Binding of [3H]ouabain by erythrocytes and levels of (Na+ + K+) ATPase, cell sodium and potassium, and plasma triiodothyronine (T3) were studied in 16 normal, apparently healthy men. The subjects were between 28 and 50 years of age. Based on their economic status, the subjects were divided into two groups of eight each. Group A consisted of subjects with a higher economic status; these subjects had a mean body mass index (weight/height2) of 24.1 +/- 1.2 (mean +/- SEM). Group B comprised subjects with a lower economic status; these subjects had a mean body mass index of 18.8 +/- 1.0. The number of ouabain binding sites per red cell in group B was higher (466 +/- 29) compared with that in group A (348 +/- 21; P less than 0.01). The dissociation constant (Kd) between the two groups was not different. The (Na+ + K+) ATPase activity per 10(10) cells in group B (0.50 +/- 0.07) was higher compared with that in group A (0.29 +/- 0.03). The increased number of pump sites and increased enzyme activity in group B were associated with a lower [Na]-to-[K] ratio in the cell (P less than 0.01). The plasma T3 level in group A (202 +/- 16.7 ng/dL) was not statistically different from that of group B (163 +/- 17.6 ng/dL). The number of sodium pump sites showed an inverse relationship with body mass index (r = -0.55; P less than 0.05) and with the cell [Na]-to-[K] ratio (r = -0.74; P less than 0.01). The number of pump sites in group A showed a positive correlation (r = 0.60) with plasma T3 levels. Such a relationship was, however, weak in group B (r = 0.36). The results lead to the conclusion that there was increased utilization of available cell energy by sodium pump activity in the subjects in group B. This may be a physiologic adaptation for efficient utilization of ingested nutrients via the sodium pump in response to marginal nutrient deficits in these subjects. Dissociation of increased pump sites from plasma T3 levels may mean that the adaptive phenomenon does not represent a wasteful loss of energy.