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Dr Michael Eades: We didn't evolve to eat meat.  We evolved because we ate meat.

Miki Ben-Dor PhD's research suggests that humans (or rather the genus Homo) have been hypercarnivores throughout our 2.5 million year evolutionary history.  Throughout human evolution, humans were apex predators that specialized in hunting very large animals, which resulted in physiological adaptions to meat consumption and especially fat consumption.  That is, humans are lipivores (fat eaters). Amber O'Hearn makes the case that humans have evolved to become lipovores (fat eaters) rather than carnivores (meat eaters) and that our large brains are the result of our lipivore diet.  The expensive tissue hypothesis explains that, as our brain increased in size, our digestive system correspondingly decreased in size in order to reduce its metabolic requirement (as dictated by Kleiber's Law), which in turn caused us to rely on nutrient-dense food.  Agriculture only began about 11,500 years ago, which coincides with the extinction of Pleistocene megafauna .  Humans may have precipatited megafauna decline as consequence of being superpredators.  Miki Ben-Dor's research suggests that the human evolutionary diet was protein-limited and he uses a physiological protein limit of 35% of caloric intake, with the balance made up from fats and carbohydrates.  As hunter-gathers, humans would have needed to to make up the remaining 65% primarily from fat.  Because animal fat content becomes increasing greater with animal size, humans became specialized to hunting large, fat-rich animals

The calorie content of macronutrients is: 4 kcal/g for protein, 4 kcal/g for carbohydrate, and 9 kcal/g for fat.  Human daily caloric requirements may be calculated from your estimated BMR.  Assuming a daily requirement of 1800 kilocalories (calories) and a body weight of 90 kg, the 35% protein limit works out to 630 calories/day or 157.5 g/day (or 1.75 g/kg BW/day).

Related to the physiological protein limit is the Protein Leverage Hypothesis.  Basically, we will eat until we have reached our physiological protein requirement, which depends upon a variety of factors and is specific to each individual.  The net effect is that, if your diet is protein deficient, you will continue to eat until you have consumed enough food to meet this need.  This can lead to obesity because protein-deficient foods will cause you to overeat and injest more calories than required.  Conversely, consuming more protein than physiologically necessary will result in the injestion of less calories than required.

Dr Ted Naiman has developed the Protein:Energy Ratio [P:E Ratio = protein/(carbohydrate + fat)] in gram units as a means of estimating the amount of macronutrients required for weight loss.  He recommends a P:E Ratio of ≥1.0 to maintain lean body mass while reducing fat mass.  Basically for the P:E Ratio calculation, your daily protein requirement in grams is your weight in pounds, which means a 90 kg person requires 198 g/day.  Assuming that we restrict carbohydrates to 20 g/day, we could consume up to the remaining 178g/day with fat.   Ben Bikman states animal-sourced food typically has around a 1:1 ratio of protein to fat so you would have to eat lean meat and/or supplement with protein isolates and/or eliminate carbohydrates completely.  The body has very little ability to store protein and the limited carbohydrates result in low insulin, which puts the body into ketosis thereby causing it to burn stored fat.  A person should lose weight even if the P:E Ratio's total daily calories are higher than what would be calculated from BMR.  However, eating primarily very lean protein for a very high P:E Ratio diet is likely unsustainable in the long-term due to rabbit starvation from exceeding the physilogical protein limit.

Stuart Phillips PhD often mentions a protein intake of at least 1.2-1.6 g/kgBW/day to maintain lean body mass as well as a physiological protein limit of 35%.  Seniors require more protein to compensate for their reduced ability to synthesize muscle (anabolic resistance).  The best way to maintain lean body mass or gain muscle is through resistance training.  Complete proteins (particularly those rich in leucine) help but it is the exercise that is most anabolic and provides 80% of muscle synthesis.

Unless you're an athlete, the practical aspect of the high protein consumption is that it can be challenging to consume 1.2 g/kgBW/d of protein and even harder to achieve the P:E Ratio's daily protein target.  For improved muscle synthesis, it is better to injest protein over several meals (eg, 3 x 0.4 g/kgBW) rather than one large meal. Examples of how much food is required to acheive 100 g/day of protein:

  • 17 large eggs: one large egg contains 6 grams of protein and 5 grams of fat
  • ½ whole chicken: one 1.4kg Costco roasted chicken contains 216g of protein and 123g of fat
  • 1½ 8oz round steaks: 3oz / 85g round steak contains 24g of protein and 12g of fat

MY Recommendation:  Limit carbohydrates to a maximum 20 g/day of low glycemic vegetables and fruit and consume animal-sourced whole foods to satiety.  Eat ONLY when you're hungry and don't when you're not. DO NOT count calories but DO consult with your doctor.