They are concerned that healthy foods and supplementation will interfere with the poison chemo. There is a grain of truth to that, but it is about 90% nonsense.
Especially with supplements and food that have anti oxidant's and anti inflammatory benefits. They will protect the healthy cells, limiting the damage to them. And the chemo will still attack the cancer cells, as they do not have the design internal design or intelligence to absorb the benefits of the good foods and the supplementation that will help protect the healthy cells. It is all about limiting the collateral damage of the chemo. Chemo is like nuking the body from the inside. It is like agent orange. It just indiscriminately destroys everything in its path.
A cancer cell basically has only two functions. Feed and multiply. It's design is not as complex as as the regular cell. That is why it does not compete with the body for most of the benefits of the chemical by products of food such as anti oxidants or vitamins or anti inflammatory's. It is not interested in those elements. It just wants to eat. It especially wants the sugar and protein from the foods. Many people who have cancer die from malnutrition, as the cancer cells have more receptors then regular cells for insulin and protein, so they deprive, or steal, the regular cells nutrition. The lack of nutrition further weakens the immune system, and allows the cancer to proliferate at a even faster rate, and so on. It is a insidious cycle.
The medical community know this. Why they discourage people from taking supplements and herbs is beyond me.