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It might be hard to believe, but it is actually very easy to get enough protein from a whole food plant-based diet. Protein is an essential nutrient. The estimated average requirement for protein is 0.5-0.6 grams of protein per kg of body weight, which is 4-5% of your total calorie intake per day. The recommended dietary allowance for protein adds two standard deviations to that to cover individual variation, making it 0.8 g protein/kg body weight, or 8-10% total calories. This means that an average size woman of 132 lbs. needs 48 grams of protein per day and an average size man of 154 lbs. needs 56 grams of protein per day. A whole foods plant-based diet easily provides enough protein to meet these requirements.
Protein has long been linked to human progress and civilization, and it is largely believed that animal protein is of higher quality than plant protein. Animal-based proteins have a higher biological value, meaning they are more efficiently utilized than plant-based proteins. Animal protein has a similar amino acid profile to humans. Plant proteins are not complete proteins, because they do not have a similar ratio of essential amino acid like animal protein does. This is a good thing though; we want our protein intake to be low with some amino acids. Incomplete protein consumption does not promote cancer. It is when protein is "completed" that cancer promotion can occur. A higher biological value (like that of animal protein) means faster growth rates, including faster cancer growth, earlier menarche, and more breast cancer.
It is not possible to consume a mostly animal protein diet and consume only 10% protein. Most meat contains about 50% protein. The meat would have to be mostly fat to not contain so much protein and eating mostly fat isn't something you want to do either. It is difficult to keep eating meat with a mostly plant-based diet because a plant-based diet will give you all the protein you need and any meat consumed would be giving you excess protein that would be harmful to your body. There is no need to combine protein or even eat certain plant proteins together. Supplementing with animal-based proteins would displace complex carbohydrates and antioxidants and add unnecessary protein and fat.
Consuming more than 10% protein of your total calories has been shown in numerous studies to actually harm the body. One study with rats injected with carcinogens showed that tumor growth could be turned on and off with the amount of protein being given to the rats. When 20% of total calories were protein, tumors grew, but when given 5% protein, tumors stopped growing and would actually reduce in size. After 100 weeks, all of the rats given 20% protein were dead, while all the rats given 5% protein were alive. Interestingly enough, when tested in research studies, plant protein, even when consuming over 20% of total calories, does not promote cancer development. It is only animal protein that seems to induce cancer development when more than 10% of total calories is consumed.
The point is that we need to stop worrying about how much protein we are getting, and start focusing on where we are getting our protein from. Plant protein is proving to be a better source of protein than animal protein. The key to getting enough protein is to simply eat a variety of plant-based foods. As long as you are doing that, you will always be getting enough protein. After all, a powerful silverback gorilla eats only plants, and no one worries about if he is getting enough protein!
The China Study by T. Colin Campbell, PhD and Thomas M. Campbell II, MD