In the United States the nutritional needs of the public are estimated and expressed in the Recommended Dietary Allowances (RDA). These were initially established during World War II to determine in a time of possible shortage, what levels of nutrients were required to insure that the nutrition of the people would be safeguarded. The RDA are established by the Food and Nutrition Board of the National Research Council, whose members come from the National Academy of Sciences, the National Academy of Engineering and the Institute of Medicine.
The first RDA were published in 1943 by a group known as the National Nutrition Program, a forerunner of the Food and Nutrition Board. Initially, the RDA were intended as a guide for planning and procuring food supplies for national defense. Now RDA are considered to be goals for the average daily amounts of nutrients that population groups should consume over a period of time.
The RDA are the levels of intake of essential nutrients considered, in the judgement of the Food and Nutrition Board on the basis of available scientific knowledge, to meet the known nutrition needs of practically all healthy persons. The NAS-NRC recognizes that diets are more than combinations of nutrients and should satisfy social and psychological needs as well.
As the needs for nutrients have been clearly defined, the RDA have been revise at roughly five year intervals. The Ninth Edition of the RDA was published in 1980. The Tenth Edition was due to be released in 1986, but controversy regarding some of its recommendations has delayed its publication.
The requirements for a nutrient is the minimum intake that will maintain normal functions and health. In practice, estimates of nutrient requirements are determined by a number of techniques including:
- Collection of data on nutrient intake from apparently normal, healthy people.
- Determinations of the amount of nutrient required to prevent disease states (generally epidemiological data).
- Biochemical assessments of tissue saturation or adequacy of molecular function.
- Nutrient balance studies.
- Studies of subjects on diets containing marginally low or deficient levels of nutrient followed by correction of the deficit with measured amounts of nutrient.
- Animal studies.
Once the requirement. for a nutrient has been estimated, the following four steps in estimation of the recommended allowances may be utilized:
- Estimation of the average requirement of a population for a nutrient and the variability of requirements within a population
- Increasing the average requirement by an amount sufficient to meet the needs of nearly all members of the population.
- Increasing the allowance to account for the inefficient utilization by the body of the nutrient consumed.
- Using judgement in interpreting and extrapolating allowances when information on requirements is limited.
As an example of how RDA are determined, the following is a simplification of the calculations utilized to determine the 1980 RDA for protein for adult males. First it must be determined how much protein the average adult male loses each day so that the amount that has to be replaced by diet can be determined. These are based on a hypothetical individual known as the reference man. The reference man is considered to be 25 years old, to weigh 70 kg (154 lb), to be moderately active and to live where the mean temperature is 20°C (68°F). The reference female is considered to weigh 55 kg (110 lb). Numerous studies indicate that the following are the average losses of protein from the body of a healthy male:
Nitrogen Loss from healthy males
|Source of Loss||
|Loss of skin, hair, etc.||
|Mnior (saliva, tec.||
The average man loses 24 grams of protein per day and hence, should need to consume 24 g/day to replace this loss. The RDA, however, attempts to meet the needs of almost all healthy people so a recommendation that was valid only for the average person is not made. Rather it is noted that when studying the protein needs of groups of males that one standard deviation is about 15%. If the need for protein are normally distributed, the two standard deviations should ensure that 97.5% of the population is receiving an adequate diet. Two standard deviations would be 30% for protein so the requirement was increased by 30%.
24 + 7.2 = 31.2 g/day
We will later see that not all proteins are equally utilized and thus while 31.2 g/day should meet the needs of 97.5% of the adult male population, this population might be consuming proteins that are not ideal. To correct for this, the RDA was increased by 30%:
Some studies indicate that protein consumed in mixed diets may not be utilized as well as those in experimental diets that these figures were based on. To be sure that this is not a problem, it is assumed that the proteins will be only 75% utilized: This is equal to about 0.8 g/kg body weight so the requirement for the reference man is:
70 x 0.8 = 56 grams of protein per day
Note that this is not a minimum requirement or an average one. Rather it has many safeguards built in and is intended to cover practically all healthy people.