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Which foods are classified as "nightshades," and why are adverse reactions sometimes associated with these foods?

Nightshades—all belonging to a scientific family of plants called Solanaceae—include several thousand species of flowering plants, and most of these plants are not edible as food. At the very same time, however, there are a good number of nightshades that have been enjoyed as staple foods in diets for literally thousands of years. Our WHFoods nightshades include: tomatoes, potatoes, eggplant, and bell peppers. Nightshades can also be found in our profiled herbs and spices, and these nightshades include cayenne pepper and chili pepper.

Like all of our 100 WHFoods, nightshades are nutrient-rich. Bell peppers, for example, are our second most concentrated source of vitamin C. Tomatoes are our second most concentrated source of biotin, as well as our second most concentrated source of lycopene (a carotenoid phytonutrient). Our nightshade foods are all very good sources of fiber, and all of our nightshade foods are ranked sources of nine or more nutrients.

Alkaloids in Nightshade Foods

Alkaloids are a chemically related group of substances found in a variety of foods, including cocoa, coffee, tea, black pepper, and honey (depending on the types of flowers found in the bees' foraging zone). Alkaloids in food can have health benefits, sometimes including antioxidant-related benefits and cancer-protective properties. These same potential health benefits have been linked in research studies to some of the alkaloids found in nightshades. Among the nightshade plants most commonly enjoyed as foods, the alkaloids in tomatoes, potatoes, and eggplant are best studied. Below is a chart showing key alkaloids in these three nightshade foods.

Nightshade Food Scientific Name Key Alkaloids
Tomato Lycopersicon esculentum tomatine, deyhdrotomatine
Potato Solanum tuberosum alpha-solanine, chaconine
Eggplant Solanum melongena solasonine, solarmargine

In animal studies, the cancer-preventive properties of the nightshade alkaloids are often related to their interactions with membranes, including the outer cell membranes (plasma membranes) and mitochondrial membranes. The solanine alkaloids in potato, for example, have been shown to influence the flow of ions across membranes, including the flow of potassium ions. Both the solanine and chaconine alkaloids in potato have been shown to have cholesterol-binding ability in lab studies, and the formation of alkaloid-cholesterol complexes at the membrane level may be involved in the ability of potato alkaloids to influence membrane function. We have not seen large-scale human studies showing indisputable disease risk reduction associated specifically with nightshade alkaloids. We have seen studies showing the substantial nutrient contribution that nightshade can make to a meal plan, including their important phytonutrient contributions.

Along with the potential health benefits associated with food alkaloids, however, are also potential problems with excessive intake. The key word in this sentence is "excessive." Potatoes provide a good example here. The strictest public health standards that we have seen for intake of TGA (total glycoalkaloids) in potato is 100 micrograms per gram of potato. The U.S. Food and Drug Administration (FDA) has set the maximum acceptable content of TGA at 200 micrograms per gram of potato. While it is possible to purchase potatoes in the U.S. that exceed the 100 microgram/gram level, we have not seen studies on U.S. potatoes for which the TGA level exceeded the 200 microgram/gram FDA level. In fact, it is not uncommon for the TGA level in potatoes to dip into the range of 25-75 micrograms/gram. So while it is possible to consume too many solanine and chaconine alkaloids from potatoes, it is also unlikely based on the research data.

Part of the problem in assessing alkaloid content in potatoes is the very large variability in potato alkaloids. Potatoes—like other nightshade plants—make alkaloids from cholesterol as part of their natural defense system. These plants take the steroid ring structure from cholesterol (in the form of the aglycone solanidine) and hook it together with a chain of three sugars. (In the case of solanine, these three sugars are galactose, glucose, and rhamnose.) The extent of alkaloid formation in potatoes depends on many factors, including the exact variety (cultivar) of potato, the climate, and exact time of planting and harvesting. All of these factors seem logical, since they are all related to the potential presence of insects and other pests that might do harm to potatoes. But these factors also make it very difficult to predict the level of alkaloids in potatoes. We've reviewed nearly a dozen studies in this area, and the data show fairly wide ranges of alkaloid content in different potatoes. At the bottom end of these ranges, potatoes contain about 25 micrograms/gram of total glycoalkaloids. At the top end of the ranges, they might contain up to 275 micrograms/gram. (And as a reminder, the FDA maximum concentration level for glycoalkaloids in potatoes is 200 mcg/g.)

In the case of tomatoes, accumulation of alkaloids appears to take place to a much greater extent in the leaves, stems, and vines than in the matured fruit. In fact, as the fruit of the tomato plant matures, its alkaloid content decreases as well. This set of circumstances may be partly responsible for the relative lack of research on tomato versus potato alkaloids, and less common mention of the tomatine alkaloids in tomato in comparison with the solanine alkaloids in potato.

Impact of Preparation, Cooking, and Storage

One aspect of potato alkaloids that is not subject to high variability is the location of these alkaloids in the potato. As part of the potato's natural protection system, it makes sense for the potato to manufacture alkaloids where it is most vulnerable: namely, in the skin, the flesh just below the skin, and the spots where it has started to sprout. The skin is obviously the most exposed portion of the potato, and the sprouting spots are places where unique growth is taking place. So it makes sense for the potato to take special protective measures in the skin and sprouting spots. This set of events also makes it relatively easy for anyone to lower the alkaloid content of potatoes if desired. We've seen a study on potatoes from Mexico in which 65-70% of the alkaloids were removed by skinning the potatoes prior to cooking. For persons wanting to lower the alkaloid content of potatoes, it would also make sense to cut out any sprouting spots prior to cooking. In general, however, since the alkaloid content of potatoes averages well below the 200 microgram/gram level and since the potato skin is an especially nutrient-rich portion of the potato, we like the idea of cooking potatoes with the skin intact rather than removed.

Cooking itself can have a variable impact on the alkaloid content of potatoes. We've seen one study in which the total glycoalkaloid content of potatoes was lowered by 40% after only 10 minutes of baking at 410°F (210°C). Longer term, dry heat cooking methods (like oven baking) seem more effective in lowering alkaloid content than wet heat methods like steaming or boiling. This difference may be related to the solubility of alkaloids like solanine in water, which is very low unless a strongly acidic substance has been added to the water in a substantial amount.

With respect to storage, studies show that exposure to light can increase the alkaloid content of potatoes—in part by increasing the metabolic events associated with sprouting. For this reason, you should be able to lessen the development of alkaloids in potatoes by storing them in a dark place that is shielded from light.

Nightshades and Specific Health Problems

It is not uncommon to find accounts from individuals who have been diagnosed with varying forms of arthritis or other musculoskeletal health problems in which elimination of nightshades vegetables resulted in a feeling of improvement. While we have no reason to dispute the experience of anyone who makes dietary changes and experiences a change in health, we also have yet to see a large-scale human research study showing health improvement of any kind following elimination of nightshades. While future research may end up shedding more light on the relationship between nightshade alkaloids and the function of our joints, musculoskeletal, or nervous systems, at present we have no research-based reason to recommend elimination of nightshades in connection with these body system problems.

WHFoods Recommendations

In the absence of a research basis for recommending avoidance of nightshade vegetables like tomatoes, potatoes, and bell peppers—and given the known nutritional benefits of these foods as well as their potential health benefits—we recommend enjoyment of nightshade vegetables along with our many other WHFoods unless your personal eating experience tells you otherwise. With respect to potatoes, we would also note that for persons wanting to lower the alkaloid content of this nightshade food, the skinning and removal of potato skin and sprouting spots can substantially lower their alkaloid content.


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