Many new diets make claims that raw food, particularly vegetables, is better for you based on the assumption that our ancestors evolved to eat food that way and cooking destroys some of the nutrients. As I write this sentence I am neutral on the claims made of consuming raw vegetables. The claims do make sense to me because I know that heat tends to destabilize the structure of proteins, essentially rendering them inactive, and alters the structure of chemical compounds (please note I use the word chemical here in a general sense to describe naturally occurring chemicals, such as those made by plants, as well as synthetic chemicals). But I also know that the process of digestion is designed to do exactly that. With that said, does it even matter if you eat your vegetables raw if everything going down your gullet will be broken down to basic molecular units?
As a starting point, I began by reading a review of the literature published in 2004 that examined 40 different studies from 1994 to 2003 that were looking at the relationship between vegetables and cancer risk. Overwhelmingly, 92% of the studies that were evaluated found a decreased risk of cancer in all groups of people who ate more vegetables then a test groups who ate less. Generally speaking there was an even greater reduction in the risk of getting cancer for the people who consumed raw vegetables.
There were a couple problems with the studies used in this review, which is not a flaw of the review itself but speaks more of the information that was available to them. First, none of the studies were directly comparing the impact of raw vs cooked vegetables but rather were interested in whether vegetables reduced the risk of different kinds of cancer. It was just a bonus that some of them did actually distinguish between cooked and raw in their data. Secondly, the portion sizes varied greatly between studies and even varied between raw and cooked quantities within studies. Lastly, the majority of the studies did very little to differentiate between the type of vegetables consumed.
Despite the limitations of those studies, there is a reason why eating raw cruciferous vegetables such as broccoli, cabbage, turnips and many others could be more beneficial than their cooked counterparts. These vegetables contain compounds called glucosinolates, which are believed to provide little to no benefit by themselves. Although there are emerging studies that may be proving this notion wrong. Glucosinolates are converted to isothiocyanates by enzymes found in the plants themselves, as well as bacteria in our intestines. Isothiocyanates are known to activate the “detoxifying” antioxidant enzymes in our body called CYP2E1 and glutathione, and the activity of these enzymes are important for preventing diseases like cancer.
The problem is that heat tends to destroy the enzymes needed to convert glucosinolates in the first place. Boiling your vegetables will lead to the majority of the glucosinolates to leach out into the water, while steaming, stir frying, or microwaving will leave them intact. Even though the cooking methods other than boiling leaves the glucosinolates intact, the heat will still destroy the enzyme needed to convert them to the more beneficial isothiocyanates. Keep in mind that your own intestinal bacteria will still work to produce isothiocyanates, but the bioavailability will still be decreased a bit when eating the cooked veggies. (Bioavailability meaning the amount that is absorbed into the bloodstream.)
Sulforaphane is a specific example of an isothiocyanate. Found in high concentrations in broccoli as well as other leafy green cruciferous vegetables, sulforaphane is known as a powerful cancer inhibitor. One study found that sulforaphane was absorbed faster and had a higher bioavailability (about 37%) in people who consumed raw broccoli over cooked. I found another study that saw a 10 fold increase in sulforaphane bioavailability in people who ate soup made from fresh broccoli over frozen broccoli. They attributed the difference to the blanching process that frozen vegetables under go before freezing, which apparently destroys the enzyme needed to convert glucoraphanin (the parent glucosinolate molecule) to sulforaphane.
So far, it sounds pretty simple. Raw is the way to go, right?
Unfortunately, it’s not that simple. A study looking at the effect of deep frying, steaming and boiling on the nutritional content of broccoli, carrots and zucchini found mixed results. In general, boiling did not effect total carotenoid levels and in some cases actually enhanced them. Side note: carotenoids are a class of compounds that serve as antioxidants and immune system boosters. Boiling decreased vitamin C content only in the zucchini and not so much in the other vegetables. Steaming had a further decreasing effect on carotenoids and vitamin C, except in broccoli, but was superior to boiling and frying in retaining polyphenols. Deep frying in peanut oil caused a big decrease in carotenoids, but was superior to boiling when it came to preserving polyphenolic content. Vitamin C content was similar between frying and steaming in zucchini but markedly decreased in the broccoli compared to other methods.
If you didn’t manage to stay with me on that last paragraph, its fine. The point I was trying to make is that its a complex issue.
Cooking can have more obvious benefits, such as killing bacteria. It can also make food more digestible by increasing soluble fiber, decreasing insoluble fiber, and modifying the structure of proteins. Legumes (ie. beans, peas, and lentis) contain protease inhibitors that act against the enzymes in your digestive system. Those inhibitors are destroyed by heat and will allow your body to digest legumes much easier.
You can choose to steam or stir fry vegetables if you’re concerned about retaining the most nutritional value while cooking. Alternatively, if you boil your veggies, find a way to use the water as it is the major reason for nutrients leaching out of the food.
Following a completely raw diet, as some diet fads suggest, would be the height of insanity. You can begin to develop health problems as a result of such a diet. Bad teeth, gastrointestinal problems, and (ironically) nutritional deficiencies are some of the issues you would eventually be faced with.
I think the take home message here is variety. Eat vegetables however you can get them and you’ll already be decreasing your odds of getting cancer. If you’re all about optimizing your chances then mixing raw food and cooked food into your diet will give you the best results in terms of health and disease prevention.