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What does it mean when a food is referred to as a "nightshade" and should I be concerned about eating these types of foods?

Quick Summary

Potatoes, tomatoes, sweet and hot peppers, eggplant, tomatillos, tamarios, pepinos, pimentos, paprika, and cayenne peppers 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.

What are nightshades?

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.

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.

What foods belong to the nightshade family?

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 known, 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.

It may be helpful to note here that black pepper, which belongs to the Piperaceae family, is not a member of the nightshade foods, and neither is sweet potato. Although 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.

In the spice category, the seasoning paprika is also derived from Capsicum annum, the common red pepper, and the seasoning cayenne comes from another nightshade, Capsicum frutenscens. Hot sauces that contain hot peppers should also be considered as a nightshade food.

How can nightshades affect my health?

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; (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.

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.

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.

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 World's Healthiest Foods, 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.

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.

How does cooking affect 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.

What practical steps should I take with respect to nightshades in my meal plan?

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:

  • Store your potatoes for 1-3 weeks only in a dark cupboard, preferably in a cool and dry part of the house such as a basement (if your basement is dry). It's important not to keep potatoes in a lighted area; the exposure to light will increase alkaloid formation.

  • Wash all potatoes before cooking so you'll be better able to spot the green areas, if any.
  • Thoroughly cut out all green areas, especially green areas on the peel, before cooking. and cook the rest for safe eating. If you're sensitive to nightshades in the first place, it's best to discard the whole potato. After cooking, if the potato tastes bitter, do not eat it.
  • Do not purchase potatoes that have been waxed or apply wax to potatoes yourself. Waxes do not help reduce greening and can increase potato decay by cutting down on gas exchange in and out of the potato.

Nightshade Foods

  • Tomatoes
  • Peppers (all Capsicum species including bell, hot and pimento peppers and all foods made from these peppers including paprika, cayenne pepper and Tabasco sauce)
  • Eggplant
  • Ground cherries
  • Tomatillos
  • Garden huckleberries
  • Tamarillos
  • Pepinos
  • Naranjillas


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