An accurately measured intake of nutrients, calories, and water is
necessary for determining metabolic demands imposed in any space flight.
There is insufficient knowledge to predict total metabolic requirements
under the numerous stresses which can be anticipated. Simulator studies
are of great importance even for short-duration flights.
The two most important variables to be considered in establishing the
minimal diet are protein and energy requirements. NASA is supporting
research at the University of California (Berkeley) to determine these
requirements and to estimate individual variation in healthy young men.
The possibility of minimizing need through biological adaptation is
being explored.
It is difficult to estimate the minimum protein requirement of an adult
man. The generally accepted criterion of minimum adequate protein
nutrition in the adult is the maintenance of nitrogen balance at minimum
intake. The minimum protein requirements depend on endogenous nitrogen
loss. Analysis of the little data available indicates a best estimate of
2 mg of nitrogen per kilocalorie of basal energy expenditure. However,
this figure is higher than that noted in experiments in some human
subjects.
After minimum nitrogen requirements and minimum amino acid requirements
have been established, studies will be directed toward investigating
caloric restriction and adaptation to restriction of calories. It has
been suggested that caloric restriction in animals and man results in
apparent decreased energy need for the same activity. This apparent
paradox has never been explained. It has been shown that there is
adaptation to repeated episodes of caloric restriction both in animals
and man, so that subsequent periods of caloric restriction result in
decreased rate of weight loss, nitrogen loss, and longer survival.
Additional experiments are urgently required to determine the metabolic
demands for minerals—in particular, the metabolic balance of calcium,
potassium, sodium, and phosphorus. Under conditions of high water
consumption, large mineral losses are to be expected. Failure to replace
these can cause an imbalance which could impair the efficiency of the
individual to the extent of endangering the flight.
Analysis of samples taken in flight, both of urine and feces, should be
made. Respiratory quotients can be determined in flight, blood samples
should be taken before and immediately after flight for analyzing
selected components (in simulator studies these could be taken
periodically), and nutritional intakes (which would be facilitated by
formula diets) must be measured and analyzed.