Protein is a key macronutrient for human reproduction, growth, development, health and longevity. While protein can support energy needs of the body, protein nourishment is key with higher requirements associated with weight loss/management, aging and sport performance. That means that accurate assessment of protein content of food and supplements is important for many people to plan their diets. Moreover, many food and supplement products are produced with a primary purpose of delivering a higher concentrated protein content. This is especially true for many products marketed to people who exercise as well as managing their weight. Based on the variety of food and supplement options that provide protein, it is important to understand how protein levels are estimated and validated through testing.
Protein is a Critical Macronutrient in Foods with One Big Unique Factor
Whether its meat, eggs, dairy food, poultry, legumes or vegetables, all intact foods will have a varying amount of protein, as well as different type of protein. General estimates of protein content of foods are known and provided in official databases, as well as used in nutrition assessment and meal planning programs. Specific testing of foods is straight forward and done by crude analysis for nitrogen content (e.g. Kjeldahl and Dumas protocols). Carbohydrate, fat (lipids), water, minerals and many vitamins do not contain nitrogen, so once the nitrogen content of a food is determined in a laboratory, the amount of protein it contains can be estimated.
In general, the grams of nitrogen determined by testing will be multiplied by a factor to estimate grams of protein. The default nitrogen conversion factor to estimate grams of protein in foods is 6.25 and based on the general knowledge that many proteins in high-protein foods commonly consumed (e.g. meats, eggs, fish) is about 16%. Meanwhile, the nitrogen conversion factor for milk, dairy foods and concentrated milk proteins like whey and casein is 6.38, because the nitrogen concentration is a little lower based on the amino acid profiles. On the other hand, the conversion factor for vegetable proteins is lower (e.g. 5.70) because the nitrogen concentration in plant material is higher.
||Percent (%) Nitrogen
||Nitrogen Conversion Factor
|Meat, Fish, Eggs
|Dairy (Milk, Whey, Casein)
Protein is a High Biological Value Macronutrient
Many believe that dietary protein is the most valuable of the macronutrients as its most vital role is the support the growth, development, maintenance and integrity of the body structural and functional tissue. Thus, dietary protein quality is often discussed in concert with quantity. Among the key factors influencing protein quality are essential amino acid composition, digestibility, assimilation to body tissue and retention. Measures such as Biological Value (BV) and Protein Efficiency Ratio (PER) have been used, alongside with Protein Digestibility Amino Acid Score (PDCAAS). More recently, Digestible Indispensable Amino Acid Score (DIAAS) has been developed and applied. PDCAAS is used as the basis of the Daily Value (DV) found on Nutrition Facts panels in North America.
Incomplete proteins are those with one or more essential amino acids in limited proportion to the other essential amino acids and as a result, have lower quality scores. Meanwhile, proteins in which some of the amino acids are irreversibly processed (e.g. proline to hydroxyproline) and/or those that are harder to digest will also have lower quality scores. Basically, less of the quantity of food or supplement protein (as assessed via nitrogen) can be efficiently converted to human protein. In general milk proteins and animal proteins have higher quality scores because of its role early in the human lifespan as well as common proteins with human.
Formulated Protein Products: Getting Protein Claims Right!
Protein is a highly valuable macronutrient in sports nutrition, fitness and weight management. Formulated higher protein foods such as protein bars, ready-to-drink shakes and powders will contain protein isolates and concentrates (e.g. whey, casein, milk protein, soy, rice or pea) to achieve desired protein targets per serving. Thus, to accurately assess the protein content of a formulated protein product with more than one protein ingredient, the proper nitrogen conversion factors need to be applied relative to the protein input from the ingredients.
In addition, the input of any additional non-protein nitrogen containing ingredients needs to be addressed appropriately as well. For instance, if a protein product contains added 1) nonessential amino acids such as glycine or glutamine, 2) amino acids not used to make protein (e.g. taurine, beta-alanine or citrulline) or amino acid-derived ingredients such as carnitine, creatine or glutathione, these should not be counted as protein in a serving. This is not to say that they don’t have other potential value; however, it is to say that if they were counted as protein the biological value will be reduced relative to their presence. Moreover, it isn’t always as simple as just subtracting the formulation input from the estimated protein based on total nitrogen. That’s because some of these ingredients have a higher nitrogen concentration than protein. For instance, glutamine and creatine are roughly 20% nitrogen.
It is a position of the iPB that protein in foods, recipe products and supplement formulations should accurately communicate protein quantity and where applicable quality. This means that food and supplement companies should apply the appropriate conversion factors, as well as discount non-protein ingredients as protein on the label.