Dear Cecil: Raw foodists claim that heat destroys digestive enzymes in fruits and vegetables, reducing the benefit we can derive from eating them. I've also heard an opposing claim: that cooking vegetables breaks down indigestible cellulose, which allows us to absorb more nutrients. What do you say? Red Ree

Cecil replies:

I can’t blame you for seeking a second opinion on this one, RR. A quick Google search for “digestive enzymes” yields reams of websites with names like Life Force and Soul Healing, all vigorously encouraging the reader to ingest extra helpings of these enzymes — whether as found naturally, in fruits and vegetables, or synthetically, via the growing supplement market. Keep clicking and pretty soon you’re reading about vaccinations, autism, colloidal silver, precious bodily fluids, etc.

In such a context, trend pieces about celebrities who’ve bought into enzymes start to look authoritative; legit medical literature is sparse at best. This is because, as far as I can gather, the alleged science in play here is so crackpot that doctors and dietitians have been disinclined to waste time refuting it. As a columnist in the Journal of the American Dietetic Association succinctly put it, “The problem with this theory is that the body already makes the enzymes needed to digest and absorb food, and the enzymes in food are inactivated by the acidity of the stomach.” In other words, you don’t need ’em, and they don’t help.

That’s not to say there isn’t a good question here apropos the value of raw foods. In this regard we have plenty of research to fall back on, not to mention 200,000 years of lived experience, the cooking of food being a reasonably significant development in the evolution of the human species. Is it better to eat your vegetables raw or cooked? Depends on both the vegetable and the cooking method. As to the former:

You’ll get more vitamin C out of a raw tomato, but cooking increases your intake of the far rarer antioxidant lycopene, released as heat breaks down cell walls. Indeed, according to a 2008 study in the British Journal of Nutrition, subjects on a raw-food diet had lower-than-average levels of lycopene in their blood.

Cooking carrots increases the amount of beta-carotene, an antioxidant and an important source of vitamin A. Antioxidants in general have been shown to be more available to human eaters when the food they’re in is cooked; see also zucchini and broccoli.

This stuff is tricky, though. Cooking broccoli at high heat, for example, damages an enzyme that releases a compound called sulforaphane, a potential anticarcinogen, but leaving it undercooked allows a rogue protein to render the sulforaphane inactive. In 2005 a researcher at the University of Illinois reported finding a “just enough” prep method to reliably split the difference: steam the broccoli lightly for three to four minutes.

Cooking food also, yes, breaks down its cellulose — that is, dietary fiber — making it easier to both chew and digest.

And, as I say, how you cook it matters too. You’ll be shocked to hear that a broad consensus abjures frying, which introduces into the equation free radicals — cell-damaging agents linked to a number of diseases, including cancer, when their levels in the body grow disproportionate; it’s antioxidants that keep them in check.

Beyond that, research has complicated the commonly held belief that steaming vegetables is better than boiling them. Again: depends on the veggie. A 2008 paper in the Journal of Agricultural and Food Chemistry reported that carrots, for instance, retain slightly more carotenoids when boiled than they do steamed. In some cases you might best ditch the old ways altogether: in a 2007 study, microwave cooking led to greater retention of vitamin C in broccoli than either steaming or boiling did.

As to the overall implications of a raw diet on a person’s health, this is where the research to date gets limited, but here’s a few things we know so far:

A 2005 study of 17 raw-food “leaders” — the movement’s evangelists, in other words, mostly also vegan — found that subjects “met or exceeded recommended intakes of vegetables, fruits, and fats and did not meet recommendations for calcium-rich foods, protein-rich foods, and grains.” In terms of shock value, this is right down there with reports that frying is bad. (Why the grains deficit? Likely because if you’re determined not to cook them, making grains human-digestion-compliant involves a bunch of soaking and sprouting — a significant hassle.)

Long-term raw, vegetarian diets have been linked to low body weight and BMI, but also to low bone mass.

Finally, a 2008 observational study tracked participants’ sense of their quality of life, anxiety, stress, etc, following a period of weeks spent at a Florida raw-foods institute. Researchers surveyed the attendees at check-in, then again three months later, and wouldn’t you know: responses broadly expressed feelings of a higher quality of life, with lower stress, and little displeasure among those who decided to keep on the diet after their stay.

Great news, of course. But I suspect it had something to do with the Florida vacation as well.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.