A: Most lymphocytes obtained by venipuncture are in a resting state in terms of cell division. Since they have a 4- to 6-month lifespan, the nutrient levels accumulated in these lymphocytes represent a history of an individual's nutrient status. This situation is analogous to using HbA1c measurements to approximate a diabetic person's glucose levels over the months preceding a test. Thus, lymphocytes provide a history rather than a snapshot of nutrient intake.

Resting lymphocytes can be stimulated by a lymphocyte-specific mitogen to undergo cell division and grow in culture. The degree of growth that the lymphocytes can maintain is directly related to the nutrients they have available. Thus, the FIA provides a functional intracellular analysis of nutrient status accumulated in human lymphocytes over their resting lifespan.

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