Medium-chain triglycerides (MCTs) are included in the fat blend of several preterm formulas because of their complete absorption and rapid oxidation. The effects of two different fat blend compositions on calcium (Ca), phosphorus (P), and magnesium (Mg) balances and plasma levels and on plasma levels of parathyroid hormone (PTH), alkaline phosphatase (AP), and 1,25-dihydroxyvitamin D [1,25-(OH)2D] were investigated in 28 healthy very-low-birth weight infants at 4 weeks of age. A preterm formula with a traditional corn oil/MCT blend containing 38% MCTs (MCT group) was compared to a new fat blend, designed to resemble human milk more, containing 6% MCTs (LCT group). There was a higher absorption of Ca in the MCT group (73% vs. 60%. p less than 0.005), and an equal absorption of P (both 92%). The excretion of Ca correlated with the excretion of fat (p less than 0.00005). The LCT group showed a higher median PTH level (MCT: 2.1 pmol/L, LCT: 4.7 pmol/L, p less than 0.01) and a higher urinary P excretion (p less than 0.001). Mg absorption was also lower with LCT, but retention of Mg exceeded intrauterine values in both groups. Mineral plasma levels were in the normal range in both groups. AP was not different between groups and in the upper part of the reference range, whereas 1,25-(OH)2D levels were above the normal range and also not different between groups. We conclude that with the LCT formula, Ca absorption is slightly lower than with the MCT formula, whereas P absorption is unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)