The nightshade family of plants (Solanaceae) is a very large one, consisting of almost 3000 different species, many of which are inedible and even toxic. The most infamous of these is belladonna or deadly nightshade, the poison used by Juliet to fake her death in Shakespeare’s play, Romeo and Juliet. Maybe this is why nightshades have become mired in controversy with some sources suggesting that humans should not be eating them at all, noting the possibility of inflammatory conditions stemming from the alkaloid constituents of nightshades. So what is the truth when it comes to nightshade plants?
The name, nightshades, is thought to have come from the preference of these plants for shady growing areas and their propensity for flowering at night. Included in the nightshade family are some fruits and vegetables including (1);
Tomatoes, all varieties
Potatoes, all varieties (but not sweet potatoes or yams)
Peppers, all varieties including sweet and hot peppers, chili peppers, cayenne, paprika (but not pepper derived from peppercorns)
Other common nightshades include tobacco and petunias
POSSIBLE DETRIMENTAL EFFECTS OF NIGHTSHADE PLANTS
Alkaloids are substances produced by plants which can have physiological influences on those who eat them, actions which might be desirable, toxic, or both (2). Vegetable and fruit nightshades are not only excellent sources of fiber, they are also rich in the beneficial components of antioxidants, vitamins and minerals (3). However, nightshades also produce alkaloids, natural pesticides that offer the plant protection from marauding insects and bacteria or viral threats and it is these substances that are at the core of the debate surrounding these plants.
Solanine is a bitter alkaloid that has been isolated from parts of many nightshade plants although not in large quantities. Even potatoes, the main source of solanine in the human diet, generally contain only small amounts of solanine, between 0.05 and 0.65 mg per 100 gm potato. Higher concentrations can occur after exposure to light which also causes the potato flesh to take on a green hue. In improper storage conditions, solanine concentrations in potatoes can increase to between 0.6 to 22 mg/100 gm of potato (4). Health Canada considers a solanine level of up to 20 mg/100 gm as safe for human use with a low risk of adverse health effects (5). Doses of 200 to 400 mg for adults (20 to 40 mg for children) have been suggested as enough to produce toxic symptoms, primarily from irritation of the gastrointestinal system. Effects can include a bitter or burning sensation in the mouth and flu-like symptoms such as vomiting, abdominal cramps and diarrhea. Much higher doses have the potential to cause impairment of the nervous system with more severe effects such as drowsiness, restlessness, shaking, weakness, confusion and disturbed vision (5).
Solanine poisoning from potatoes, though theoretically possible, is actually very rare with few cases reported and Health Canada does not consider it a health concern (5). Many studies corroborate this finding (4,6).
Why is this?
Firstly, a very large quantity of potatoes eaten at one time would be necessary to reach high enough levels of solanine to cause problems. In a 2007 report to The Times magazine, Alexander Pavlista, a professor of agronomy and horticulture at the University of Nebraska, stated that a 100-pound person would need to eat about a pound of completely green potato flesh to consume enough solanine to cause illness (7).
Secondly, solanine is poorly absorbed in the gastrointestinal tract and rapidly metabolized in the stomach to solanidine, a less toxic form. Excretion from the body also occurs quickly (8).
Finally, even green potatoes examined for solanine content usually remain within safe limits for human consumption (9).
In any case, it is prudent to trim any sprouts and green flesh from potatoes before consuming them (10). Keep in mind that the longer potatoes are exposed to light, the more they are likely to turn green. Choosing fresh potatoes whenever possible will be tastier and contain a minimal amount of solanine.
Tomatoes contain the alkaloid tomatine in the stem and leaves but have no appreciable amounts in the ripe fruit (11).
The Arthritis Foundation in the US advises that there is no research to back the claim that nightshades can increase inflammation (12). In fact, studies have shown that eating nightshade plants can actually reduce markers of inflammation (13).
LEAKY GUT SYNDROME
Leaky gut syndrome, aka increased intestinal permeability, is a fairly recent addition to medical vocabulary. This problem occurs within the thin membrane lining the intestine in which the normally tight barrier develops small spaces through which harmful substances like partially digested food, toxins and bacteria enter the bloodstream. It is thought that leaky gut can result from conditions such as poor gut health, nutrient deficiencies, a diet low in fiber and high in sugar, chronic stress, excessive alcohol intake and long-term use of NSAID medications (non-steroidal anti-inflammatory medications).
Some medical practitioners believe leaky gut syndrome contributes to the development of certain chronic diseases such as autoimmune disease. However the link between nightshades and the development of a leaky gut is very weak. Studies in mice with Inflammatory Bowel Disease have suggested that alkaloids in potatoes can increase inflammation and intestinal permeability (14,15). Other components of food plants such as lectins and saponins have also shown possible detrimental effects on the gut barrier in animal studies (16,17,18). But there is no evidence that this is the case in humans. Further research is needed.
OTHER DIGESTIVE ISSUES
Gastrointestinal problems such as bloating, diarrhea and constipation can oftentimes be relieved by avoiding FODMAP foods, foods that contain short-chain fermentable carbohydrates that are not undergoing complete digestion in the large intestine. The gas that results can cause pain and bloating (19). Nightshade foods can also contain FODMAPs and so they are drawn into the list of foods suggested for avoidance to relieve digestive symptoms. However, this effect is not due to their alkaloid content. In fact, an alkaloid in hot peppers known as capsaicin has been shown to decrease abdominal pain and bloating in Irritable Bowel Disease after six weeks of treatment (20). There is no evidence that nightshades increase gastrointestinal symptoms unless a FODMAP intolerance exists.
Anecdotal evidence proposes that avoiding nightshades can lessen the severity of some autoimmune diseases. Some research also suggests that, in people with a higher genetic risk of rheumatoid arthritis, lectins can be a cause of leaky gut and trigger the expression of rheumatoid arthritis, an autoimmune disease (18). Once again, little to no research has examined nightshade plants specifically. Further research is needed.
Calcitriol, the active form of Vitamin D has been associated with higher calcium blood levels and the deposition of calcium into joints, a condition fairly common in sufferers of rheumatoid arthritis (21). Though nightshade plants contain a low level of calcitriol, most of this nutrient is produced from the reaction of skin to sunlight and there is no evidence that eating nightshades results in high blood calcium. Other medical conditions such as hyperparathyroidism, sarcoidosis, cancer or very high dose Vitamin D supplementation are the common causes of high blood calcium.
SENSITIVITY AND ALLERGY TO NIGHTSHADE PLANTS
Nightshade foods are a healthy and safe part of the diet for most people. However, like many other foods, it is possible to be sensitive to or even develop an allergy to nightshades, though this is rare. An allergy to nightshades shows itself with the common symptoms of any allergy such as hives and skin rash, itchiness, headaches, nausea, vomiting, excessive mucous production, breathing difficulties, achy muscles and inflammation. An intolerance to nightshades is also possible if the enzymes needed to digest them are lacking. A nightshade intolerance can cause symptoms such as bloating and gas, heartburn, nausea and diarrhea.
DIAGNOSING NIGHTSHADE INTOLERANCE OR ALLERGY
The internet is a powerful source of information and a quick search reveals many different allegations surrounding nightshades. You may be wondering if some of your own symptoms might be due to nightshade plants. If so, there is an easy way to check this out.
Try a period of abstinence from nightshade food plants to see if it results in any lessening of your symptoms. Three months is ideal but even a break of 4 weeks should give you an idea about how these foods are affecting you. During this time, keep a food journal, writing down any symptoms that you display and any change in symptoms over time. After the abstinence period you can begin to reintroduce nightshade foods one at a time. Wait for a few days after each addition, observing any change in symptoms and noting them down in your journal.
If there has been no change in symptoms during the absence of nightshades in your diet then you can safely continue to eat them. However, if symptoms disappeared during the avoidance of nightshades and returned after starting to eat them again, then you may be one of the few people with an allergy or intolerance to nightshades and it is probably a good idea to continue to avoid them.
Nightshade allergy can also be diagnosed or confirmed by a physician through diagnostic tests such as a skin prick test or a blood test.
BENEFITS OF NIGHTSHADE PLANTS
Plant alkaloids are known to have beneficial effects in humans. A 2013 study found that anatabine, an alkaloid found in peppers, potatoes, tomatoes and eggplant has powerful anti-inflammatory effects in people with joint pain and stiffness, including those with autoimmune conditions and osteoarthritis when taken as a supplement (22). A 2015 review examined the state of science worldwide on the effects of alkaloids from eggplants, potatoes and tomatoes on cancer cells. Results showed that the growth of cancer cells is inhibited both in the test tube and in actual tumours by the consumption of these foods from the nightshade family (23).
Let’s look at some specific nightshade food plants and the benefits they offer us.
Tomatoes are rich in antioxidants. In fact they are our most concentrated dietary source of the antioxidant lycopene and are also high in anthocyanins. Anthocyanins are a type of flavonoid which is associated with a lower risk of death from cardiovascular disease, even when they are present in relatively small amounts (24).
Tomatoes are also our best source of biotin, a good source of Vitamin A, C and K and rich in the mineral, potassium. (3) These nutrients reduce oxidative stress and inflammation and lower the risk of chronic diseases such as some cancers and heart disease (25,26).
Evidence for a protective effect from the lycopene in tomatoes against prostate cancer is strong (27).
Lycopene also has many benefits for the cardiovascular system, showing activity against atherosclerosis, hypertension, metabolic disorders and in reducing stiffness of the arteries by protecting their endothelial lining (28).
Animal and test tube studies of the tomato alkaloid, tomatine, are elucidating some promising benefits of this alkaloid on cholesterol. In hamsters, tomatine and cholesterol form an insoluble complex that is excreted in the feces, effectively lowering blood cholesterol levels. This may be why very little dietary tomatine is absorbed from the digestive tract into the blood stream, consistent with the low reported oral toxicity of tomatine. In this experiment, the more tomatine available, the more LDL-cholesterol level is decreased (29).
Additionally, Esculeoside A, another alkaloid isolated from tomatoes, causes significant reduction in blood levels of cholesterol, triglycerides and LDL-cholesterol and reduced severity of atherosclerotic plaques in mice (30).
In test tube studies, tomatine-containing green tomato extracts show ability to induce death in human cancer cells including breast, colon, stomach and liver cancer cells (31).
Potatoes are an excellent source of many healthy nutrients including fiber; vitamins B6, C, thiamine, niacin and folate; and minerals such as potassium, manganese, magnesium, copper, phosphorus and iron (3).
Consumption of potatoes has been found to reduce inflammation in humans. A 2010 study showed that subjects eating yellow and purple potatoes daily for six weeks had reduced markers of inflammation in their blood and lowered their level of DNA damage (13).
Test tube experiments using purple potato extracts in a simulated gastrointestinal tract found inhibition of several species of pathogenic bacteria and slowed the growth rate of certain cancer cells (32).
On a side note, low levels of alkaloids in potatoes impart desirable flavours to the potato flesh (5).
Hot and Sweet Peppers
Peppers are a rich source of Vitamin A, B6, C and folate as well as the antioxidants lycopene and anthocyanins (3).
Hot peppers such as chili peppers contain an alkaloid called capsaicin which has been shown to decrease pain and inflammation from osteoarthritis, rheumatoid arthritis and fibromyalgia (33), reduce heartburn symptoms (34) and help with weight loss by reducing calorie intake (35). Capsaicin has also been shown to increase satisfaction and feelings of fullness after eating and tends to suppress overeating (36).
A study of patients with Irritable Bowel Syndrome (IBS) indicated that administration of red pepper powder was significantly more effective than placebo in decreasing the intensity of abdominal pain and bloating in this disease (20). On the other hand, another study found no effect at all of capsaicin on abdominal pain in Irritable Bowel Disease (37).
In 2013 interesting research looked into the relationship between the alkaloid, nicotine and its effect on Parkinson’s Disease. It has been noted that exposure to nicotine from tobacco smoking appears to cut the risk of Parkinson’s Disease in half (38) and so this study was created to test whether the risk of Parkinson’s might also be affected by consumption of other nicotine-containing members of the nightshade family. Though their nicotine content is very low, nicotine is present in peppers, tomatoes and potatoes and the amount required to affect the incidence of Parkinson’s Disease is small. Results of the study linked significantly decreased risk of Parkinson’s Disease to the consumption of edible nightshades. Consumption of all other vegetables combined did not result in lowering of risk. Peppers have the highest nicotine content among the nightshade food plants and ingestion of peppers alone was also associated with lower risk for Parkinson’s Disease. These effects occurred mainly in the subjects who had never used tobacco or smoked cigarettes (39).
At 2.5 gm per cup, eggplants are very high in dietary fiber, a nutrient that is lacking in many modern diets. Fiber promotes healthy bowel function and is linked to lower risk of heart disease and colon cancer (40).
Eggplants also contain the antioxidants lycopene and anthocyanins.
Additionally, eggplants contain a unique antioxidant, nasunin, that neutralizes free radicals and, in so doing, reduces damage to cells from inflammation (41).
Eggplants are also a good source of Vitamin K (3).
Okra is high in Vitamin C and K.
Gooseberries and ground cherries
Fruit members of nightshades are also rich in healthy nutrients. They contain the antioxidants lycopene and anthocyanins; Vitamins A, C, niacin and thiamine; along with minerals such as iron and manganese (3).
For most people, eating nightshade plants is not only very safe but also extremely healthful. Scientific evidence of any link between nightshades and health problems is weak at best. There have been few if any nightshade studies that actually used nightshade plants as their focus and were performed on human subjects. Most use animals or test tubes as their study arenas and the material studied is high doses of substances that appear in nightshades. Additionally, the amount of alkaloids in nightshade food plants is very low and would require the ingestion of massive quantities to cause any adverse effects.
On the other hand, there is plenty of research that has revealed benefits of not only the constituents present in nightshades but of the whole plants themselves.
If you are still unsure of the effect of nightshades on yourself, clearing your diet completely from nightshade foods, along with diligent use of a food journal, as described above, will tell you what you need to know. It is not wise to deprive yourself of foods from this nutrient-dense family of plants without good reason.
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