Protein – anabolism, catabolism and nitrogen balance
Protein anabolism (protein synthesis) and protein catabolism (protein breakdown) occurs within an individual simultaneously, involving up to 50% of the body protein at any one time. Nitrogen balance refers to the incoming and outgoing food nitrogen in an organism long-term. Nitrogen equilibrium exists in an individual when the nitrogen intake (protein nitrogen) equals the waste nitrogen output. The nitrogen balance is zero (not positive or negative) in healthy adults with an adequate diet.
During growth, pregnancy and lactation, or during rehabilitation after a disease or malnutrition, total anabolism exceeds total catabolism. More dietary nitrogen is retained to support body protein synthesis and less is excreted as nitrogenous waste (POSITIVE NITROGEN BALANCE).
When total catabolism exceeds total anabolism, body protein is broken down in excess; more nitrogen is excreted in the urine than is utilized from food and a NEGATIVE NITROGEN BALANCE results. This excessive breakdown of body proteins occurs during systemic diseases, after injuries, burns and surgery. A diet rich in good quality proteins and protein sparing calories (enough carbohydrates and fats) is desirable before and after surgery, a major disease or injury, or doing long distance sport competitions, to counterbalance this loss of protein. A nitrogen loss from dietary causes occurs when the protein intake is inadequate or when the intake of calories in the diet is insufficient. The breakdown of tissue protein for the creation of energy (insufficient carbohydrates and fats) is wasteful and undesirable.
Insufficient intake of proteins and calories, or an inadequate supply of all the essential amino acids, is the major malnutrition problem in the world today. Protein plays a vital role in all life processes, the symptoms of PROTEIN DEFICIENCY include: decreased resistance to disease, slow convalescence, fatigue easily, loss of weight and in children slow, stunted growth. Continued protein deprivation leads to edema, liver damage and low blood protein levels, including hemoglobin.
HOW MUCH PROTEIN SHOULD WE EAT?
Unless you consume lots of dairy products as a counterbalance, meat proteins can leach calcium from your bones. A high-protein diet places undue strain on your kidneys, which excrete metabolized proteins. Your body uses only the amount of protein it can consume and much of the excess must be excreted; some excess is turned into sugars and fat. Studies have shown that EXCESS protein consumption does not promote health, weight loss or the building of muscles. How much protein do we need?. The RDA recommends that we take in 0.8 g – 0.9g of protein for every kilogram we weigh (0.36 – 0.41 g/pd). For non-vegetarians, it is about 14-18% of daily calories. Protein content is highest in meat, poultry, fish, dairy, soybeans and nuts – all of which contain 15-40% protein of their total caloric content. Beans, cereals, lentils and peas have a protein content under 15%. If your protein is entirely from vegetable sources, it is important to choose a wide array of foods to ensure you get all the essential amino acids.
EXCESS PROTEIN CONSUMPTION
Proteins in excess of actual need cannot be stored by the body and are burned as fuel for energy; proteins as energy food are inferior to carbohydrates or fats. The digestion of proteins in excess of actual need leaves toxic residues of metabolic waste products, which contributes to self-intoxication and consequent disease.
Too much protein can cause:
- Toxic residues in the tissues
- Biochemical imbalance in tissues, resulting in over-acidity.
- Accumulation of uric acid, urea and toxic purines in the tissues (these meat poisons interfere with muscle and nerve function)
- Diminished strength and endurance
- Intestinal putrefaction, resulting in constipation and auto-toxemia
- Vitamin B6 deficiency
- Arteriosclerosis, heart disease and kidney damage
- Arthritis – faulty protein metabolism may be the contributing causes of arthritis.
A good rule: eat enough protein, not too much!
Studies have documented that daily protein intake above 95 grams increases excretion of calcium in the urine. A study has shown that, in young men, calcium balance could be achieved with 500 mg daily, provided the daily protein level was 47g. However when the protein intake was increased to 142g daily, 30 of the 33 young men could not achieve calcium balance even at 1400 mg calcium daily. Consumption of 95 g protein daily along with 500 mg calcium would produce a bone loss of 21 g calcium yearly. Also, the higher protein intake caused a greater absorption of calcium, which caused the higher calcium excretion. Another study has shown that dietary protein reduces the reabsorption of calcium in the kidney during a four-hour period after meals, thus increasing calcium excretion.
What appears to be an adequate protein supply, but still having a feeling of starvation and a need for sugar uptake, may in fact be due to insufficient protein uptake. A small amount of extra protein may stop this desire for more sugar. If the diet contains lots of grains, legumes and vegetables, adding more protein, could increase the desire for more sugar. It is wise to experiment with protein levels.