As mentioned earlier, there are some important exceptions to the generalized conclusion about raw versus cooked food and digestion—that the overall risk of compromised digestion can be greater for raw versus cooked foods when thorough chewing is difficult to achieve or simply not achieved because of an "eating on the run" lifestyle.
First, it is possible to prepare raw foods in such a way that they become partially "pre-digested." Fermenting, and in some cases the marination of raw foods, is an example of partial "pre-digestion" of those foods. (In the case of marinated foods, changes in food structure depend on the composition of the marinade, thus leaving a lot of room for variable impacts with this approach.) While large-scale research in this area of raw food preparation methods is not available, small-scale human studies (as well as animal and lab studies) have shown improved absorption of select nutrients from select fermented foods. In some ways, we believe it makes sense to think about certain fermented, sprouted, and marinated foods as "partially pre-digested."
Second, raw foods can be mechanically prepared in such a way as to exceed the degree of breakdown that would come from chewing. For example, a raw food could be pulverized in a blender to create a juice whose overall particles were smaller than the particles created through chewing. It is not necessarily the case that smaller food particles automatically provide us with a better change of nourishment for all individual nutrients found in food since mechanical processing can result in the creation of some particle fragments that are different from the fragments produced through chewing. Although little research in this area is available on which to base a conclusion, dietitians have fairly extensive experience with the use of blenderized and elemental foods in hospital settings, with improved nourishment as a common result.
With respect to digestion, it is important to note that the functioning of our gastrointestinal tract is not the same over the entire course of human life. In the very early years of life, bacterial populations in our large intestine are still evolving in terms of complexity, and our overall digestive capacity is also still evolving. In later life, changes in digestive function are also the norm. Secretion of digestive fluids (including stomach acid secretion and pancreatic enzyme secretion) is typically lower, as is muscle movement of food contents through the large intestine (called "propulsive motility"). Age-related changes can make thorough digestion of whole, raw foods more challenging than might be the case in earlier life circumstances.
The overall takeaway points about raw food, cooked food, and digestion are: (1) as a general rule, cooking (but not overcooking) can help contribute to good digestion, even though cooking is not essential for good digestion and does not lessen the need for thorough chewing; (2) the fermenting, sprouting, and marinating of raw foods can help increase their digestibility; and (3) the choice of raw versus cooked foods partly depends on individual life circumstances and the status of a person's digestive tract function.
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