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

Introduction

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 WHF 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 WHF, 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.

WHF 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 WHF 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.

Overview - the basics about nightshade foods

Potatoes, tomatoes, sweet and hot peppers, eggplant, tomatillos, tamarios, pepinos, pimentos, paprika, cayenne, and Tabasco sauce are classified as nightshade foods. A particular group of substances in these foods, called alkaloids, can impact nerve-muscle function and digestive function in animals and humans, and may also be able to compromise joint function. Because the amount of alkaloids is very low in nightshade foods when compared with other nightshade plants, health problems from nightshade foods may only occur in individuals who are especially sensitive to these alkaloid substances. Since cooking only lowers alkaloid content of nightshade foods by about 40-50%, highly sensitive individuals may want to avoid this category of food altogether, while non-sensitive individuals may be able to eat these foods, especially in cooked form, without problem. Green and sprouted spots on potatoes usually reflect high alkaloid content, even though the green itself involves the presence of chlorophyll, not alkaloids. For this reason, sprouted areas should always be thoroughly removed before potato cooking, or the potatoes should be discarded altogether.

Nightshades—a description

Nightshades are a diverse group of foods, herbs, shrubs, and trees that have fascinated scientists, doctors, and nutritionists for centuries. “Nightshade” is actually the common name used to describe over 2,800 species of plants, many with very different properties and constituents. All of the plants, however, belong to a scientific order called Polemoniales, and to a scientific family called Solanaceae. To give you an idea of the diversity associated with this group of plants, consider the fact that tobacco, morning glory, potato, and tomato are all classified as nightshades.

Pharmaceutical nightshades

Nightshades are actually more famous as drugs than as foods. The best-known nightshades when it comes to pharmacy include mandrake (Mandragora officinum), tobacco (Nicotiana tabacum) and belladonna, also called deadly nightshade (Atropa belladonna).

What has interested scientists most about nightshades in a pharmacological sense is a group of compounds in them called alkaloids. The alkaloids found in nightshades are not only the basis for consideration of nightshades as drugs, but also for understanding adverse reactions to nightshades when they are eaten as food. Adverse reactions to nightshade alkaloids are discussed further in the health effects section of this nightshade profile.

Foods considered to be nightshades

Nightshade vegetables and fruit

The most famous food members of the nightshade family include potatoes (Solanum tuberosum), tomatoes (Lycopersicon esculentum), many species of sweet and hot peppers (all species of Capsicum, including Capsicum annum), and eggplant (Solanum melongena). Less well know, but equally genuine nightshade foods include ground cherries (all species of Physalis), tomatillos (Physallis ixocapra), garden huckleberry (Solanum melanocerasum), tamarillos (Cyphomandra betacea), pepinos (Solanum muricatum), and naranjillas (Solanum quitoense). Pimentos (also called pimientos) belong to the nightshade family, and usually come from the pepper plant Capsicum annum. Pimento cheese and pimento-stuffed olives are therefore examples of foods that should be classified as containing nightshade components. Although the sweet potato, whose scientific name is Ipomoea batatas, belongs to the same plant order as the nightshades (Polemoniales), it does not belong to the Solanaceae family found in this order, but to a different plant family called Convolvulaceae.

Nightshade spices

The seasoning paprika is also derived from Capsicum annum, the common red pepper, and the seasoning cayenne comes from another nightshade, Capsicum frutenscens. Tabasco sauce, which contains large amounts of Capsicum annum, should also be considered as a nightshade food. It may be helpful to note here that black pepper, which belongs to the Piperaceae family, is not a member of the nightshade foods.

Ways in which nightshades may affect health

Alkaloids - The chemistry of nightshades

Most of the health research on nightshades has focused on a special group of substances found in all nightshades called alkaloids. In chemical terms, alkaloids are easy to identify because they all have at least one ring-like structure that contains the element nitrogen. Plants produce alkaloids as a regular part of their biochemical activity, and these alkaloids are primarily designed to help protect the plants from insects that would otherwise eat them.

Four basic types of alkaloids are found in nightshade plants. These types are: (1) the steroid alkaloids, which contain a fairly complicated fused ring structure and are found in most food nightshades including potato and tomato; (to compare the value of one of the most notable steroid alkaloid -solanine-in the foods in which it is most concentrated, please refer to Table 1)(2) the tropane alkaloids, all originating from the simple amino acid ornithine and found in fewer of the overall nightshades, but more extensively researched due to their strong drug-like properties; (3) the pyrrolizidine alkaloid and (4) the indole alkaloids, both important groups from a drug standpoint.

Table 1

Vegetable

Solanine contentmilligrams per 100 gram serving

Common peppers

7.7-9.2

Eggplant

6.1-11.33

Potatoes

2-13

Effect of steroid alkaloids on the nervous system

The steroid alkaloids in potato - primarily solanine and chaonine - have been studied for their health effects in two areas. First is their ability to block activity of an enzyme in nerve cells called cholintesterase. Many of the alkaloids found in nightshades possess this kind of activity, called cholinesterase inhibition. If the activity of cholinesterase is too strongly blocked, the nervous system control of muscle movement becomes disrupted, and muscle twitching, trembling, paralyzed breathing, or convulsions can result. The steroid alkaloids found in potato have clearly been shown to block cholinesterase activity, but this block does not usually appear strong enough to produce nerve-muscle disruptions like twitching or trembling.

Effect of steroid alkaloids on joint health

A second type of problem potentially related to the potato alkaloids involves damage to the joints caused by inflammation and altered mineral status. Whether alkaloids can contribute to joint damage of this kind is not clear from current levels of research. Some researchers have speculated that nightshade alkaloids can contribute to excessive loss of calcium from bone and excessive depositing of calcium in soft tissue. For this reason, these researchers have recommended elimination of nightshade foods from the meal plans of all individuals with osteoarthritis, rheumatoid arthritis, or other joint problems like gout.

Effect of nicotine alkaloid on health

Just as there is no firm research evidence for the impact of the steroid alkaloids in nightshade foods on the nervous system or joints, there is also no solid research evidence for impact of the more drug-like alkaloids in nightshade foods on body systems. But to the surprise of many people, nightshade foods do contain very small amounts of drug-like alkaloids that have long been fascinating to medical and drug researchers.

Consider, for example, the most famous of the one-ring type alkaloids (monocyclic alkaloids) found in the highest concentrations in tobacco (Nicotiana tabacum): nicotine. This alkaloid is found not only in non-food nightshades like tobacco, but also in the food nightshades including eggplant and tomato.

But there is one important difference here: while alkaloids like nicotine are definitely found in nightshade foods, the amount involved is dramatically less. Even in the case of eggplant, which is the food nightshade that appears to have the highest nicotine content after tobacco, the amount of nicotine is far lower than the amount found in tobacco. In the case of green tomatoes, which also contain nicotine, the amount is even less. The levels of nicotine in all nightshade foods are so low that most healthcare practitioners have simply ignored the presence of nicotine in these foods as a potential compromising factor in our health. At the WHF, we both agree and disagree with this conclusion. While we agree that the amount of nicotine in nightshade foods is very, very small, it still seems possible to us that some individuals might be particularly sensitive to the alkaloids found in nightshades, and that even very small amounts might compromise function in the bodies of these individuals.

Increased alkaloid content of green and sprouting potatoes

It’s important to point out that green spots on potatoes, or sprouting on potatoes, usually correspond to an increased alkaloid content, and this increased alkaloid content is one of the main reasons for avoiding consumption of green or sprouted potatoes. (The green color itself is chlorophyll, and helpful to our health, but unfortunately, it’s also accompanied by the increased alkaloids that we can’t see). Interestingly, in one study conducted with hamsters who were fed the sprouted portions of potatoes, increased alkaloid content did not seem to impact the nerves or joints nearly as much as the digestive system itself. The researchers focused on damage to the stomach and intestines when trying to understand the problems caused by ingestion of potato sprout material, and concluded that there were reasons to avoid this material based on digestive system evidence alone. A bitter taste in potatoes after the potatoes have been cooked is usually a good indication that excessive amounts of alkaloids are present.

The effect of cooking on the alkaloid content in nightshade foods

Steaming, boiling, and baking all help reduce the alkaloid content of nightshades. Alkaloids are only reduced, however, by about 40-50% from cooking. For non-sensitive individuals, the cooking of nightshade foods will often be sufficient to make the alkaloid risk from nightshade intake insignificant. However, for sensitive individuals, the remaining alkaloid concentration may be enough to cause problems.

Practical tips

First, if you are an individual with existing joint problems like osteoarthritis, rheumatoid arthritis, or gout, temporary 2-3 week elimination of nightshade foods from your meal plan may be a worthwhile step to determine if these foods could be contributing to your joint problems. This same recommendation would apply to individuals with existing nervous system problems, particularly nerve-muscle related problems.

Second, even if you are an individual with no existing health problems potentially related to nightshade intake, you will want to take precautions to avoid excessive intake of alkaloids from these foods. Handling of potatoes is especially important in this regard, and the following practices will help you avoid excessive intake of potato alkaloids:

References

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