A Conversation on Human Dietary Design and Adaptation

Two colleagues were discussing some of the stuff we’ve written about the growing scientific evidence in favour of plant-based in comparison with traditional meat-based diets. I have cleaned up the conversation and provided links to some of the points in the discussion. The discussion went over the usual arguments:

“There’s no way you can get enough protein on a vegetarian diet. You need animal protein for that!”

“Not really … The evidence highlights that a varied diet with nuts and grains provides most of the amino acids you need. In any case, with fortified foods nowadays, most people can get all the amino acids they need if they buy and eat intelligently. It also happens to be true that we don’t really need as much protein as we tend to think we do. And the evidence actually points to plant proteins being better for you.”

“How about vitamins? Aren’t you bound to be deficient in certain vitamins if you’re vegan or vegetarian? I think vitamin B is an issue.”

“It depends whether you are referring to vegetarians or vegans. Vegetarians eat products containing eggs and/or animal milk. Thus they are likely to be getting at least some animal protein. Vegans can always get vitamin B from fortified cereals, nutritional yeasts and a wide variety of sources. It’s mostly Vitamin B12 that people are talking about and this is available from a range of sources.” (Check out this link for examples.)

“Well … The biggest and strongest animals are carnivores.”

“‘Animals’ are a very broad category, but taking that wide ranging view, it also depends on your perspective. In water it’s probably true that the blue whale, killer whales, sharks and larger sea creatures all eat animals; but arguably there is not enough plant life in the deeper parts of the ocean or the poles to support a plant-based diet for animals of that size. On land, however, it’s hard to argue that elephants, giraffes, bison, cows, moose and the larger antelopes are not bigger and stronger than most carnivores.”

“Well … Why do human beings have just one standard stomach? If they were meant to be vegetarian they would have four compartments like ruminants (cows, goats etc)! The fact that we have one stomach shows that we should eat meat like carnivores.”

“I see you’re narrowing it down to mammals. Interesting that you should choose to look at the stomachs. There are actually at least two classes of herbivorous mammals – the ruminants (with four compartments) and the non-ruminants, like the rabbit, that have what you’d call a standard stomach. You can see that perhaps it’s not so useful to use the type of stomach as a basis for comparison. An arguably more useful basis is the length of the digestive tract. Generally speaking, herbivores have significantly longer digestive tracts than carnivores. In fact, according to evolutionary theory,

The morphology of mammals’ digestive tracts reflects their evolutionary adaption to different diets. The digestive tract of herbivorous mammals is generally much longer than that of carnivores. The increased length – especially of the small intestine – allows for more time for the cellulose of plant cell walls to be broken down by microorganisms.

Linzey, Donald W. Vertebrate Biology. McGraw-Hill Science/Engineering/Math, 2000

“Okay. I presume human beings are more like cats and dogs than herbivores then …“

“Human beings are actually more like herbivores than carnivores. In fact, humans and elephants have the same ratio of digestive tract length to body length. Of course these ratios must not be taken as absolutes, since there are outliers on either end of the scale – herbivores with uncharacteristically short intestines or carnivores with long digestive tracts. However, the evidence does point to humans as more suited to a plant-based diet than one based on meat.”

“Well … I’m not a fan of evolutionary theory myself. I believe more in intelligent design.“

“Funny you should say that. The Genesis account of Creation, which Christians, Jews and Muslims believe, actually teaches that human beings were designed for a plant-based diet.”

“You mean the stuff about the apple in the garden? I remember that Adam and Eve just ate various kinds of fruit.“

“Actually it’s slightly more complicated than that. Before the whole incident with the snake (Gen 3:1-7), it appears that the diet prescribed for humans comprised both seed-bearing and fruit-bearing plants i.e. grains, nuts and fruits (Gen 1:29). This makes sense as it allows for proteins and healthy fats from grains and nuts as well as vitamins and sugars (carbohydrates) from fruits. Animals were to eat ‘green herbs’ i.e. vegetables (Gen 1:30). The vegetables were added to the human diet after the incident with the forbidden fruit (Gen 3:18), presumably because of a decline in the quality of the nutrient value of food as a result of the curse on the ground. Meat appears to have been added much later, after the flood (Gen 9:3,4), presumable because of the absence of plant food.”

“So I guess you’re saying that humans are probably best suited for a plant-based diet?“

“Yes … Whether you want to adopt a materialistic scientific or religious perspective, the evidence seems to be pointing that way.”