There seem to be just as many opinions as there are ways of preparing our veggies. I have always enjoyed both raw and cooked vegetables, feeling that each has something to offer. But is there a best way to eat your veggies?
A review paper from 2004 showed that when examined, some cancers were higher in people who ate cooked veggies over raw and others were higher in people who ate raw veggies more often, but the consensus seems to be that for most cancers, the risk is lowest in people who eat more raw veggies:
Raw versus cooked vegetables and cancer risk (2004):
It is clear from this review that both raw and cooked vegetables are inversely related to several epithelial cancers, particularly those of the upper gastrointestinal tract, and possibly to breast cancer. Although more of the studies showed a statistically significant inverse relationship between raw vegetables and cancer than either cooked or total vegetables, the literature is too varied to compare definitively. Studies on diet and cancer need to differentiate between raw and cooked vegetables in their methods of food recall and in their analyses. In addition, more consistency is needed regarding the types of vegetables assessed in each category. In the meantime, the public should be encouraged to increase their vegetable intake and to consider eating some of them raw.
The underlying theme of this and other studies is that we simply need to eat more vegetables. Period.
There are, however, good reasons to cook certain veggies:
Some tubers and legumes contain enzyme inhibitors which can lead to gastric disturbances. Most of these are destroyed by cooking (and also sprouting, fermenting, or soaking).
β-carotene in plants (a precursor to Vit. A) becomes more bioavailable when cooked. Carrots in particular. And also a substance called "lycopene" found largely in tomatoes, is a similar Vit A precursor which is more available after cooking.
Yet, cooking also destroys some vitamins, Vitamin C, for instance, as it's water soluble.
Raw Veggies as Antibiotics
Recently a Dr. Peter Turnbaugh from Harvard gave a presentation, Gut Microbial Promotion of Energy Gain on Processed Diets, in his discussion, he brings up the valid point that when we eat raw veggies, we also ingest the plant's antimicrobial defenses.
In an experiment, Dr. Turnbaugh looked at the gut microbes of rats after eating raw sweet potato.
Diets of sweet potato (Ipomoea batatas) fed raw versus cooked produced dramatic changes in the composition of the distal gut microbiota, as indicated by 16S rRNA gene sequencing of fecal samples. Raw diets led to lower proportions of bacteria from the Firmicutes versus Bacteroidetes phyla, a microbial phenotype consistent with the weight loss observed in raw-fed hosts. Flow cytometry employing physiological stains indicated that raw diets were associated with increased rates of cell damage and lower proportions of highly active cells, a finding consistent with our observation of profoundly lower mRNA yields from gut microbial communities sampled from raw-fed versus cooked-fed hosts. To assess whether these effects were specifically attributable to food-derived compounds, we recently repeated the study with standard chow diets and water administered with or without one of two key antimicrobial compounds present in sweet potato: caffeic acid and chlorogenic acid. Our results suggest that such food-derived compounds can indeed promote bacterial cell damage within the gut, a result confirmed by in vitro assays of the effects of these compounds on gut microbial isolates. Our emerging results encourage a view beyond therapeutics in considering the role of xenobiotic compounds on host-microbial interactions in human health.
"Bacterial cell damage" is his way of saying that bacteria were harmed in the completion of this study. What we don't know, and I assume Dr. Turnbaugh is studying, is whether or not this antibacterial effect of raw veggies is in any way damaging to a healthy human. Since humans are omnivores that evolved alongside plants, bacteria, and fungus, I assume it's all part of the "grand plan" for us to ingest the antimicrobial component of plants.
Also note that Dr. Turnbaugh does not mention the fungal component of the gut microbiome, which we all know, also responds to a plant's antimicrobial defense. This type of study will be interesting to follow.
Raw Veggies as Probiotics
The ever-vigilant Gemma shared this one with me: The Edible Plant Microbiome: Importance and Health Issues (2014).
Here we see that the bacteria living freely inside, and helping protect plants from harm, may also convey the same benefits to whomever eats that plant!
However, even though plants are a substantial part of a balanced diet including raw-eaten vegetables, fruits and herbs, the plant-associated microbial diversity has been largely ignored in this context. We hypothesize that the edible plant microbiome and its diversity can be important for humans as (i) an additional contributor to the diversity of our gut microbiome, and (ii) as a stimulus for the human immune system.
The topic of exactly where our microbiome originates comes up often. When I wrote about irradiated foods we broached the topic that modern processing destroys lots of the value in foods, such as the clinging microbes. Cooking can also destroy the probiotic value of foods, but also the pathogenic load, so it may be a toss-up. Commercially raised and manually harvested vegetables have been implicated in numerous outbreaks of deadly infections, and will undoubtedly continue to do so. Even more reason to start your own vegetable plot, no matter how small!
Raw Veggies as Prebiotics
Much of a plant's fiber is made more digestible by cooking. This can have both beneficial and negative aspects on one's diet. Cooking can also reduce "antinutrients" often found in beans and legumes.
Resistant starch is nearly completely destroyed by cooking, although cooling and reheating forms a new type of RS (RS3).
Inulin and Beta-glucans, however, are not destroyed by conventional cooking methods, so enjoy your hot oatmeal and dandelion or chicory coffee!
As humans have been eating a combination of raw and cooked (and also soaked, sprouted, and fermented) veggies since time immemorial, there is no reason to think we should do otherwise. Some foods are never seemingly eaten raw, ie. kidney beans, but most veggies can be eaten raw if you like, and it may even be beneficial to eat some types of foods, ie. potato, raw from time-to-time.
Mix it up! It's the veggies that are important. But for best results, learn to prepare them in a variety of ways.