The variables that go into determining how much is enough protein vs. carbohydrate intake in a diet is: weight, activity level, and goals. This issue has been on debate for decades, and will likely continue.
The average sedentary American takes in 2000-2,500 calories a day. This is too much from an anti-aging perspective and should be reduced to 1500 to 1800 calories a day, depending on the activity level.
Protein once absorbed into the blood is filtered by the kidneys and if not used to build and repair muscle tissue, is converted to energy or stored as fat, but how much is enough protein? At 1500 calories a day intake, no extra protein is stored as fat. This is the equilibrium level. If you ingest more, fat is produced. The RDA for protein for adult males is 63 grams per day and 50 grams for female. Athletes can maintain protein equilibrium (muscle building equals muscle breakdown) on 1 gram of protein per kilogram of body weight per day.
Most persons can achieve protein equilibrium (positive nitrogen balance) at 0.6 to 1.2 grams of protein per kilogram per day. Protein should comprise 15-20% of a healthy diet (+/- a few %). If you eat 1500 calories per day, then you should eat about 56 grams of protein. (Take 1500 calories times 15%, and then divide by 4 calories per gram). If you are consuming more calories, say 2000 calorie, then the amount of protein/kg should increase to 1.2 gm/kg in order to maintain the 15-20% protein ratio as percentage of total calorie. Protein requirements of athletes are 1.2-1.5 gram of protein per kilogram of body weight. It is well known that excessive protein is not good for you.
The real question is: if excessive protein is not good, and carbohydrate is bad, then what should you eat to make up the calories? The insight of this lies in understanding the kinds of carbohydrates we take in. The average American takes in the carbohydrates like potato, bread, rice, and wheat products that are all high glycemic index food that converts into sugar easily. These provide quick energy, but also is a fast track to diabetes and aging. The better complex carbohydrates green leafy vegetables. They have a lot of antioxidants, but these do not provide a lot of calories.
Trying to tell American to go off pasta is very hard. On top of that, to get enough calories from just green leafy vegetables are even more difficult. So how do you get the calories?
The easiest message for the low glycemic way is to increase protein source for the mass public. Most people think of steaks and poultry as protein. This is certainly a welcome news compare to vegetables and fruits. High protein intake is better than high carbohydrate intake. What you do is trading diabetes for kidney overload. What is your choice? Neither of course.
The secret in deciphering all this lies in the details. I propose to you based on my studies that at the end of the day, 50% carbohydrate is needed, no more than 20% protein, and balance in fat is a good balance. The key is to know what kind of carbohydrate to take in. When you ingest foods that are high in glycemic index, make sure you ingest it with some proteins. This blend will then lower the glycemic index of the food. The amount of protein should continue to be restricted at 20% or thereabouts, depending on activity level.
This way you avoid protein overload that damage your kidneys, glucose overload that damage your pancreas, at the same time maintaining adequate calorie under a calorie restriction mode consistent with maximum longevity. Regardless of which type of diet and what plans or whose plan, the goals are the same and very clear - prevent high blood sugar spikes, reduce organ overload, reduce oxidative stress.
To accomplish this, you need, in my view, once again:
These are the 3 pillars of a well-balanced anti-aging diet and the key to longevity. Calorie alone and % alone of macronutrients simply do not tell the whole story, although it makes the explaining a lot easier than what I have written above which not everyone can understand.
I think if you understand the principals of how much is enough protein, they will guide you automatically to choose the right food. Furthermore, If you are on an anti-aging calorie restriction program, you would not have to worry about the percentage too much as you are not going to eat too much anyway (compare to the average American diet). Most of us eat by impulse, and eating is an addiction that is socially acceptable. The concepts I have outlined above will guide to you a healthy diet. Don't worry about the percentage. Eating is not an analytical exercise, although it is mentally very stimulating.