Protein

Functions

Protein is found in all living cells. Different proteins play very different roles; some proteins provide structural properties while others have functional roles. Protein is made up of long chains of building blocks called amino acids. In total there are twenty amino acids found in human protein, of these 11 amino acids can be made by our bodies but the other nine can't, so are needed in the diet - these are referred to as the 'essential amino acids'. 

Proteins within the body are continually broken down and rebuilt, a process referred to as protein turnover. In addition to amino acids playing a role in synthesis of proteins they are also used as a source of dietary energy, just as fat and carbohydrate are. However, proteins are far more complex and variable than either dietary fats or carbohydrates.

Nine Essential Amino Acids

Histidine, isoleucine, leucine, lysine, metionine, phenylalanine, threonine, tryptophan and valine.

Food Sources

Proteins are found in both plant and animal derived foods. Animal proteins are closer in their amino acid profile to humans, however all the necessary amino acids can be provided by a vegetarian diet. Meat, fish, poultry and beans are all rich sources of protein. It can also be found to a lesser degree in dairy products, nuts, wholegrains, vegetables and fruits.

Typical Contributions from Food

Food	Protein (g)
Beef, sirloin steak, grilled (100g)	28.1
Lamb, leg steak, grilled (100g)	26
Chicken, drumstick flesh, grilled (100g)	26.4
Chicken, breast flesh, grilled (100g)	31.2
Snapper, flesh, baked (100g)	25.2
Salmon, red, canned, (100g)	20.3
Milk, trim, (1 cup)	8.6g
Cheese, edam, (20g)	5.3g
Icecream, vanilla (1 cup)	5.5g
Yoghurt, plain, unsweetened (1 cup)	4.4g
Yoghurt, soy, (150g pottle)	7.5
Baked beans, in tomato sauce (1 cup)	12.7g
Mushrooms, raw (1 cup)	1.6g
Banana (one)	1.4g
Almond nuts (10)	2.5
Peanut butter (1 tbsp)	5.0

Source: The Concise New Zealand Food Composition Tables 5th Edition.

Recommended Dietary Intake (RDI) Levels

Following are the recommended dietary intake (RDI) levels for New Zealand and Australia.

Protein RDIs – New Zealand and Australia

Life Stage	Age	Males (g/day)	Females (g/day)
Children	1-3 yr	14	14
	4-8 yr	20	20
	9-13 yr	40	35
	14-18 yr	65	45
Adults	19-70 yr	64	46
	71+ yr	81	57
Pregnancy	14-18 yr	-	58
	19+ yr	-	60
Breastfeeding	14-18 yr	-	63
	19+ yr	-	67

Acceptable Macronutrient Distribution Ranges to Reduce Chronic Disease Risk

The New Zealand and Australian governments recommend that between 15% and 25% of the energy in our diet comes from protein.

For a moderately active young man (height 1.8m) a daily energy intake of 12,000kJ would be normal. Hence, between 1,800 and 3,000kJ of energy should ideally come from protein for this man. This equates to around 110-190g of protein each day for a man. For a  moderately active young woman (1.6m tall) energy requirements are typically lower at around 8700kJ/day. At this energy intake level, protein contribution would ideally be between 80-140g.

Deficiency and Toxicity

A protein deficiency is uncommon in New Zealand and Australia. However, worldwide protein-energy malnutrition is a common problem that is responsible for over half the deaths of young children. Protein-energy malnutrition impairs immune response and in young children results in stunted growth and wasting.

Controlled clinical trials have shown that high protein intake results in increased urinary calcium excretion. However, there is no evidence to suggest that this has any effect on bone health in real life situations. There has also been some suggestion that excessive protein intake may impact renal function, but a number of short-term studies involving athletes consuming up to 2.8g protein/kg body weight per day have revealed no adverse effects on kidney metabolism in athletes.

An intake of at least 15% dietary energy from protein is necessary in the opinion of the New Zealand and Australian health authorities in order to provide all the associated micronutrients found with protein in foods. And until such time as more research has been conducted on the effects of a high-protein diet on long-term health it is also considered prudent to limit protein intake to providing a maximum of 25% of dietary energy (see above for calculation).  

References

Ministry of Health. (2006). Nutrient reference values for Australia and New Zealand. Canberra: Commonwealth of Australia.

Last Updated: 21 October 2008

The material provided by Thinking Nutrition Ltd on this website is for information purposes only. It is not a substitute for appropriate health advice from a qualified medical practitioner.