Protein nutrition, digestion, absorption and nitrogen balance

Question 1

Draw the basic structure of a protein.

Answer 1

NH2 - CH - COOH (with an “R” side group coming off the central carbon atom

Question 2

What gives a protein its unique properties?

Answer 2

The side group (the ‘R’ group)

Question 3

How many essential amino acids are there? (ones that are required to be taken up in our diet)

Answer 3

9

Question 4

How much of our daily energy intake is recommended to come from protein?

Answer 4

Approx 20%

Question 5

What is the recommended daily intake of protein per kg of body weight for adults?

Answer 5

women = 0.75g/kg
men = 0.85g/kg

However if you are an athlete/are training for something then you protein turnover is much higher and so then you need more protein

Question 6

Can a high protein diet increase muscle mass alone?

Answer 6

No - protein allows for muscle fibres to be rebuilt/provides the building blocks for muscle growth but you must actually exercise/lengthen/strengthen the muscle fibres in order to gain muscle mass.

Question 7

What two factors influence protein quality?

Answer 7

Its digestibility - animal protein is very easily digestible whereas plant protein is much less digestible.

Its amino acid composition - your dietary protein must supply the 9 essential amino acids as well as sufficient nirtrogen so that the other amino acids can be synthesized in the body

Question 8

Describe the digestion of protein from the point where it enters the mouth to where it arrives in the small intestine,

Answer 8

1. When ingested, the chewing of the teeth and the mixing of the food with saliva forms a moistened ball
2. As the protein rich food arrives in the stomach it is broken down by the acid in the gastric juice and the acid then activates the inactive pepsinogen secreted by the Chief cells to become the activated pepsin. Together the acid and pepsin convert the protein to small polypeptides.
3. Once in the small intestine the chyme stimulates the release of pancreatic enzyme precursors which are in their inactive state. They are activated by enteropeptidase (on the duodenal cell lining) which converts trypsinogen to trypsin and then trypsin can activate the other inactive enzyme precursors.

a series of proteases convert polypeptides to peptides and then a series of peptidases convert peptides to amino acids.

4. Absorption of the chopped up amino acids can then occur through the gut enterocyte (di and tripeptides re cotransported with H+, small peptides pass across via trascylosis, and amino acids are cotransported with Na+)

Question 9

What is nitrogen balance?

Answer 9

When the rate of nitrogen intake is equal to the rate of nitrogen expenditure

Question 10

What happens when the amount of ingested nitrogen falls below ‘normal’? eg due to a decrease in the protein quality

Answer 10

Initially the body will go into negative nitrogen balance for a few days but then the liver and other organs will adapt to the lowered nitrogen and nitrogen balance can be restored.

Question 11

What are some possible factors that can cause negative nitrogen balance? (net loss of N)

Answer 11

• when dieting/starvation (decreased protein intake) over the short term
• as a result of trauma/surgery the body will always go into negative nitrogen balance
• as a result of burns (a lot of burn victims actually die as a result of negative nitrogen balance as they are unable to synthesize enough protein to repair the damage)
• cancers (caechexia due to muscle wasting)
• during lactation (breastfeeding)

Question 12

What are some factors that can cause the body to go into positive nitrogen balance?

Answer 12

• Increased protein intake (over short term)
• growth
• pregnancy
• recovery from a surgery/illness

Question 13

What is the main way the body gets rid of excess nitrogen?

Answer 13

Through the urine. Urea, uric acid and creatine are all vehicles for excess nitrogen excretion.

Question 14

What happens to excess protein (excess amino acids in the body)?

Answer 14

They cannot be stored and so are deaminated to give ammonia and residual carbon skeletons. Ammonia is toxic and so is detoxified to urea.

Question 15

What is Kwashiorkor?

Answer 15

Usually common in young children not babies. Occurs due to a lack of protein in the diet and an excess of carbs. A lot of the time this happens in third world countries when the infant is taken off breast feeding when the second child comes along.

It prevents with a little bit of muscle wasting but much more prominently with a lot of bloating/oedema of the stomach.

It is treated by adding protein in the diet.

Question 16

What is marasmus/ protein energy malnutrition?

Answer 16

Marasmus is caused by a severe nutritional deficiency in general. It is usually found in very young infants and very young children. It can be prevented by breastfeeding. It is actually caused by the total or partial lack of nutritional elements in the food over a period of time.

Can be identified as children will have lost fat on normal areas such as the buttocks and thighs and they will be depressed and have anxiety.

Treatment is a nutritious diet and vitB supplements.