Anyone who has observed the rate at which carnivorous and herbivorous species chew, cannot fail to see how slow herbivores chew - indeed many carnivores hardly chew at all before swallowing. According to Lucas (ibid., p.148) humans chew at a rate of 1.3 per second. In contrast pigs and dogs (archetype omnivores), chew at a rate of 3.03 and 3.16 per second respectively, whereas mountain goats chew at a rate of 1.28 per second -a rate consistent with a range of herbivores of a similar size to humans. Humans seem to eat too slowly when compared to the archetype omnivores.
According to Jordano(5), In order to receive sufficient protein from a high fruit diet, strong frugivores must consume large quantities of fruit which they digest and eliminate rapidly. Chivers concludes that the human digestive system is adapted for rapidly digested foods(1). Milton's paper(7) claims that the human digestive system is still in its ancestral slower digesting herbivorous state. If humans really are adapted to meat eating, and as alleged by Chiversmeat is rapidly digested, then why do the subjects in Milton's study only digest slowly like herbivores?
According to Milton, mean transit time for liquid markers in chimps fed high fibre diets (a more natural scenario) is 35.1±2.3 hours, whereas in humans the figure is 38.9 to 61.6 hours for high and low fibre diets respectively (results for particle markers are similar). Evidently, despite the relative lack of haustration in the human colon, the human does not digest the cultural "omnivorous" diet faster than a chimp that has to break down tough leaf matter for a day and a half. According to Burkitts figures(13) only rural villagers on a high fibre diet have transit times comparable to those of the chimpanzee. In contrast, those that eat more processed Western diets had transit times from 42.4 hours for UK vegetarians to 83.4 - 144 hours for naval persons. It's worth pointing out that contemporary human "omnivores" suffer appendicitis, diverticular disease, cancer of the colon, indeed some 40% of the UK population suffer from constipation(14), while haemorrhoids affect about a third of the population, and about 2 thirds of the older population. It is not clear how these medical issues affect comparative digestive studies that include humans.
In Miltonsstudy, all of the great apes studied and humans had a time of first appearance of digestive markers of roughly 24 hours. In the archetype "omnivore", the dog, mean transit times of 37.4 hours reduced to 28.7 when more fibre was added to their diet(15) - however time of first appearance may be much faster. Milton gives figures for some carnivores, which show that they digest and eliminate faster than humans can on high-fibre diets.
Research on digestive transit times seems to fit uncomfortably with the findings on digestive anatomy. Domestic dogs don't seem to digest meat centred diets faster than chimpanzees fed high fibre diets or humans fed high fibre diets, although they do so when more fibre is added to their diet. It may be that mean transit times are more reflective of diet and body size than anatomy. Reasearch on humans has also found that the addition of meat to the diet correlates with reduced transit times (17), a risk factor for cancer (17,18). Research on transit times seems limited to too few species and observations. However, Miltons study is not consistent with humans being efficient meat eaters. Futhermore, digestive transit times for meat in humans may be artificially lower due to artificial extraction of blood, from which the high iron content promotes constipation.