Oxalic Acid and the Nightshade Story

The plant family known as nightshade is of prime concern. These are potatoes (not sweet potatoes, they are OK), eggplants, tomatoes, peppers, tomatios and tobacco. All contain a substance called solanine, which protects them from being eaten by insects and other living things before they are ripe. Solanine interferes with enzymes in the muscles of humans and can cause us to experience pain. The solanine levels are highest in unripe vegetables but drop as the vegetable ripens. A vine ripened tomato will not cause problems while a tomato picked green and ripened off the vine will. Be sure to buy vine ripened tomatoes and not get tricked by on the vine tomatoes. Bell peppers are problematic. Green bell peppers are not ripe; therefore they are high in solanine. Bell peppers turn red when they are ripe and red bell peppers will not cause a problem. This goes for hot peppers also, red is good, green is not. Potatoes are always a problem. When they are harvested they defoliate the above ground portion first. The plant then puts more solanine into the potato to protect itself from the predator. As far as eggplants go, they should be vine-ripened for the solanine levels to drop.
Solanine should be avoided by everyone but especially those with arthritis and back pain.
Oxalic acid is a substance found in high concentrations in eggs, fish, and certain vegetables (Balch and Balch pg. 483). Diets high in oxalic acid have been linked to the formation of kidney stones in some individuals. The foods that contain oxalic acid should also be avoided by those with joint problems (Balch and Balch pg. 278). Oxalic acid also reduces iron absorption.
The vegetables that I usually avoid which contain oxalic acid are: spinach, beet greens, Swiss chard, sorrel, and rhubarb. When I eat them they leave an unpleasant and somewhat astringent feeling in my mouth. Controversy still exists whether or not oxalic acid is more problematic in raw or cooked foods. I get the same unpleasant feeling when those vegetables are consumed raw or cooked, so I avoid both. There are other foods which contain oxalic acid but they do not cause the unpleasant feeling in my mouth and I believe that they have very small amounts of oxalic acid, so I will not list them here. If you have kidney stones, obtain a complete list from Prescription for Nutritional Healing Third Edition. More important than that is avoiding a diet high in animal protein because if you are prone to kidney stones, the consumption of animal protein has been strongly associated with oxalate absorption (Balch and Balch pg. 484).
Each one of us is different. If you are curious as to whether the oxalic acid-containing vegetables are right for you, then I suggest a simple test. On an empty stomach, eat a portion of the vegetable that you wish to test (and only that vegetable, no dressing) and see how it makes you feel.