by David Greenwalt
What are nightshades?
Despite their rather mysterious-sounding name, “nightshades” are simply a family of plants. Their scientific name is Solanaceae. The family includes over 2,000 species, ranging from vines and herbs to shrubs and trees.
Most nightshades are inedible, but a few are commonly eaten: potatoes, tomatoes, bell peppers, chili peppers, and eggplant. Less common foods from the nightshade family are ground cherries, tomatilloes, and red goji berries.
Nightshades fruits and vegetables:
- Bell Peppers
- Chili Peppers
- Ground cherries
- Red goji berries
Like all vegetables and fruits, edible nightshades contain an abundance of vitamins, minerals, phytochemicals, and fiber.
So why do some people blame these foods for health problems like arthritis, inflammation, and Irritable Bowel Disease?
Do nightshades cause arthritis?
The talk of a possible link between nightshades and health problems seems to have started in the 1970’s with a plant scientist named Norman Childers. He decided to stop eating nightshades and using tobacco (also a member of the nightshade family) in an effort to help relieve his diverticulitis.
According to Childers, eliminating nightshades not only improved his diverticulitis symptoms, it also relieved his arthritis (1).
Eventually Childers wrote a book promoting a nightshade-free diet to help arthritis.
Over the next 20 years, he surveyed people who had bought his book. Childers’ last survey reported that 68% of respondents said the diet had helped their arthritis (1).
Keep in mind, however, that there was no control group for this study. In addition, only 8% of people who were mailed the survey even responded (1). Those who did answer the survey might have been those who felt more positive about the diet.
Childers’ survey also reported that 79% of respondents had been treated with drugs for their arthritis, and 80% of those reported temporary relief (1). There is no way to know whether improved symptoms were due to the diet, or to drugs, a placebo effect, or something else. This study simply was not high enough quality to prove a cause-effect relationship.
Fear about nightshades may also have been fueled by reports that they cause minerals to be deposited in the soft tissues of animals.
For example, when grazing cattle in South America were found to have mineral deposits in their soft tissues (as can occur with arthritis), researchers found that a plant in the nightshade family was to blame (1). Cattle were eating the leaves of Solanum malacoxylon – a leafy flowering plant that humans do not eat. Researchers discovered that the form of vitamin D in this plant was being modified in the rumen of cattle in such a way that made it toxic to them.
Human digestion and metabolism of nutrients including vitamin D is quite different than that of cattle. There is no evidence that the vitamin D from nightshades is harmful to us in any way.
The bottom line:
There is no high-quality evidence that nightshades contribute to arthritis.
Do nightshades cause Irritable Bowel Disease?
Some animal studies have suggested that nightshades could aggravate inflammation in individuals predisposed to Irritable Bowel Disease (IBD) (2, 3).
Normally, there is a barrier of cells lining our intestines that keeps things in our intestines that are supposed to stay in, and keeps out what is supposed to stay out. In people with IBD, it is thought that disruption of that intestinal barrier is what can cause an uncontrolled inflammatory response and physical symptoms of IBD.
Glycoalkaloids, found in potatoes and other nightshade vegetables, seem to be able to disrupt this barrier (2).
In an animal model of IBD, mice fed fried potato skins containing glycoalkaloids had higher levels of inflammation (3). The researchers concluded that eating glycoalkaloids from potatoes could play a part in starting or worsening IBD.
The bottom line:
While their effect on humans is not known, there is some animal evidence that glycoalkaloids from potatoes may increase inflammation in IBD.
While those with IBD may want to pay attention to whether nightshades irritate their symptoms, the vast majority of people will get many important health benefits from eating nightshade vegetables.
Nightshades can lower inflammation
A study from Washington State University found that men who ate purple or yellow potatoes for 6 weeks had reduced markers of inflammation and less oxidative stress.
This is important because cellular oxidation increases risk of chronic diseases (4). Inflammation is also involved in many chronic diseases (5, 6).
Recently, one small study showed that young women who drank tomato juice daily for 2 months had reduced levels of inflammatory markers secreted by fat tissue (7). The reduction was not related to any weight loss or changes in body fat, suggesting that the tomato juice itself was able to decrease inflammation.
Certain compounds in nightshades may actually help prevent arthritis, rather than causing it. Diosgenin, a substance in nightshades, has been shown to have anti-inflammatory properties (8). Researchers have suggested that diosgenin may be beneficial in the treatment of osteoarthritis (8).
Capsaicin, a phytochemical found in chili peppers, also has anti-inflammatory effects. Animal and test tube studies have both shown that red pepper extracts can reduce markers of inflammation (9, 10).
The bottom line:
Numerous studies have shown that nightshade vegetables can decrease inflammation.
Nightshades and Cardiovascular Disease
Potatoes, and especially the more deeply colored yellow or purple potatoes, are rich sources of antioxidants (11). Eating foods that are rich in antioxidants lowers risk of chronic diseases such as cardiovascular disease (12).
Tomatoes are also high in antioxidants, especially lycopene. Studies have found that people who eat the highest amounts of tomatoes have reduced risk of cardiovascular disease, (13). This may be due to lycopene. Observational studies have shown that as levels of lycopene go up, risk of cardiovascular disease goes down (14, 15, 16).
In a clinical study, subjects were fed high-fat meals known to cause post-meal oxidative stress on two different occasions (17). One meal contained tomato products; the other did not. Following the tomato-containing meal, subjects had significantly lower levels of oxidative stress and inflammatory markers.
Since oxidation and inflammation both play a role in cardiovascular disease, the authors suggested that tomatoes may be protective against cardiovascular disease (17).
The bottom line:
There is an abundance of evidence showing that diets high in fruits and vegetables are protective against cardiovascular disease. There is no evidence to suggest that nightshade vegetables are an exception. In fact, colored potatoes and tomatoes, in particular, are thought to lower risk.
Nightshades and Cancer
Eating foods that are rich in antioxidants also lowers risk of cancer (18).
Some studies have suggested that the glycoalkaloids produced by nightshades may help fight cancer. Glycoalkaloids from eggplants, potatoes, and tomatoes inhibited the growth of cancer cells in test tube studies, as well as the growth of tumors in animal studies (19).
Another study showed that liquid extract from eggplants decreased a type of inflammation that is associated with cancer in mice (20).
Red peppers and chili peppers contain capsaicin, a phytochemical that has been shown to have anticancer effects (21). Capsaicin has been shown to alter the expression of genes involved in cancer cell survival and growth. Animal and cell studies have indicated that capsaicin may have potential in the treatment of a variety of cancers, including breast, lung, and prostate (22).
High consumption of tomatoes also reduces risk factors for some types of cancer, including prostate cancer (23, 24). In one study of over 47,000 men, those who ate 2 or more servings of tomato sauce per week had a 23% lower risk of getting prostate cancer than those who averaged less than 1 serving per month (24).
The bottom line:
Nightshades contain antioxidants, glycoalkaloids, and phytochemicals that may lower risk of cancer.
Nightshade fruits and vegetables are a rich source of vitamins, minerals, fiber, antioxidants, and other phytochemicals. Increased consumption of edible nightshades, especially tomatoes and colored potatoes, has been linked with lower risk of cardiovascular disease and some types of cancer.
Researchers are continuing to investigate phytochemicals in nightshades, including capsaicin and diosgenin, for potential in fighting inflammation and chronic diseases such as cancer.
For the vast majority of people, nightshade vegetables are likely to provide a variety of health benefits.
People with IBD or other autoimmune diseases may want to be cautious with their nightshade consumption. Pay attention to your symptoms after eating these foods to see whether they cause any irritation. In addition, peeling potatoes and removing any sprouts may help, since that is where most of the glycoalkaloids are concentrated.
If you think eating these foods contributes to your IBD symptoms, speak with your physician or dietitian.