Nightshades.

"Nightshade" is actually the common name used to describe over 2,800 species of plants, many with very different properties and constituents. All of the plants, however, belong to a scientific order called Polemoniales, and to a scientific family called Solanaceae. To give you an idea of the diversity associated with this group of plants, consider the fact that tobacco, morning glory, potato, and tomato are all classified as nightshades.

Nightshades contain substances known as alkaloids. These substances can impact nerve muscle function and joint function. Cooking nightshades lowers the alkaloid content by about 40-50% The steroid alkaloids in potato - primarily solanine and chaonine - have been studied for their health effects in two areas. First is their ability to block activity of an enzyme in nerve cells called cholintesterase. Many of the alkaloids found in nightshades possess this kind of activity, called cholinesterase inhibition. If the activity of cholinesterase is too strongly blocked, the nervous system control of muscle movement becomes disrupted, and muscle twitching, trembling, paralyzed breathing, or convulsions can result. Nightshade has widely been known to aggravate arthritis and other joint diseases. Some people are very sensitive to alkaloids and find that cutting out nightshades relieves such symptoms.

If you think you may have a sensitivity to alkaloids then you should try to cut them out of your diet and see if you notice any difference :)