to recommend against them even if you have an inflammatory condition!

To top it off, there is literally zero scientific evidence that people with inflammatory conditions do better when they remove nightshades from their diet. There are no studies, not even uncontrolled epidemiological ones. The single reference I could “find” was not on PubMed—it wasn’t anywhere, actually—and the various sites that use it don’t even agree on when it was published, which is a pretty good indication that they, too, couldn’t find the article and are just using the damning headline (“A relationship of Arthritis to the Solanaceae (nightshades)”) to bolster their argument—they know most people won’t bother to check the references.

So no, there is no need for anyone to avoid nightshade veggies, not even if they have a serious autoimmune disease or some other inflammatory condition.

Nightshade Veggies Are Healthy and Delicious

If you got bored in the middle of the article and skipped ahead to the end, let me summarize what we’ve covered:

• There is NO evidence that the alkaloids from nightshades can cause inflammation.
• There is NO evidence that nightshades can increase permeability of the intestine and lead to food hypersensitivity and subsequent inflammation.
• There is NO evidence that nightshades can increase joint calcification.
• There is NO evidence that nightshades are bad for anyone, even someone with an autoimmune disorder or some other inflammatory condition.

In short, there isn’t a single compelling reason to remove nightshades from your diet.

Let’s be real for a second—nightshades are great veggies! They’re easy to use, healthy, and perhaps most important of all they are delicious. I guarantee that you’re more likely to improve your physical wellbeing if you include more of them in your diet, not less.

The bottom line is this: the nightshade-inflammation link is bad science, and I don’t think bad science is worth changing your diet over—do you?

Addendum 7/15/2015: Allergic Reactions

A reader brought an inadvertent omission to my attention: the possibility of allergic reactions to tomatoes, potatoes, or other nightshades. Though food allergies to nightshade veggies are extremely uncommon, they do occur occasionally and can be extremely serious—even life-threatening.

Allergies are immune reactions to specific food proteins, so a person with a specific nightshade allergy (such as a tomato allergy) may or may not react with other nightshades depending on whether the specific protein they are allergic to is present. The most common allergen in nightshade veggies is shared by tomatoes and potatoes (the protein “patatin”), so cross-reactivity is common between potatoes and tomatoes. Interestingly, latex contains a protein similar to patatin, so many tomato or potato allergy sufferers will also react to latex.

If you don’t have an allergy to any nightshade veggies, there is no reason to avoid them; allergenic proteins are harmless except to those with IgE antibodies to them. IgE-type allergic reactions are rapid in their onset, so if you do have an allergy you will experience it within minutes to an hour of eating the offending food—there is no such thing as a delayed-onset IgE allergic reaction (actually, the scientific evidence for any type of delayed-onset immune reaction is low, but that’s an article for another day).

So do keep in mind that some people may react to some nightshades, but still not in a way that could lead to the type of inflammation a climber would worry about.