A reader recently asked a good question about which cooking methods cause the most loss of nutrients in kale; specifically he was wondering about the relative benefits of raw versus steamed kale. Reading a number of research studies investigating this question in Brassica vegetables shows a variety of answers. However, here are abstracts from two recent articles:
(1) Journal of Zhejiang University SCIENCE B, 2009 Vol. 10(8):580~588, Yuan GF, Sun B, Yuan J, Wang QM. Effects of different cooking methods on health-promoting compounds of broccoli.
In general, the steaming led to the lowest loss of total glucosinolates, while stir-frying and stir-frying/boiling presented the highest loss. Stir-frying and stir-frying/boiling, the two most popular methods for most homemade dishes in China, cause great losses of chlorophyll, soluble protein, soluble sugar, vitamin C, and glucosinolates, but the steaming method appears the best in retention of the nutrients in cooking broccoli (I suspect the same will be true for kale and other Brassica vegetables).
This next abstract shows the many points between raw, cooking, consumption, and absorption that influence the potential outcome for health-promotion from eating Brassica vegetables.
(2) Proc Nutr Soc. 2007 Feb;66(1):69-81. Effect of cooking brassica vegetables on the subsequent hydrolysis and metabolic fate of glucosinolates. Rungapamestry V, Duncan AJ, Fuller Z, Ratcliffe B.
The protective effects of brassica vegetables against cancer may be partly related to their glucosinolate content. Glucosinolates are hydrolysed by plant myrosinase following damage of plant tissue. Isothiocyanates are one of the main groups of metabolites of glucosinolates and are implicated in the preventive effect against cancer. During cooking of brassica the glucosinolate-myrosinase system may be modified as a result of inactivation of plant myrosinase, loss of enzymic cofactors such as epithiospecifier protein, thermal breakdown and/or leaching of glucosinolates and their metabolites or volatilisation of metabolites. Cooking brassica affects the site of release of breakdown products of glucosinolates, which is the upper gastrointestinal tract following consumption of raw brassica containing active plant myrosinase. After consumption of cooked brassica devoid of plant myrosinase, glucosinolates are hydrolysed in the colon under the action of the resident microflora. Feeding trials with human subjects have shown that hydrolysis of glucosinolates and absorption of isothiocyanates are greater following ingestion of raw brassica with active plant myrosinase than after consumption of the cooked plant with denatured myrosinase. The digestive fate of glucosinolates may be further influenced by the extent of cell rupture during ingestion, gastrointestinal transit time, meal composition, individual genotype and differences in colonic microflora.
Bottom Line? Enjoy all varieties of kale and other Brassica vegetables using all methods of preparation, however, it seems prudent to consume as much as possible either raw (well-chewed or put into a smoothie) or lightly steamed to maximize their cancer-fighting potential.
Where kale is more than decoration on my plate!
Diana Dyer, MS, RD