FREE subscriptions for Australia and New Zealand users... click here The nitrogen balance is a simple index of whether physiological protein requirements are being met. Urinary urea is taken as a rough indicator of protein degradation and loss from the body. Oral protein intake is quantified. Protein and urea are assumed to be rough indices of physiological nitrogen. A negative nitrogen balance occurs when urinary nitrogen losses are greater than oral nitrogen intake. It may occur in:
A positive nitrogen balance occurs when urinary losses are less than intake. It may occur in:
Both positive and negative nitrogen balances are indications for dietary supplementation after a full assessment of which particular amino acids are deficient, with or without treatment of any underlying cause. Last reviewed 03/2022 Links:
August 16, 2019 This was written in response to a post on the PSSM Forum on Facebook.
Negative nitrogen balance is a normal process that is helpful under the right circumstances. It is not confined to PSSM horses. You have certainly experienced it yourself. There are some circumstances under which you cannot consume enough dietary protein to meet your needs for amino acids. When you have the flu, for example, your immune system is working hard and you need a lot of amino acids to synthesize proteins. Your body tears down muscle in order break protein down into amino acids for use elsewhere. Some of the amino acids are burned for energy.  All amino acids have a chemical structure with a central carbon atom bonded to four different parts: an amino group, a carboxyl group, a hydrogen atom, and an "R" group that is a different group for each amino acid. When an amino acid is burned for energy, one of the first steps is the removal of the amino group. If this is not used to synthesize one of the nonessential amino acids, it is discarded as waste. To do that, it enters a metabolic cycle called the urea cycle and is incorporated into urea, a nitrogenous compound that ends up in urine. You can measure the amount of nitrogen going into a person or a horse as dietary amino acids (the bulk of the nitrogen that we take in is in amino acids). You can measure the amount of nitrogen going out as urea. Normally, these two quantities are about the same. A bodybuilder lifts weights to induce muscle damage; when the muscle rebuilds, it is built bigger. So a bodybuilder is in positive nitrogen balance: they take in more nitrogen in those protein shakes than they excrete as urea. A person with the flu is in negative nitrogen balance: they are burning more amino acids than they take in through their diet. When you have recovered from the flu, you have probably noticed that you don't have your normal strength for a while. Your muscles rebuild over the next week or two (you go into positive nitrogen balance) and you get back to normal. A horse with PSSM2 cannot rebuild muscle as easily, which is why there are cases of a PSSM2 horse going into a real downward spiral if they get a hoof abscess or a viral infection. The cause is illness, injury, or surgery. The symptoms are muscle wasting that is grossly apparent. You reverse it through dietary protein supplementation with complete (whey) or complementary (pea or soy) protein, or with amino acids typically lacking in plant proteins that are the bulk of a horse's diet (lysine, methionine, and threonine). In the case of Immune-mediated myositis (IMM), steroid hormone therapy has proven useful.
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Learning Objectives
The recommendations for protein for different groups are listed in Table \(\PageIndex{1}\).
Using the information in the table above, how much protein does a 150 pound sedentary adult need? Steps to answer the question include:
Most Americans meet or exceed the RDA for protein.
The appropriate amount of protein in a person’s diet is that which maintains a balance between what is taken in and what is used. The RDAs for protein were determined by assessing nitrogen balance. Nitrogen is one of the basic elements contained in all amino acids. When proteins are broken down and amino acids are catabolized, nitrogen is released. Remember that when the liver breaks down amino acids, it produces ammonia, which is rapidly converted to nontoxic, nitrogen-containing urea, which is then transported to the kidneys for excretion. Most nitrogen is lost as urea in the urine, but urea is also excreted in the feces. Proteins are also lost in sweat and as hair and nails grow. The RDA, therefore, is the amount of protein a person should consume in their diet to balance the amount of protein used up and lost from the body. For healthy adults, this amount of protein was determined to be 0.8 grams of protein per kilogram of body weight. You can calculate your exact recommended protein intake per day based on your weight by using the following equation: (Weight in lbs. ÷ 2.2 kg/lb) × 0.8 g/kg The National Academy of Medicine used data from multiple studies that determined nitrogen balance in people of different age groups to calculate the RDA for protein. A person is said to be in nitrogen balance when the nitrogen input equals the amount of nitrogen used and excreted (Table \(\PageIndex{1}\)). A person is in negative nitrogen balance when the amount of excreted nitrogen is greater than that consumed, meaning that the body is breaking down more protein to meet its demands. This state of imbalance can occur in people who have certain diseases, such as cancer or muscular dystrophy. A person healing from a severe wound may also be in negative nitrogen balance because protein is being used up to repair tissues. Someone who has a low-protein diet may also be in negative nitrogen balance as they are taking in less protein than what they actually need. Positive nitrogen balance occurs when a person excretes less nitrogen than what is taken in by the diet, such as during child growth or pregnancy. At these times the body requires more protein to build new tissues, so more of what gets consumed gets used up and less nitrogen is excreted.
The protein food group consists of foods made from meat, seafood, poultry, eggs, soy, beans, peas, nuts and seeds. Fruits and vegetables are not high in protein, but help to spare protein by providing carbohydrate. The overall suggestion is to eat a variety of protein-rich foods to benefit health. Different protein-containing foods provide different nutrients. For example, animal proteins contain Vitamin B12 and plant proteins contain a higher amount of fiber. In addition, recommendations suggest choosing leaner/less fatty cuts of meat.
While protein is contained in a wide variety of foods, it differs in quality. High-quality protein contains all the essential amino acids in the proportions needed by the human body. The amino acid profile of different foods is therefore one component of protein quality. Foods that contain all 9 essential amino acids are called complete protein sources, or high-quality protein sources. Foods that are complete protein sources include animal foods such as milk, cheese, eggs, fish, poultry, and meat, and a few plant foods, such as soy and quinoa. Foods that do not contain all of the essential amino acids in sufficient amounts to support growth and health are called incomplete protein sources. Most plant-based foods are deficient in at least one essential amino acid and therefore are incomplete protein sources. For example, grains are usually deficient in the amino acid lysine, and legumes do not contain methionine or tryptophan. Because grains and legumes are not deficient in the same amino acids, they can complement each other in a diet. The process of combining two or more incomplete protein sources to make a complete protein is known as mutual supplementation. Some examples of mutual supplementation are provided in Table \(\PageIndex{2}\). Complementary protein sources do not have to be consumed at the same time—as long as they are consumed within the same day, you will meet your protein needs.
Key Takeaways
References
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What is negative protein balance
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