Tobacco is clearly the nightshade with the thickest rap sheet with respect to human health problems. Tobacco smoke is known to contain more than 4000 chemicals, and more than 70 of them are implicated in cancer.
Among nightshade foods, the potato is most obviously associated with human health problems. The case against solanine at high levels is clearly documented, but chronic lower-level exposure has not been sufficiently studied. As shown above there may be other compounds in potatoes that interact with the immune system or other human organ systems.
The connection between human health and the other major foods in the Solanaceae group is even cloudier. The most obvious culprit is the glycoalkaloid solanine, which has reportedly been isolated from all members of the family.1 Solanine levels vary dramatically in different parts of the plant, and over the lifetime of the plant. The presence of solanine in a plant may not mean that there are detectable levels in the part of the plant that we eat.
We can infer from the presence of anatabine across common nightshades, and the chemical signatures of other plant families, that there may in fact be other chemicals shared across multiple species within the family.
Back in Roddick’s day intense chemical analysis was common and published because that was an important technique for scientists tracing the family relationships of these plants. That taxonomic work is now more easily and reliably done using genetic analysis. Chemical inventories of plants rich in natural products are clearly still done, but that detail is probably now more closely held as proprietary knowledge.
That leaves us with scraps and shards to work with in the scientific literature. Following is a brief summary of some of the reported chemistry of possible interest in other nightshade foods.
Nightshade Peppers (Capsicum annuum, C. chinense, C. baccatum, C. frutescens, and C. pubescens)
Most dietary nightshades come from the Solanum branch. Peppers derive from a more distant branch on the Solanacea family tree, and so cary the genus name Capsicum. Biochemically they are known for the capsaicinoids, a group of related compounds including the principle and namesake, capsaicin. The fire behind the nightshade peppers, capsaicin is what we measure with Scoville units.
The capsaicinoids are big enough players in our biological systems—particularly the nervous system—that they can be used both to treat and cause pain. So do peppers help relieve arthritis pain? Or are they players in autoimmune conditions like arthritis? Does it lead to GI disease, or is capsaicin a powerful gastroprotective agent? The cumulative data suggests both are possible. (Look back to section on side effects for how that might work.)
Let’s start with a quick summary on toxicity: “Capsaicin is a powerful irritant; initial administration causes intense pain. Prolonged treatment causes insensitivity to painful stimuli and induces selective degeneration of certain primary sensory neurons. Capsaicin is considered a moderate irritant to human skin and a strong irritant to gastric mucosa. Irritating to mucous membranes; produces severe gastritis and diarrhoea.”2
Capsaicin also has intriguing potential for health promotion. For some people it has unquestioned biomedical utility, already being used in creams, ointments, and patches as a topical pain reliever to treat pain, particularly that resulting from neuropathy and arthritis. Research is ongoing to exploit its antioxidant and antimicrobial activity, and also to explore potential treatments for cancer, diabetes, gastrointestinal diseases, and metabolic syndrome.3
But capsaicin’s charisma also makes it controversial. “Even though widely consumed, capsaicin has a long and checkered history as to whether its consumption or topical use is carcinogenic,” caution researchers. “Conflicting epidemiologic and basic research studies suggest that capsaicin could have a role in either preventing cancer or causing cancer.” While a battery of basic research studies show that capsaicin impedes a variety of cancer cells in lab settings, animal studies have been more ambiguous, showing a mix of carcinogenic and protective effects. Some human epidemiologic studies, while limited, hint that regularly eating hot peppers may increase some cancer risks, particularly of the gallbladder and stomach. The researchers conclude: “Thus, a complete consensus as to whether the primary effect of capsaicin is cancer prevention or promotion has not yet been reached.”4
Eggplant (Solanum melongena)
Not surprisingly, Mendel Friedman has extensively reviewed the scientific record for eggplant. As expected, they have a rich complement of bioactive compounds, including anthocyanidins, flavonoids, saponins, and glycoalkaloids. “Individual or combinations of the bioactive compounds are probably responsible for the reported analgesic, antianaphylactic, anti-inflammatory, antioxidant, antipyretic, intraocular pressure reducing, central nervous system depressing, hypolipodemic, and hypotensive properties of eggplants,” he wrote. The primary glycoalkaloid-related compunds isolated from eggplant are alpha-solamargine, alpha-solasonine, and solasodine. When tested across different varieties, different stages of ripeness, and different cultivation regions the glycoalkaloid levels ranged widely. These same glycoalkaloids, isolated not from eggplant but from other nightshade species, have also shown some promise as anti-cancer agents.5 More recent research shows some extracts of eggplant have protective benefits against DNA damage.6
The eggplant glycoalkaloids appear to be more like the tomato than the potato. “Human toxicological studies have shown that solamargine and solasonine used at certain levels cause such toxic effects as cell-membrane disruption, acetylcholinesterase inhibition, liver damage, heart damage, teratogenicity and embryotoxicity,” summarizes a recent review. “However, it should be mentioned that with a moderate use of these glycoalkaloids, their ability to bind with sterols and complex cholesterol may have beneficial effects.”7
Other nightshade fruits and vegetables
Pepino (Solanum muricatum Aiton)
Closely related to the tomato and potato,8 this mildly sweet and juicy melon-like fruit originates, like the potato, from the Andes. Pepino was an important crop for the Incan Empire, and it’s now experiencing renewed interest.9
As a Solonaceae and close relative of the potato and tomato, a closer examination of its of its chemical potential appears to be underway. It’s been studied for compounds that may have potential as an herbicide10 and in the lab for its anti-cancer and anti-inflammatory properties.11 I could find no breakdown of possible glycoalkaloid content, but there is some indication from related lab tests that the pepino fruit may run light in this regard.9
Goji Berry (Lycium barbarum, Lycium chinense)
The goji, or wolfberry, is native to China and Tibet. It is touted as a superfood, particularly as a rich source of healthful plant compounds including flavan-3-ols, flavonols, monoterpenes, phenolic acids, coumarin, and phenolic amides.12 While it has been published elsewhere that all Solanacea plants contain solanine, I could find no record of solanine in goji. One study focused on devising a new testing protocol for finding glycoalkaloids (chaconine, solanidine, and solanine) and tropane alkaloids (hyoscyamine, scopolamine) detected none in goji berries at the low threshold set by the test.13 A 2006 report of eight samples from China and Thailand found atropine in all samples, but at “far below toxic levels”14 A more recent study of different levels of soil fertilization on goji fruit chemistry reported no tropane and steroidal alkaloids detected.15
One review did summarizes the recommendation of several Traditional Chinese Medicine texts “that patients suffering from diarrhea, fever, arthritis and strong inflammatory conditions should avoid the consumption of the fruit.”16
Susumber berry (Solanum torvum)
Known by many names—devil’s fig, fausse aubergine, gully bean, sundakkai, turkey berry, and soda apple—this plant natively ranges through South America and into the Caribbean, and is a part of Jamaican cuisine. The berries can become toxic when stressed. Poisoning is rare, but has been recorded several times in the literature. Toxicology reports from one incident found the glycoalkaloids solasonine and solamargine as the likely culprit.17 Some work has been done in an effort to isolate potentially beneficial compounds.18
The foods in this branch of the Solanaceae family tree all have a similar visual presentation: a small fruit like a cherry tomato wrapped in a papery husk, or calyx. The naming schemes get somewhat confusing, as all appear to be called ground cherry in at least some areas. The intriguing chemistry here is a steroid-related group called withanolides. A recent review: “This group has received considerable attention based on the potential health benefits associated with their biological activity as it relates to properties that include anti-inflammatory, anti-microbial, anti-tumor, antifeedant, and immunosuppressive. Not surprisingly, Physalis has been used in traditional medicine as treatment for certain maladies.”19 Research specifically on the withanolides from Physalis species has been dwarfed by the research related to ashwaghanda. (See below.)
Tomatillos: (Physallis ixocarpa or P. philadelphica)
Also called Mexican ground cherry or Mexican husk tomato. A native of Mexico and popular throughout Central Americas. Contains withanolides.
Golden Berry (Physalis peruviana)
Also Cape Gooseberry, Peruvian ground cherry, Pichuberry, this common medicinal plant comes from Peru and is now grown all over the world. It is used in many parts of the world for flavoring, as a fruit, and as a juice. The goldenberry—like the goji berry—is sometimes marketed as a superfood. The goldenberry contains more withanolides than other edible Physalis species and there are, predictably, a few studies of antioxidant and anti-inflammatory properties.20 Overall the research is sparse. “It is interesting that the toxicity studies on the fruits of Physalis peruviana L.—a widely used plant in different geographical regions all over the world—are lacking,” reported one research group from 2013.21
Groundcherry (Physalis grisea, P. pruinosa, P. pubescens)
These three species are native to North America and grow wild, though some are cultivated in gardens and small farms. There are efforts to further domesticate. Possible names include: strawberry-tomato, downy groundcherry, husk tomato, low groundcherry and hairy groundcherry. Like other members of the Physalis genus, the plants, including the fruit, contain apparently unique withaneroles.
Ashwagandha (Withania somnifera)
Is ashwagandha—the Indian ginseng—the nightshade exception that breaks the rule? With more than 3,000 years22 of use in traditional healing and within the Indian practice of Ayurvedic medicine, the herbal remedy is intriguing. In multiple searches of academic databases I can find no mention of solanine, the toxic glycoalkaloid common to many nightshades and one suspect in nightshade sensitivity. The plant is a “tremendously rich reservoir of a wide range of secondary metabolites” including about 40 withanolides which appear to be at the center of its biological activity against inflammation, pathogens, and cancer. Despite a significant publication record there is little mention of toxicity or adverse effects. In 2017 the academic publishing giant Springer International Publishing AG rolled out an ebook more than 500 pages long detailing the promising track record and future benefits of ashwagandha.23
- page 646, “4.16 – Human–Environment Interactions – Taste” , in Comprehensive Natural Products II
- “Biological Activities of Red Pepper (Capsicum annuum) and Its Pungent Principle Capsaicin: A Review”
- “Potential Phytopharmacy and Food Applications of Capsicum spp.: A Comprehensive Review“
- “Toxic Phytochemicals and Their Potential Risks for Human Cancer”
- “Chemistry and Anticarcinogenic Mechanisms of Glycoalkaloids Produced by Eggplants, Potatoes, and Tomatoes”
- “Eggplant fruits protect against DNA damage and mutations”
- “The steroidal glycoalkaloids solamargine and solasonine in Solanum plants”
- “The first de novo transcriptome of pepino (Solanum muricatum): assembly, comprehensive analysis and comparison with the closely related species S. caripense, potato and tomato”
- “Phenolic Profile and Biological Activities of the Pepino (Solanum muricatum) Fruit and Its Wild Relative S. caripense”
- “Phytotoxicity and cytogenotoxicity of hydroalcoholic extracts from Solanum muricatum Ait. and Solanum betaceum Cav. (Solanaceae) in the plant model Lactuca sativa”
- “Solanum muricatum Ait. inhibits inflammation and cancer by modulating the immune system”
- “Superfruits: Phytochemicals, antioxidant efficacies, and health effects – A comprehensive review“
- “Densitometric TLC analysis for the control of tropane and steroidal alkaloids in Lycium barbarum”
- “HPLC-MS Trace Analysis of Atropine in Lycium Barbarum Berries”
- “Impact of Nitrogen Fertilizer Levels on Metabolite Profiling of the Lycium barbarum L. Fruit”
- “Goji (Lycium barbarum and L. chinense): Phytochemistry, Pharmacology and Safety in the Perspective of Traditional Uses and Recent Popularity”
- “Susumber berries: Unexpected cause of cholinergic poisoning”
- “Protective effect of Solanum torvum on doxorubicin-induced nephrotoxicity in rats”
- “A review of nutritional properties and health benefits of Physalis species”
- “A review of nutritional properties and health benefits of Physalis species”
- “Acute and Subchronic Toxic Effects of the Fruits of Physalis peruviana L.”
- page 717, “Chapter 52 – Ashwagandha: Multiple Health Benefits”, Nutraceuticals Efficacy, Safety and Toxicity
- page 26, Science of Ashwagandha: Preventive and Therapeutic Potentials