Protein, one of the three macronutrients, is made up of small molecules called amino acids – about 20 types of different amino acids in various combinations give various proteins.
We distinguish between essential (9 types) and non-essential (11 types) amino acids. "Essential" means the amino acids cannot be made naturally by our body – we have to get them from food. Your body can produce the "non-essential" ones.
Protein's main function is to build and repair cells, as well as making hormones, enzymes and keeping other biochemical processes working smoothly.
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Different Kinds of Protein
Protein can be of animal or plant origin.
Animal protein (e.g. meat, poultry, eggs, dairy products) is generally absorbed easily by the body and contains all essential amino acids.
However, many of these sources can contain a significant amount of fat (particularly saturated fat) and they generally lack fiber. Animals can also be raised with hormones and antibiotics to increase production, so if possible you should look for organic / hormone-free options.
Plant protein (e.g. lentils, beans, certain whole grains) is generally harder to absorb and does not contain all the essential amino acids.
Most vegetables are deficient in the amino acids methionine and lysine, however, these can be found in soy and quinoa so a vegetarian diet would ideally combine protein from vegetables as well as either soy or quinoa to deliver all essential amino acids.
Out of the 3 macronutrients, we generally need the least amount of protein (12-15% of your total daily calories). If you generally have a healthy diet, deficiencies will be pretty unlikely. For example (putting your muscle-building aspirations aside for a moment) the average 130 lb woman would be fine with 1.5-2 oz of protein a day.
On the other hand, excessive amounts can lead to osteoporosis if you are not getting enough calcium. When broken down, protein releases acids which the body neutralizes with sodium or calcium. Long-term calcium insufficiencies can lead to a weakening of the bones, so if you are upping your protein, make sure you have enough calcium in your diet!
Protein In Children's Diets
The main role of protein in children's nutrition is to assist growth and development. Until ~6 months, breastmilk/formula satisfies all protein needs. Once solids are introduced, children should be eating proteins that provide all the essential amino acids.
This will mean either including animal protein or a good combination of plant proteins for a child on a vegetarian or other restrictive diet.
Protein deficiencies are rarely an issue with healthy babies in developed countries – excess consumption is a more frequent problem.
Average protein requirements for children can be divided into 2 parts: (a) requirements for maintenance and (b) requirements for growth.
Maintenance requirements are the same for all ages and sexes: 0.66 grams of protein per kilogram of body weight per day. Growth requirements are highest when we are younger, gradually reducing over time.
The average daily protein requirements for children are as follows:
|Age (years)||Requirements for maintenance|
(g/bodyweight in kg)
|Requirements for growth|
(g/bodyweight in kg)
- Gallagher ML. The Nutrients and Their Metabolism. In: Mahan LK, Escott-Stump, S. Krause’s Food & Nutrition Therapy. International Edition, 12e. 2008. pg. 39-143.
- WHO/FAO/UNU (World Health Organization/Food and Agricuture Organization of the United Nations/United Nations University) (2007) Protein and amino acid requirements in human nutrition. Report of a Joint WHO/FAO/UNU Expert Consultation. WHO Technical Report Series, No 935, WHO, Geneva, Switzerland.
- Heaney RP, Layman DK. (2008) Amount and type of protein influences bone health. Am J Clin Nutr. 87:1567-1570.