Human biology is built for endurance hunting. We are not structured to eat meat with every meal, but some of our essential vitamins are only
This write-up is in response to some claims that humans were not built to eat meat.
Human biology is built for endurance hunting. We are not structured to eat meat with every meal, but some of our essential vitamins are only naturally available from meat.
Plant carbohydrate is the only fuel which causes multiple metabolic dysfunctions when consumed in excess of immediate needs. The shift from fat to carbs in 1977 is bulk energy recommendation coincides with the sharp beginning of the obesity epidemic.
Grains, pulses, roots, etc are not naturally a primary food source. They require substantial technology to prepare. The vegetable foods we can eat are not available year round without major technological advancements.
Some of our survival does absolutely rely on plants as well, including non-digestible plant matter, and other vitamins.
It's not that we "can" eat both. It's that we truly are adapted to require eating both. Technology allows us to get around this requirement, but it is not in any way "natural".
We're also not adapted for having food instantly available all the time, and we are not adapted to using carbohydrate as a primary source of energy.
Long rambly supporting info:
We, and our ancestors back to previous species, have a biology designed to follow prey around to the point of the prey's exhaustion. While many animals can move faster than us in the short term, very few can travel as far as we can in a few hours, or even in a day, without becoming incapacitated by fatigue. At that point, we simply walk up to the prey and run it through with a stick.
We are upright, which gives us a reduced thermal load from spending a long time moving in the sun.
It also supports both wooded foraging and coastal hunting.
Compared to our ancestors, we devolved upper body strength and evolved lower body strength to support hunting, at the expense of being able to harvest and climb trees to the same level as other primates. We lost our opposable big toes, and our knees angled towards the middle a bit. This is an adaptation to extended running or walking, and use of tools instead of brute strength.
Our digestive tract is designed for meat occasionally, but not constantly. The gallbladder stores and concentrates bile, and our bile is hydrophobic (eg, is used to digest fats).
Some of our vitamins only come from meat, or bioengineered foods. The most critical is Vitamin B12.
The canine argument is non-scientific. Canines, and pointed teeth, and claws, are for killing rapidly, both for territorial purposes and for hunting. Many species of primates have huge canines, even the ones that don't eat much meat.
Humans don't kill with our teeth and claws. Not a factor for what we should eat. Whales don't have canines either, and they eat tons of small shrimp and fish every year.
True herbivore teeth generally continue to grow throughout life, because it's extra hard on teeth. We have two sets of teeth, one to fit our small head, and one set to fit our grown up heads. Birth size and brain growth come into play there. We live twice as long as we would in the wild, and we're still dealing with people who, despite dental precautions, still have to have dental appliances.
Our front teeth have fairly sharp corners, and a chisel edge, designed for biting out big chunks at a time. Basically, once we get food, we need to be able to eat a bunch of it fast, because more dangerous predators would come along soon.
Note that what society considers meat is very limited. In the wild, primates would eat birds, lizards, insects, crustaceans, worms, and anything else available. Some primates eat other primate species on a monthly basis.
Also compare our digestive tracts past the teeth. Our stomach is designed to help break down meat, and liberate protein. Our duodenum is designed to begin the breakdown of fats. Our jejunum is designed to absorb carbs, fats, and proteins that have been liberated above. Our ileum is designed to absorb trace nutrients, excess bile, vitamins, and whatever wasn't absorbed above. Our colons are designed for residual fermentation and moisture absorption.
Our colons are half the size of our more vegetarian cousins among primates. We also have a very small appendix. These are related to having a lower capacity to break down cellulose. In other words, we can eat some vegetation, but not the same variety as some other species.
Our stomachs are elastic, and can hold quite a lot of food. Human gastric juice is specifically designed to break down proteins.
When food is low in animal protein, food passes fairly quickly through the stomach. When it's very meaty, it hangs out in the stomach for multiple hours.
Info about gastric emptying rates:
* Sorry this last one is not better quality. I have not been able to find research, nor older references.
This delay is so that we can absorb all of it, not because we're having a problem. Problem foods either come up, or pass through rapidly.
Herbivores, such as cattle, use extended duration fermentation to break down plant matter.
Basically, our structure is specifically adapted to handle different types and amounts of foods, depending on whatever is available at the time.
A major exception is that we're structured to not eat large amounts of carbohydrates except when we are recovering from, or in the middle of endurance exercise. We're better at it than our distant ancestors, because we've been doing agriculture for a LONG time, way longer than written language.
If we have been out hunting all day, then eating the equivalent of 10 refined peaches in 5 minutes is not so bad. It goes into the muscles and the liver to replenish glycogen, repair damage, and keep feeding ATP systemwide.
When we are recovered, this is not the case.
Our insulin has a half-life of about 10 minutes naturally, Natural insulin levels oscillate by a factor of 8 over 3-6 minute periods. When our livers and muscles are replete with glycogen (locally stored carbohydrate), those tissues refuse to absorb more glucose.
In response, blood glucose levels go up, and the body releases even more insulin. The loop happens until our fat cells kick into overdrive, and our livers kick into overdrive, and all of the glucose gets stored as fat.
During this process, our muscle cells down-regulate their sensitivity to insulin, because obviously, something is wrong. Muscles are already full, so why are you trying to feed them? Literally, GLUT4 receptor production rate is related to the energy needs of the cell. If the needs are low, then the levels can be reduced enough to induce insulin resistance.
Also, insulin is transported into a cell along with the transport protein. Endocytosis of the GLUT4 receptor is faster than regeneration and exocytosis.
Sherwood, Lauralee; Klandorf, Hillar; Yancey, Paul (2012-01-01). Animal Physiology: From Genes to Organisms. Cengage Learning. ISBN 1133709516
Muscle cells have secondary functions to absorb glucose when needed anyway, so it's just a courtesy that they listen for insulin at all.
Along the same lines, the pancreas down-regulates the number of islet (beta) cells, because obviously, such high levels of insulin must mean we have too many cells. https://www.ncbi.nlm.nih.gov/pubmed/15655713
Unfortunately, in its panic, the liver has to pack it into very low density lipoprotein myceles (VLDL), which is particularly difficult for our body to deal with. This gets stuffed into the walls of our arteries just to keep it from killing us on the spot. This leads to necrosis, clots, hardened arteries, etc.
None of this cascade of metabolic hormone dysfunction happens from eating excess meat, though excess of any food can lead to obesity and long-term insulin resistance. Protein takes longer to convert to glucose, but does have a nitrogen load on the kidneys. There are limits to how much can be acquired "in the wild", and this tends to be much better self-regulated due to the longer gastric clearance times.
* more sources needed here, but I'm tired.
Eating high amounts of animal fat or cholesterol also does not cause a lipid problem for the blood. Dietary fats are packaged as High Density Lipoprotein (HDL) and carried to the liver for further processing, or to fat cells for storage. HDL has the benefit of also scavenging free fatty acids out of the blood, and off of the walls of blood vessels.
80% of the body's cholesterol is produced by the liver, and excess is excreted by being bound in non-digested plant matter in the intestines.
Note that the epidemic of obesity in the US aligns closely with the low-fat claims. The recommendation for low-fat diets coincides with higher carbohydrate consumption (how else do you get energy?). This shift happened in 1977. You can see this coincides with a sharp uptick in obesity rates here:
In summary, we're supposed to eat alternating meat and plant meals, but not so much in the way of carbohydrates. When we do eat carbohydrates, it would be perhaps one or two small pieces of fruit during a hunt, or as a special treat during the week that the small fruits were ripening on the tree, but not demolished by birds, or perhaps a handful of grass seeds or beans in passing.
* Further discussion could be had on the health, social, and environmental implications of using technology to bypass these physiological limitations.
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