The typical plant contains a total of a percent or more of such substances. Compared to the amount of synthetic pesticides we consume, we eat about 10,000 times more of the plant pesticides. It has long been known that virtually all chemicals are toxic if ingested in sufficiently high doses. Common table salt can cause stomach cancer. Ames and others have pointed out that high levels of chemicals cause large-scale cell death and replacement by division.
Dividing cell are much more subject to mutations that quiescent cells.
Repair mechanisms exist, but they are not perfect. Ames has stated that oxidative DNA damage is a major contributor to aging and to cancer. He points out that any agent causing chronic cell division can be indirectly mutagenic because it increases the probability of endogenous DNA damage being converted to mutations. If chemicals are administered at doeses substantially lower than MTD they are not likely to cause elevated rates of cell death and cell division and hence would not increase mutations. Thus a chemical that produces cell death and cancer at the MTD could be harmless at lower dose levels.