James Roddick produced much of the nightshade focused research of the 70s and 80s. Over the last 20 years the undisputed leader of nightshade research is Agricultural Research Service chemist Mendel Friedman, based at the USDA’s Western Regional Research Center. With a minimum of 30 publications involving nightshade elements I’ll try to synthesize and summarize the most relevant health details.
Friedman’s work reinforces that nightshades are chemically a very diverse plant family, and each species harbors a wealth of biologically active compounds. Some of these compounds, and perhaps many, are nutritionally beneficial for most people. Some may have food science or pharmaceutical value beyond nutrition. And some may cause health problems for some people.
Every plant is made up of thousands of chemicals, and their ultimate chemistry is determined by a combination of environment and genes.1 Farmers manipulate both to influence the final product. Potatoes, for example, are monitored for glycoalkaloid content. Too many glycoalkaloids and the tuber becomes inedible. But potato growers benefit from glycoalkaloids in the leaves because they’re a natural pesticide, warding off disease and troublesome insect pests. Breeders, who are always trying to improve working plants, have to balance this cost-benefit equation. They don’t always get it right: In 1970 in Canada and the US a new potato breed was brought to market, then removed because of high solanine content. (It remains in use for breeding, but not for food production.)2 In 1986 glycoalkaloids ran alarmingly high in a Swedish potato cultivar.3
Friedman’s research reflects this balancing act. He records the known difficulties, particularly with glycoalkaloids, while also prospecting for benefits. Overall he documents more benefits than harm.
Consider the tomato. It’s a great source of the red pigment lycopene, and epidemiological studies suggest that people with high lycopene levels in their blood have a lower risk of breast, colon, lung, and prostate cancer. Lycopene traps chemical radicals that damage DNA, cells, and tissues, which may explain these health benefits. Tomatoes also contain many other antioxidants like ascorbic acid, beta-carotene, chlorogenic acid, rutin, plastoquinones, tocopherol, and xantophylls. Friedman posits these active ingredients may work with lycopene to prevent cell damage, because eating the whole tomato appears more effective in cancer prevention than pure lycopene.4
So what about tomatine, the primary tomato glycoalkaloid? Friedman notes that “high-tomatine tomatoes are consumed without apparent acute toxic effects; therefore, tomatine appears to be much safer for humans than are potato glycoalkaloids: “This conclusion is reinforced by the widespread consumption of high-tomatine ‘pickled green’ and ‘fried-green tomatoes’ without apparent ill effects.” Tomatine does inhibit acetylcholinesterase, though at a lower level than potato glycoalkaloids.4
Tomatine also binds with cholesterol, activity long considered good from a dietary health perspective. Less well understood is its impact on other cholesterol-dependent processes, including the role this may play in intestinal permeability and the whole ‘leaky gut’ hypothesis. (Briefly: Failing structural integrity of the lining of the intestine allows some large molecules to slip into the bloodstream, inciting an immune response. Intestinal permeability is undeniably compromised in inflammatory bowel disease. Later in this essay I explore some research that indicates this may also play a role in the the genesis of other autoimmune conditions. While that research is not quite ready for prime time, the ‘leaky gut’ idea has taken on something of a life its own in alternative health.)5
In rat studies, tomatine removes cholesterol from mucosal cells that line the intestine.6 Does this affect their function, particularly their ability to maintain a barrier?
Nutritional science up until now has parsed out these benefits at a population level. We can say with some certainty that for large populations a diet with tomatoes is better for overall health than a diet without tomatoes. But that body of knowledge does not necessarily translate to individuals. If 99 people are healthier eating tomatoes, and one person suffers from it, none of the nutritional science from the last three decades will show that harm. Nutritional science and it’s resulting dietary recommendations has been almost entirely about population averages. Sensitivity is a more individualized phenomenon.
One last nugget of intrigue comes from the world of vaccine adjuvants. These are “molecules or compounds that have intrinsic immunomodulatory properties and, when administered in conjunction with an antigen, effectively potentiate the host antigen-specific immune responses compared to responses raised when antigen is given alone.”7 And tomatine, as is turns out, is “widely recognized as a strong immunostimulator.”8
The emerging science of nutrigenomics may help us compare our insights about population health with individual variations. But until those methods emerge, we’re left with more questions than answers about if and how tomatoes might cause health problems.
The evidence is more visible in the potato.
NEXT: The Potato Problem
- “Effect of Genotype and Environment on the Glycoalkaloid Content of Rare, Heritage, and Commercial Potato Varieties” & “Differential responses of five cherry tomato varieties to water stress: Changes on phenolic metabolites and related enzymes”
- “Potato Glycoalkaloids: Chemistry, Analysis, Safety, and Plant Physiology”
- “High Levels of Glycoalkaloids in the Established Swedish Potato Variety Magnum Bonum”
- Tomato Glycoalkaloids: Role in the Plant and in the Diet”
- “Leaky gut: mechanisms, measurement and clinical implications in humans” & “Leaky Gut As a Danger Signal for Autoimmune Diseases”
- “Tomato Glycoalkaloids: Role in the Plant and in the Diet”
- page 1183, Chapter 61: Mucosal Adjuvants: New Developments and Challenges, in Mucosal Immunology.
- page 212, Chapter 11: “Natural Vaccine Adjuvants and Immunopotentiators Derived From Plants, Fungi, Marine Organisms, and Insects” in Immunopotentiators in Modern Vaccines (Second Edition).