The word protein originates from the idea that proteins are central to life and the first nutrient. Vitamins – vita meaning life and amin meaning protein – got their name from the misconception that amino acids, the building blocks of protein, were the essential components for maintaining life.

Proteins are found in animals and plants, but the mixture of amino acids – the building blocks of the protein found from different sources – varies. As a result, there are 21 common amino acids consisting of 12 nonessential and nine essential amino acids. Essential amino acids are those that cannot be synthesized from other amino acids, but must be consumed in the diet. The usual way that nonessential amino acids are formed is by metabolism of other amino acids. All amino acids have a basic structure of an alpha-amino nitrogen and carboxylic acid.

Maintaining the amounts of protein in muscles and organs is essential to life and is the main objective of the adaptation to starvation. In fact, loss of more than 50 percent of body protein is incompatible with life. The protein is stored in organs and there is no labile compartment.

The Importance of Protein
There is evidence that modestly increasing the proportion of protein in the diet, while controlling total calorie intake, may:

* Improve body composition.
* Facilitate fat loss.
* Improve body weight maintenance after weight loss.

Fat Retention
Mankind is very well adapted to malnutrition and starvation, and this adaptation is reflected both in the way the body stores energy and how it uses these stores of energy when food intake is reduced or eliminated altogether. In the average 70 kg (154 lbs) man:

* The largest store of calories is in the form of fat in adipose tissue with approximately 135,000 calories* stored in 13.5 kg (30 lbs) of adipose tissue.

*A dietary calorie is 1,000 calories or a kcal, but for simplicity will simply be noted as calories. You may also see dietary calories capitalized as “Calories.”

This storage compartment can be greatly expanded with long-term overnutrition in obese individuals.

There are approximately 54,000 calories stored as protein both in muscle and organs, such as the heart and liver. Only half of these calories can be mobilized for energy, since depletion below 50 percent of total protein stores is incompatible with life. In addition to being an energy source, protein plays a functional role in many organs, including the liver, and depletion is associated with impaired immunity to infection. In fact, the most common cause of death in an epidemic of starvation is typically simple bacterial pneumonia. Conservation of protein is an adaptation tightly linked to survival during acute starvation.

Meal Replacement Shakes and Weight Maintenance
Studies show that meal replacement shakes are a viable way to maintain weight, as recognized by the European Food Safety Authority, and that increasing the protein to about 30 percent of resting metabolic rate, as estimated by bioelectrical impedance, leads to greater loss of fat with retention of lean body mass.