Occurrence and Requirements

When a healthy man (A) eats a proteinfree diet for 7-10 days, his daily N loss in all compartments (urine, feces, skin, hair, etc.) stabilizes at an average of 54 mg/kg. This corresponds to a daily loss of 0.34 g protein/kg (24 g/d for a 70 kg body). To include 97.5 % of all people of this age group two standard deviations have to be added: the resulting 0.45 g/kg are called the minimum protein requirement. This (approximately 31 g protein/d), however, is sufficient only based on an assumed 100% absorption. Since, in practical terms, this can be achieved only under standardized dietetic conditions, an additional safety margin for the overall population is needed.

In many countries, as in the U.S., the presently recommended intake for adults is 0.8 g/kg body weight (see also AMDR for proteins, on this page). In the case of many diseases (e. g., burns, nephrotic syndrome) actual protein requirements may be much higher; in the case of others (e.g., renal insufficiency, liver disease) restriction of protein intake to the minimum requirement may be indicated.

A growing third trimester fetus has the highest protein need but also receives a preselected, ideal AA mixture directly into its blood. Preemies are deprived of these optimal conditions; their protein requirement may rise to 3.8 g/kg when fed enterally (B). Since overall fluid volume of neonates is limited, such high protein intakes can often be achieved only by feeding protein-enriched mother's milk.

A nursing infant's requirement of 2.2 g/ kg, on the contrary, can be met by feeding mother's milk alone.

Actual intakes are often far above recommended amounts in Western industrialized nations. In the U.S., people have been consuming 80-125 g protein/d since the beginning of the twentieth century and those intakes have not changed. Protein sources, though, have changed: whereas in the past most of the protein was of vegetable origin, today, animal proteins make up more than 70 % of total protein intake.

There has never been proven toxicity of elevated protein intakes. Accounting for this fact, the recently established

Acceptable Macronutrient Distribution Range (AMDR), lists 10-35% of total calories from proteins, with an upper range that exceeds by far the amount considered adequate according to the existing RDA (-10% of total calories, depending on activity levels).

However, a high intake of animal protein is always associated with high intakes of other substances (e. g., fat, cholesterol, purines). Positive effects of elevated protein intakes could also not be proven. It has been shown that increased intakes lead to heightened turnover of muscle protein. Based on such findings, millions of people eat protein concentrates and AA mixtures in the hope of benefiting by an increase in muscle growth. However, heightened protein metabolism in muscle is not automatically equivalent to increased muscle growth. The question should be whether or not increased protein metabolism is better or worse for the body.

Occurrence and Requirements 135

- B. Recommendations by Life Stage and Gender Groups (DRI, 2001)

Life Stage and

Age

AI

EAR

RDA

RDA

Gender Group

(g/kg/d)

(g/kg/d)

(g/kg/d)

(g/d)

Infants

0 - 6mo

1.52

7 - 12mo

1.52

1.1

1.5

17

Children

1- 3y

0.88

1.1

13

4 - 8y

0.76

0.95

19

Boys

9 - 13 y

0.76

0.95

34

14 - 18 y

0.73

0.85

52

Girls

9 - 13 y

0.76

0.95

34

14 - 18y

0.71

0.85

46

Men

> 19 y

0.66

0.80

56

Women

> 19 y

0.66

0.80

46

Pregnancy

14 - 50 y

0.88 or + 21 g

1.1 or + 25 g

Lactation

14 - 50 y

1.05 or + 21.2 g

Physical activity

No evidence of increased requirements

- C. Protein Content in Food-

56g protein is contained m: 3 kg vegetables

- A. Requirements and Intake

Healthy adult male, 70 kg

24

Loss with protein-free diet

31

FAO/WHO = minimum protein requirement

40

Including safety margin for variable intestinal use

56

Including safety margin for variable BV, RDA etc.

55 - 62 Q

Actual intake in the U.S.-women, age-dependent

71 - 101 ยก2

Actual intake in the U.S.-men, age-dependent

Protein (g/d)

400g cold cuts 270g fish 200g meat 170g smoked ham 150g hard cheese

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