Kale (Brassica oleracea var. sabellica) is frequently promoted as a cancer-preventive food in natural therapy circles. This recommendation is primarily grounded in scientific evidence, though the strength of this evidence is modest and largely preclinical. Kale contains high levels of glucosinolates, which are sulfur-containing compounds that, when broken down (e.g., by chewing or digestion), produce biologically active metabolites such as sulforaphane and indole-3-carbinol. These compounds have been shown in cell culture and animal studies to have anti-cancer properties, including the ability to modulate carcinogen metabolism, induce apoptosis (programmed cell death) in cancer cells, and inhibit tumor growth.

Epidemiological studies suggest that populations with higher intakes of cruciferous vegetables (including kale) have a reduced risk of certain cancers, such as lung, colorectal, and breast cancer. However, these studies are observational and cannot definitively prove causation. Clinical trials in humans specifically using kale are limited, and most data come from studies of cruciferous vegetables as a group. Thus, while there is a plausible biological mechanism and some supportive population data, direct evidence for kale as a cancer therapy in humans is lacking.

In summary, scientific evidence supports the idea that kale contains compounds with anti-cancer activity, but its use as a treatment rather than as part of a preventive diet is not strongly validated. More robust clinical trials are needed to confirm its efficacy in cancer therapy.