Hot Topic

For a city not known for its winters, Austin has been putting in a good effort this week. Daytime highs are struggling to get above 40ºF and nighttime lows have dipped to temperatures I would prefer not to discuss. It’s cold out there. I plan to make soup later this evening, as few things in this world taste better than a big bowl of soup on a freezing January day. Probably I’ll opt for gazpacho, because nothing compensates for the brutality of winter like a heaping serving of raw puréed tomatoes mixed with seasonings and then chilled….Did I lose you? Were you hoping for hot soup? Well you’re not alone in your preferences. Though I’ve not yet been to Spain, I’m told that it’s difficult to find a bowl of gazpacho during frostier months, the reasoning of restaurateurs being “what lunatic would want to eat cold soup in the middle of winter?” But not everyone agrees that winter gazpacho is such folly. Raw foodists, loosely defined as those who consume 75-100% of their food uncooked, would readily choose cold soup over boiled minestrone, arguing that the former is nutritionally superior. Raw food is a growing and lucrative branch of the restaurant business (as demonstrated by the exorbitant price of my favorite Daily Juice smoothie) but supporters of broiling and barbequing aren’t swayed by the sales pitch. It was one of these individuals that brought to my attention the belief that cooked food was instrumental in human evolution. “I don’t get these raw food people,” My source lamented “Don’t they know that eating cooked food is what allowed us to develop larger brains than other animals?” This alleged knowledge was news to me, but as my boyfriend would say, the idea “Googled well”* and so here we are….

Heat Wave

Here is the claim in a (dry roasted) nutshell: humans have a larger brain, relative to body size, than other mammals. Large complex organs are energetically costly to maintain, and yet our species’ basal metabolic rate † is not significantly higher than that of similarly-sized animals with smaller brains. Since higher metabolisms are not fueling our enormous brains, the additional energy required for such stately organs must be coming from somewhere else, and that somewhere else, according to certain biologists, is a decreased gut size. ‡ The human gastrointestinal tract is about 60% smaller than expected for a primate of our size. Supporters of the “expensive-tissue hypothesis” believe that the innovation of cooking, which increases the available calories in plant-source foods, drove this important change in our anatomy.§ Over time, they argue, with less effort needed to digest tough fibrous vegetation, the gut shrank and the brain grew, eventually yielding the dimensions of our current species, Homo sapiens. Furthermore, because of these changes, it is now difficult for us to obtain enough energy (calories) from raw food sources alone.

You Can’t Start a Fire Without a Spark

If you heard a supporter of this hypothesis speak on the subject, they might present it as though it were a given, possibly a launching point for another argument. But the idea is far from universally accepted. One problem is that scientists don’t agree on how long humans and their ancestors have been able to control fire. Some estimate that the technology arose about 250,000 years ago, some 600,000 and others have suggested dates as far back as 1.9 million years. This is a rather important detail to sort out. Homo sapiens as a species is purported to have been in existence for only about 200,000 years. Cooking, and thus fire, would need to predate this considerably if it is to be the accepted cause of our being the smartypants species we are today.

Previous connections between diet and physical proportions have focused on the adoption of meat consumption. While meat is more difficult to chew in its raw form (and more likely to be teeming with bacteria), the availability of its calories is not strongly affected by cooking, which makes this hypothesis less dependent on the ability to get a campfire going.

Other critics have cited non-human species that have smaller guts or higher metabolic rates and yet have failed to develop large brains as evidence that the connection between these elements is not an obvious one.

Smoke and Mirrors

Meanwhile proponents of raw foodism, avoiding all this evolutionary biology nonsense, maintain that cooking food removes valuable nutrients and replaced them with various toxins. They’re right, of course, to some extent. We’ve all heard before that the delicious charred stuff on grilled vegetables (and meat, if you’re into that) and toasted marshmallows contains carcinogens. Additionally, as any raw food website will cheerfully inform you, cooking creates dietary advanced glycation end products (dAGEs) which are believed to contribute to maladies such as diabetes and cardiovascular disease. However, the increase** in dAGEs varies among food types and cooking techniques. Meat is more affected by heating than are vegetables, and high temperature/low-moisture cooking environments have the greatest potential for harming food items of either ilk. In terms of dAGEs, boiling is not such a big deal, and a crock pot is barely worse than a dehydrator (though I still refuse to purchase either of these silly devices).

It bears mentioning that certain vegetables are toxic in their raw form. Recently, I helped to prepare taro root, a tuber unfamiliar enough to necessitate Internet research in order to cook. It turns out that taro is not only inedible when raw (calcium oxalate) but that it really shouldn’t even be touched with bare hands until it has at least been microwaved, as it makes some people’s skin itchy. Other raw items to avoid adding to your salad in mass quantity include parsnips, kidney beans, buckwheat greens and, of course, raw chicken, tempting though it may sound.

Too Many Cooks

A major downside of trying to figure out what happened thousands and millions of years ago is that we can’t actually do experiments to confirm our hypotheses. We can argue about interpretations of the fossil record, but we can’t just retreat to the lab and subject animals to similar conditions for a few millions years to observe what happens. Nobody has that kind of funding. Most educated people accept the theory of evolution by natural selection, but specifics of cause and effect along the road to the present are difficult/impossible to prove. It may be fun to speculate about possible explanations, but there’s no getting around the fact that we weren’t there. Multiple hypotheses exist for numerous anatomical quirks. For instance, the existence of lactose intolerance after weaning in some populations has been attributed to more than one possible causative factor. One argument states that the loss of the lactase enzyme is simply another limited energy issue; why continue to produce the enzyme if there was no use for it in the pre-agricultural era. But another argument attributes the discarded enzyme to the “parent-offspring conflict” – the idea that parents, who wish to reserve enough resources to produce more than one child, have different goals than their children, each of which cares only for its own survival and would gladly postpone weaning indefinitely if it were enzymatically possible. Which hypothesis is correct? I have no idea. Just pick your favorite viewpoint and hope for the best.

What’s For Dinner?

One way of testing at least part of the expensive-tissue hypothesis, the idea that humans now lack the ability to get enough energy from uncooked food, would be to see if anyone can thrive on a raw food diet. This is harder to establish than you would think. Not a lot of studies have been done on the subject and, as with so many human health studies, they are correlation studies rather than laboratory experiments. The subjects in articles on raw foodism are people who chose this diet rather than having it randomly assigned to them in double-blind controlled study. Some studies have found that certain nutritional deficiencies exist in those adhering to raw food diets. Low B-12 levels and low serum HDL cholesterol (the “good cholesterol” you often read about) were observed in one study. Another reported low body weight and lack of normal menstruation in women. Also bothersome is that fact not that many people eat a 100% raw diet (thus the 75-100% guideline for qualifying as a raw foodist). To my knowledge a long-term study on humans consuming exclusively raw foods has yet to be done.

But would a diet of entirely cooked food be a good thing? While cooking may have helped our ancestors get enough calories in times of scarcity, most of us live in quite different conditions today. Easy calories abound and humans are now more likely to be malnourished than undernourished. The deficiencies in consumers of mostly cooked foods are vitamins and fiber, a problem that has been linked to more diseases than I have the patience to list. The same article that reported low HDL cholesterol levels in raw foodists also reported low LDL levels (aka “bad cholesterol”) in the same subjects. Cooked starches and meats may be a fine way to avoid starvation, but they don’t necessarily promote longevity. Recall that for an adaptation to succeed it need only help its bearers live long enough to produce and raise offspring. Fitness in old age is a luxury of modernity.

So, as is often the case, the best route might be a compromise between two extremes. A varied non-partisan diet of cooked and raw foods may be the most sensible solution to our dining dilemmas. Soup and salad rather than soup or salad.

* More novel slang for you. To “Google well”, means to garner enough hits when typed into one’s search engine to merit further investigation. It can also be used in the negative to express skepticism when receiving information second hand, “That doesn’t sound like it would Google well.”

† Basal metabolic rate is the minimum amount of energy required by an organism just to sit still and not die (running around costs extra). It is higher in mammals like ourselves than in retiles due to our sophisticated physiological methods of thermoregulation.

‡ The crude term “gut” refers to the alimentary tract, which includes the stomach as well as the various portions of the intestines.

§ The cell walls of plants are made of the polysaccharide cellulose, which is a real pain to break into smaller molecules. Cows have bacteria-produced enzymes to accomplish this, but humans are less fortunate. However, heat can also break cellulose into smaller, more digestible units.

** dAGEs exist in raw foods as well and are especially high in meat.

Who told you this?

Wrangham, R. and Conklin-Brittain, N. 2003. “Cooking as a biological trait.” Comparative Biochemistry and Physiology 136: 35-46.

Aiello, L. and Wheeler, P. 1995. “The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution .” Current Anthropology 36: 199-221.

Krebs, J. R. 2009. “The gourmet ape: evolution and human food preferences.” American Journal of Clinical Nutrition 90: 700S-711S.

Pennisi, E. 1999. “Did Cooked Tubers Spur the Evolution of Big Brains?” Science 283: 2004-2005.

Gibbons, A. 1998. “Solving the Brain’s Energy Crisis.” Science 280: 1345-1347.

Uribarri, J. et al. 2010. “Advanced Glycation End Products in Foods and a

Practical Guide to Their Reduction in the Diet.” Journal of the American Diabetic Association 110:911-916.

Garcia, A. L. et al. 2008. “Long-term strict raw food diet is associated with favourable plasma b-carotene and low plasma lycopene concentrations in Germans.” British Journal of Nutrition 99: 1293–1300.

Koebnick, C. et al. 2005. “Long-Term Consumption of a Raw Food Diet Is Associated with Favorable Serum LDL Cholesterol and Triglycerides but Also with Elevated Plasma Homocysteine and Low Serum HDL Cholesterol in Humans.” The Journal of Nutrition 135: 2372–2378.

Koebnick, C. et al. 1999. “Consequences of a Long-Term Raw Food Diet on Body Weight and Menstruation: Results of a Questionnaire Survey.” Annals of Nutrition & Metabolism 43:69-79.