In some people eating potatoes, tomatoes, peppers and aubergines can trigger psoriasis

potatoes are a member of the nightshade family

Everyone processes food in different ways. For some people, members of the Nightshade family of fruits and vegetables—which includes potatoes, tomatoes, peppers and aubergines—can be be an inflammatory agent which scientists believe can trigger the highly discomforting and unsightly skin disease, psoriasis.

Psoriasis a chronic disease of the skin. Cells in normal skin mature and replace dead skin every 28-30 days but psoriasis causes skin cells to mature in less than a week. In this group because the body can’t shed old skin as rapidly as new cells are rising to the surface, inflamed areas develop on the arms, back, chest, elbows, legs, nails, folds between the buttocks, and scalp which are characterised by patches of raised, reddish skin covered with silvery-white scales.

Intolerance to nightshades is not a ’cause’ of psoriasis because many people who eat them don’t suffer; but foods and products produced by this family of plants can certainly trigger or worsen the symptoms of psoriasis in some people who ingest or who are exposed to them.

Why would this be?

Potatoes, tomatoes, peppers and aubergines belong to the family of Solanaceae plants of the Solanum genus. There are more than 2,500 species in the nightshade family and they are widely used in both foods and medicines. The Nightshade family includes potatoes (but not sweet potatoes), peppers and chilli (but not the pepper you grind on your food), tomatoes and tomatillos, aubergines and goji berries. garden huckleberries, ground cherries and cape gooseberries (but not normal gooseberries nor blueberries).

All nightshade plants contain the alkaloid compound Solanine at various levels. Solanine comprises a natural defence system for them, acting as a nerve poison on insects that try to eat them and is particularly concentrated in the stems and leaves of nightshade plants and in green potatoes.

Levels of tolerance vary: some people (like me) can eat moderate amounts of tomatoes and (non-green) potatoes without much ill effect but have an immediate reaction if they handle the growing plant, for example as a grower or harvester. Others, like Puraglow’s founder Lesley Wall, will get an immediate reaction if they ingest even very small quantities of potato in prepared food, for example as potato starch.

Nightshades contain two primary toxins: Saponins and Lectins.

Scientists believe for some susceptible groups, ‘anti-nutrients’ in nightshades interfere with their digestion causing intestinal permeability, or “leaky gut”. This leaves unprotected holes in the intestinal lining, an open invitation to many of our modern auto-immune diseases. Both saponins and lectins play a major role in increasing intestinal permeability laying the groundwork for a variety of conditions which seem to have become more common in recent times, largely as a result of modern patterms of consumption.

Saponins are natural chemicals in some plants that can impair health by creating holes in the intestinal lining. A perforated intestine is vulnerable to any microbes and toxins that may enter the bloodstream. Foods high in saponins are potato skins and potato chips with the skins. Ripe tomatoes have low levels of toxic saponins. But green tomatoes and “hot house” tomatoes, or those that are harvested before they are ripe, are high in saponins as some saponins stay in the fruit if they are ripened off the plant.

Lectins are natural proteins in plants that are cell code breakers. Human cell walls are covered with chemical receptors to protect and ensure entry of only the right compounds. Lectins can bypass defences travelling through the body. Lectins can penetrate the protective mucus of the small intestine, promoting cell division at the wrong time. Lectins can perforate the intestinal wall, or trick the immune system to thinking there’s an intruder and when this happens the result is an allergic reaction.

The human immune system defends against disease and fights infection and in doing so produces T-cells. T-cells normally travel through the body to detect and fight invading germs such as bacteria, but in people with psoriasis they start to attack healthy skin cells by mistake.

This causes the deepest layer of skin to produce new skin cells more quickly than usual, which in turn triggers the immune system to produce more T-cells.

The attack itself carried out by cytokines, which are proteins that help control the immune system’s inflammatory response. Cytokines trigger inflammation, causing the blood vessels to expand and send more immune cells to different parts of the body. In psoriasis, this inflammation happens in the skin, leading to the red, itchy and scaly patches known as plaques. In psoriatic arthritis, this inflammation happens inside the body, leading to swollen and painful joints and tendons.

Drugs have been developed which can counter this but they come with many side effects. For people who suffer a problem with nightshades like Puraglow’s Lesley Wall, the easiest strategy might be to eliminate them from the diet.

Meanwhile scientists are continuing to study the complex relationship between the immune system and psoriatic disease and working to identify the antigens that trigger the autoimmune response in psoriasis and psoriatic arthritis, to better understand the role played by different kinds of immune cells in psoriatic disease, and develop new therapies that target cytokines or other parts of the immune system.

See: Psoriasis: Symptoms, Causes, Triggers and Treatments

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