Our conclusion from the nutritional, ecological and social perspective is that humans are designed to eat fats, protein, and carbohydrates, probably in that order and that for most of human pre-history, we ate a combination of foods with these ingredients as and when we could. Few early human societies persisted without a significant source of animal protein for part of the year.
But because we also think about what is good ecology, we know that humans are omnivorous for a reason. A mixed diet gave us the evolutionary edge. Those nutrient bombs from hunting and scavenging—and in modern times from feedlots and battery houses—are a huge advantage. But the more comprehensive plant-based nutrition gives us flexibility and, through that, survivability.
Our biology makes us generalists, and our brains give us the remarkable adaptive capacity to become general almost anywhere on the planet.
‘Good ecology’ tells us that animal fat and protein cost a lot. They are energetically expensive. Energy is lost in the conversion of plants to animals, and energy is required to maintain the metabolism of the animals. They are energy sinks, while plants are energy sources, or perhaps more like energy factories fuelled by the sun.
If all humans were vegetarians, then the energy used to maintain animals and the energy lost in that process would be saved. In real terms, that means less land to feed a human vegetarian that a human omnivore.
This is the logic of the argument that eating plants is how we save the planet.
‘Good ecology’ also tells us that it is never this simple. If it was, we would have done it already.
In a neat study that simulated the land requirements of 42 simulated diets that varied in the amount of meat and fat, researchers at Cornell University established that the average annual land requirements per person for each diet ranged from 0.18 ha to 0.86 ha.
In general, per capita land requirements increased with increasing quantities of meat in the diet, while the influence of the quantity of fat varied depending on the quantity of meat consumed. In addition, the proportion of land in perennial crops was greater in higher meat treatments than in lower meat and vegetarian treatments.Peters, C. J., Wilkins, J. L., & Fick, G. W. (2007). Testing a complete-diet model for estimating the land resource requirements of food consumption and agricultural carrying capacity: The New York State example. Renewable Agriculture and Food Systems, 22(2), 145-153.
Fair enough and consistent with the ecology—meat in the diet needs more land than no meat. But the relationship is not linear and is affected by the amount of fat consumed.
Eating fats, even on a vegetarian diet, needs more land than no fat. Eating meat needs more land than no meat, but as the fat consumption increases so the effects compound.
Here is the graph using carrying capacity as the measure of land needed.
Based on these analyses, Peters et al. (2007) estimated that the land base for agriculture and average yields of crops and livestock from New York State could feed between 6.08 million people (0 g meat, 52 g fat) to 2.04 million people (381 g meat, 52 g fat). So, in general, lower meat diets supported more people than higher meat diets, but the differences between diets with different meat levels decreased as fat increased. Fat makes a big difference.
This study was completed when the population of New York city was 8 million people. At that time, the carrying capacity of New York State could feed between three-quarters and a quarter of the residents depending on what they ate.
Remember that humans can convert fat into energy and protein into carbohydrates. We don’t have to eat plants. And fat is still an efficient source of energy.
What sustainably FED suggests
Will vegetarianism save the planet? Not really, for two reasons.
The first is our design. We are fat and meat eaters. Our metabolism handles both these food types efficiently and effectively. We are flexible and can live on plants alone, but our bodies process energy and nutrient-dense foods well.
There was a reason our ancestors lived near the water on the coast and shores of lakes and rivers. Water bodies are a great source of energy-dense food. Ask an Inuit.
The second is irony.
Suppose we follow the logic and the evidence that the human carrying capacity of the land is more significant when we only eat plants. The 8 billion people would use less land for food or, more likely, there would be more food for people to eat. All the grains fed to livestock and chickens could be made into pasta. This is a delightful example of Jevon’s paradox.
What happens next is more people.
Peters, C. J., Wilkins, J. L., & Fick, G. W. (2007). Testing a complete-diet model for estimating the land resource requirements of food consumption and agricultural carrying capacity: The New York State example. Renewable Agriculture and Food Systems, 22(2), 145-153.