Mostly through soils, but also through the manufacturing process.

“Even living in the tropics, there are many possibilities for diets rich in signal-disrupting substances, including iron, and in high latitudes there are opportunities for reducing our exposure to them. As a source of protein, milk is uniquely low in its iron content. Potatoes, because of the high quality of their protein, are probably relatively free of toxic signal-substances. Many tropical fruits, besides having relatively saturated fats, are also low in iron, and often contain important quantities of amino acids and proteins. In this context, Jeanne Calment's life-long, daily consumption of chocolate comes to mind: As she approaches her 121st birthday, she is still eating chocolate, though she has stopped smoking and drinking wine. The saturated fats in chocolate have been found to block the toxicity of oils rich in linoleic acid, and its odd proteins seem to have an anabolic action.”

Ray Peat