Introduction

Prolonged dietary inadequacy alters the biochemical milieu of the body, and consequently enzymatic activities. in advance of the appearance of clinical symptoms and signs. Laboratory measurements of the nutrient adequacy of body fluids or tissues, therefore, can provide objective, specific, and sensitive indicators of nutriture.

These measurements, judiciously selected for some nutrients, can provide subclinical information useful in evaluating the nutritional impact of nutrition interventions.

Depending on the specific nutrient in question, adequacy may be measured biochemically by:

• direct quantitative assay of the concentration of a nutrient or metabolite in a body fluid or tissue (e.g., albumin or ferritin levels in serum or the ascorbic acid level in leucocytes);
• in vitro measurement of the level of a nutrient-dependent enzyme or its activity before and after addition of the nutrient cofactor (e.g., stimulation of the activity of the thiamine-dependent transketolase enzyme or the riboflavin-dependent glutathione reductase enzyme in red blood cells;
• in vivo measurement of a nutrient-dependent functional response to a stimulus (e.g., reticulocytosis following folic acid or iron supplementation).

Which laboratory approach is appropriate for measuring adequacy for a particular nutrient will depend on an understanding of its basic biochemical role, the distribution among body compartments during periods of dietary lack and sufficiency, and how this distribution is influenced by short and long-term changes in the physiological environment, e.g.. acute and chronic infections, and hormonal imbalances and variations.