As you might imagine, I get a ton of questions about broccoli. Here’s one that is pretty typical.
Q: Does cooking broccoli really alter its protective potency? If so, how should I cook it, because I don’t like it raw?
or sometimes the question is more like this:
I really like broccoli and I already eat a lot of it. . . . how much is too much? . . . or enough?
A: These are loaded questions, so let me pick them apart and answer them in pieces:
Fresh broccoli is highly variable with respect to the amount of phytochemicals that it contains. Market-stage broccoli can vary in its levels of a variety of phytochemicals including glucoraphanin (GR), the precursor of sulforaphane (SF), two phytonutrients (phytochemicals) that I’ve spent much of the past quarter century working on. We demonstrated that this was largely due to the genetics of the plants with some influence from the environment in which the broccoli is grown and how it is handled after harvesting (e.g. soil and agronomy, weather, pests, fertilizer, climate, post-harvest refrigeration, etc.). Unfortunately, there is no way that a consumer can tell (e.g. no signal such as color, smell, or texture) which broccoli has higher concentrations of GR. Likewise, agronomic practices don’t govern GR levels in predictable ways. For example, organically grown broccoli is just as variable, since it depends on the cultivar being grown. We do know that broccoli sprouts and seed provide far higher concentrations of GR than market stage broccoli, but they too vary from cultivar to cultivar – this was a critical finding from our Cullman Chemoprotection Center at Johns Hopkins Medical School.
The broccoli plant naturally contains both glucoraphanin (GR) and the hydrolyzing enzyme myrosinase. They are in separate compartments within the plant's cells. When this enzyme comes into contact with GR because the cells are broken (for example, as it is chewed), it converts GR to sulforaphane, which is what’s biologically active in humans and other animals.
Heating / cooking the broccoli can thermally deactivate the myrosinase, depending on time and temperature… so most cooked broccoli contains only GR - which is then converted to SF by the body’s microflora. All of us contain many species of bacteria in our guts (lower G.I. tract) that possess myrosinase activity. The balance of these bacterial species with other bacteria in our gut that do not have myrosinase is a complete black box at this point. We’re not ready to try to predict, and likewise, there are no supplements that will stably or safely change the myrosinase content of your intestines.
The amount of sulforaphane that enters the digestive tract is generally higher if you consume fresh broccoli than it would be if you were to consume that same portion of broccoli, cooked. However, the extensive epidemiologic evidence supporting the health benefits of broccoli largely reflects the fact that the vast majority of broccoli consumed in the USA is cooked – and therefore its myrosinase has been inactivated. Thus, the epidemiology primarily reflects consumption of GR, not SF. To further complicate this, there is significant inter-individual variability on how the body converts GR to SF, as well as intra-individual variation due to the presence of other dietary components, metabolic shifts, circadian rhythms, amount of sleep, hormone swings, and on and on and on. Again, one of the things we know is that there is still an enormous amount about this variability that we don’t yet understand. In fact, one of the main metabolic pathways through which SF acts is under strong circadian control . . . so now we could even consider WHEN we eat our broccoli. But please don’t! Just eat more of it.
O.K. Let’s get even deeper in the weeds here. Broccoli also contains an epithiospecifier protein (ESP) that guides the enzymatic conversion of GR to produce sulforaphane nitrile rather than sulforaphane. There is nothing bad about the nitrile form, but it is a much, much weaker inducer of the cytoprotective and antioxidant response in humans, so if there is conversion to nitrile, you aren’t getting the full benefit of the GR in the broccoli. The presence of ESP may vary from broccoli cultivar to cultivar, so this is also a bit of a black box, but work by others tried to compensate for this by suggesting cooking conditions that would selectively inactivate ESP, but not myrosinase. (They’re both proteins and they’re both denatured or rendered inactive at some point between “very warm” and “boiling hot.” This is an extremely difficult balancing act, and even I don’t try to thread that fine temperature line when I cook broccoli (yes, I eat plenty of it). Some have suggested microwaving to a certain temperature for a very specific amount of time…. In short, it is very difficult to control at home, and I think that the juice just isn’t worth the squeeze.
Across the scientific literature there is no consensus about some of the things I’ve commented on -- probably because each experiment started with different broccoli and a different set of base conditions. Add to that the fact that a number of commercial labs don’t properly measure GR and SF separately, nor do some university labs. One way around this guessing game is to add a little exogenous mustardseed (which does not contain ESP) to cooked broccoli as the source of myrosinase.
And finally, please remember. . . and remind anybody talking about glucoraphanin and sulforaphane, that whereas all crucifers contain glucosinolates, only broccoli contains an appreciable amount of glucoraphanin, which is its predominant glucosinolate. Many nutritionists and popular press articles suggest otherwise, but they haven’t done their homework carefully, and they are wrong!
. . . so if your haven't guessed already, this is not easy. There are dozens of papers that attempt to develop “best practice” cooking techniques for broccoli that preserve the myrosinase or maximize the conversion to SF. Some involve short cooking times, some steam, some microwave, etc. and thoroughly reading the scientific literature does not lead to one definitive answer.
So, what’s a broccoli lover to do?
My practical advice on this is as follows:
Eat as much broccoli or broccoli sprouts as you want or as much as you can, and you will be getting the benefits of vegetables – fiber, nutrients, vitamins, minerals, and other phytochemicals (remember that glucoraphanin is a phytochemical or phytonutrient). You will also be crowding out less healthy food choices, including highly processed foods that are unquestionably bad for you.
Eating broccoli fresh is likely the best strategy – no matter if the level of GR is low or unknown, you’ll get the most benefit because the active myrosinase will make it as available as possible.
If you want to cook broccoli, go ahead. Even if the myrosinase is deactivated, you will still be getting GR, and the body will naturally convert it to SF – as supported by most of the epidemiological studies that include a lot of cooked broccoli. Probably less cooking is better because less GR will be leached out or lost in the cooking process, but it’s extremely difficult to say that one cooking method is optimal.
If you boil the broccoli, be sure to capture the liquid fraction, as GR is water-soluble and may end up in the leftover liquid – the ‘pot liquor.' For those of us who suffered from years of boiled broccoli at school lunches, many of the good phytochemicals ended up in the smelly leftover water and were poured down the drain.
If you really want to get measured and reliable levels of certain phytochemicals, supplementation is a good way to go. Without being self-serving, a company that I co-founded (Brassica Protection Products), and that I now serve as a scientific advisor to, has spent many years developing and ensuring that its TrueBroc® broccoli seed extract has meaningful and standardized levels of GR. This standardized extract is used in some of the better nutritional supplements presently on the market. Co-delivering TrueBroc with a good source of myrosinase – e.g., mustard or wasabi – increases the amount of GR that will be available to the consumer -- although there is still inter-individual variability. Most importantly, taking such a supplement should not in any way preclude eating plenty of broccoli and other cruciferous vegetables – it will just ensure that you are getting a meaningful and standardized amount of GR, and of course it is a work-around for those who dislike broccoli.
Do I suggest that you take nutritional supplements to achieve reasonable amounts of each of the literally tens of thousands of phytochemicals that may be good for you? Of course not. However, glucoraphanin/sulforaphane has so many well documented health benefits, and is now the subject of almost 100 clinical studies, that I think it is one of the phytochemicals for which the insurance policy of supplementation is worthwhile.