A recent (and very readable) article in the journal Nature reviews the various reasons why it is so difficult to make sense of research (even well-designed and implemented research) that tries to evaluate any one food consumption's effect on cancer development (or even most aspects of health for that matter) and then follow-up with responsible health and dietary guidelines for the public.
Differences of content of various nutrients or "bio-active constituents" (a generic term used for all molecules in food that have a metabolic action in our body, even if they are not traditionally considered a nutrient) in the food itself (due to seed variety, growing conditions, food processing, shipping, etc) PLUS differences in how any one individual processes these molecules (basic DNA differences, epi-genetic differences by factors that influence the DNA expression, to differences in content and action of gut bacteria, etc) all play roles here.
The article uses broccoli sprouts as a food created for and being promoted for cancer prevention (a relative of kale, thus the relevance for sharing this article on my kale blog) as an example of how a food has become a 'functional food', defined by The Institute of Medicine as "those foods that encompass potentially healthful products including any modified food or ingredient that may provide a health benefit beyond the traditional nutrients it contains".
The article is informative by itself, however, the comments always add other perspectives, so I recommend reading them, too.
Thus, it is currently not known 'how much to eat' of kale or any other food as a recommendation to any one person for any one situation or 'health goal', and frankly, I don't expect that information to ever really available or even necessary. While it might seem 'snazzy' in this day of 'apps' available for nearly everything we want to know and know fast, to get to that point would require millions and millions and millions of dollars for technological research within a mind-set and supported by an industry that promotes 'nutrition reductionism'.
My mind-set and bottom-line regarding 'how much to eat' is to go back several steps to the basics just like Michael Pollan's catchy phrase summarizes: "Eat food, not too much, mostly plants".
Here is what I would add to Mr. Pollan's mantra:
1) Grow as much food as possible. A basil plant on a windowsill is great! So is growing kale within your landscaping. One of my son's did this at his apartment complex.
2) Cook (prepare at home from food ingredients) as much of your meals as possible.
3) Don't go 'overboard' on any one food, including kale. One serving per day, 5-7 days per week is just great and likely 'enough' within a diet that has a wide variety of foods.
4) Try eating new foods. There are multiple Brassica vegetables in addition to kale, but seek out other new foods or varieties, too. For example, we are growing 42 varieties of garlic this year, all of which have different characteristics, meaning different 'bio-active constituents' that, guess what?, influence taste besides influencing metabolic function in the body.
5) ENJOY food. I cannot emphasize this enough. Please don't only think of food as 'functional medicine'.
I cannot end without adding that I hope you will seek out food raised organically and as much as possible that is raised organically by your local farmers, plus I hope that you will find some niche/passion/action to help provide 'good food for all' in whatever way you can, wherever you live.
Good food, local food, organic food is not elitist but a fundamental right for all people.
Where kale is apparently far more than just decoration on my plate. :-)
Diana Dyer, MS, RD