Last week, I presented a compelling theory that supports the position that humans are meat-eaters – that the increase in brain size was balanced by an equivalent reduction in the size of the GI (gastrointestinal) tract. It’s not enough to be satisfied with a ‘gut-feeling’ that we are meat-eaters. It is imperative to seek supporting evidence, so as to offer factual, unbiased nutrition advice.
In Part 2 of this series, I argue that the length of our colon (large intestine) reflects an adaptation to a predominantly carnivorous diet, but that we have thrived as a species by being opportunistic omnivores.
There are around 100 trillion microbes in the human colon, consisting of more than 700 species of bacteria, fungi, and protozoa. The microbiome diversity varies by geography and diet. The colon extracts water from solid waste before it’s eliminated from the body. It’s the only part of the digestive tract in which bacterial fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a major role in the absorption of foods and nutrients.
These symbiotic bacteria digest fibre into fats (short-chain fatty acids) by a process called fermentation. One gramme of fibre can yield about two calories of fat. This is not a significant energy source for humans, but it is for herbivores, who derive 70 per cent to 80 per cent of their calories from these fats. These bacteria also produce vitamin K and vitamin B7. Although this source of vitamins provides only a small part of our daily requirement, it can make a significant contribution when dietary vitamin intake is low.
Here’s the million-dollar question. Are modern-day humans carnivores, herbivores or omnivores? What does the totality of the available evidence suggest? I would say that we are opportunistic omnivores.
Humans have the same general digestive structure as apes; a stomach, a small intestine, a caecum, appendix, and a colon. Compared to other primates, humans have the shortest colon (large intestine). The large intestine takes up approximately 20 per cent of our total gut volume, whereas this is more than 50 per cent in chimpanzees, who are primarily frugivores – eating raw fruits, succulent vegetables, roots, shoots, nuts, and seeds.
Gorillas also have a much bigger colon than humans do. They are omnivorous frugivores, mainly eating a low-quality, high-fibre diet. In fact, three-quarters of a gorilla’s diet is vegetable fibre. A longer colon is, therefore, necessary to allow fermentation of the fibrous fruits, stems, and leaves, in order to release the nutrients in the diet.
Most of the edible part of a plant is cellulose. In human nutrition, cellulose is a non-digestible constituent of insoluble dietary fibre which is very difficult to break down. Ruminants such as cattle and sheep have complex stomach structures which are perfectly adapted for this process. These animals have a whole army of microbes that digest cellulose and convert it into an energy source (fatty acids). In other words, the fibre is digested into fat, which the animal uses for energy.
Humans can digest some forms of fibre, but much of it passes straight through us without delivering any nutritional value. The small size of our colons precludes the sheer numbers of bacteria that would be required to effectively ferment high-volume fibrous foods. Humans are anatomically adapted to eat high quality, low-fibre foods. This is why we have a larger small intestine (more than 50 per cent of the gut volume), which is primarily taxed with the absorption of food and nutrients.
Herbivores get their nutrition by eating copious amounts of low-quality food. Carnivores get their nutrition by eating smaller amounts of high-quality (animal-sourced) food. Humans are resilient opportunistic omnivores that have survived and thrived by adapting to whatever the environment had to offer – meat, fruit or vegetables – we made food of it all.
What is undisputed is the fact that humans must have some form of concentrated (high-quality, animal-sourced) nutrition in order to thrive. An important distinction should be made between what we can eat versus what we should eat. We are, physically, well adapted to process foods that need the least amount of energy to digest, to optimise assimilation of nutrients.
Next week, I’ll conclude this series by discussing the nutritional inferiority of vegan diets. Many micronutrients are inadequately catered for with vegan diets, and without supplementation, serious deficiencies can and do occur.
UK-based consultant physician and obesity management expert.
www.insulean.co.uk/ [email protected]/ Facebook: Nyambura Mburu Svendsen, or @insuleanmedical
