DF; 80% DIETARY Ca FROM DAIRY PRODUCTS (ALL AGES) OR=O7 xX 0.13 * 0.25=0.023 (Sr°FCa), | tsa, *° 20% DIETARY Ca FROM PLANTS R=0. x O.7=0475 7=0, OR=0.25 (sr°%a), os (Sr°%a)p v OR BONE TO SOIL VIA DIET (Sr?Yca) Ut = 025 (0.8 x0023)4(0.2 *0.175)=0.05 (Sr94%Ca)p _ ro * (sZare 04 DF, t * DF, : | GES MOMMAMMM 60) /:\ (Sr*/Ca), aes (Sr°%oa). = 0.08 SOIL 7 Fig. 3— Ecological discrimination against Sr* with respect to calcium (United States). t wisialy The fraction of dietary calcium derived from dairy products varies widely among the various populations. A general expression for the ecological discrimination factor is: (ORpone-soil) = (Mt x 0.025) + (Rg x 0.175) in which Mg and Rg are the fractions of dietary calcium derived from dairy products and from other sources, respectively. Discrimination ratios of Sr*®/Ca for various countries, derived from per capita consumption of principal foodstuffs?®»™ and their average calcium content, °! are given in Table 5. The over-all discrimination ratio varies from about 0.04 for countries with high milk consumption (New Zealand, Switzerland, Sweden) to about 0.15 for Far Eastern countries that consume little milk. Discrimination ratios were weighted for the population densities of the various countries to give weighted average values of 0.1, 0.06, and 0.12 for the north temperate lati- tudes, south temperate latitudes, and rest of the world population, respectively. The discrimi- nation ratios for the various areas are only superficially adjusted for differences in population dietary habits and make no allowance for individual variations in calcium metabolism and for the fraction of Sr® entering the food chain through direct fallout on vegetation. They may be conservative, however, because they are derived on the basis of complete availability of the deposited Sr® and on the assumption that all of man’s dietary calcium comes from the top 2.5 in. of the soil. Average maximum Sr® equilibrium bone levels in the world’s population postulated from ecological considerations are given in Table 6. These data suggest the average maximum level of Sr* in the bones of the population of the United States would be about 3.1 pe per gram of Ca, if they were in ecological equilibrium with the 1957 soil deposition levels. The average of the north temperate population belt would be about the same as the United States because of the lower ratio of milk to cereals in the diet of the heavily populated countries of the Far East. The population weighted world average is only slightly lower than the average for the north temperate latitude, which is not surprising since over 80 per cent of the world’s population lives in that region. 292 *p