Bone resorption varies as a function of time of day and quantity of dietary long chain polyunsaturated fatty acids

It is unclear whether dietary arachidonic acid (AA) and docosahexaenoic acid (DHA) alter the circadian rhythms of bone turnover markers, plasma osteocalcin (OC) and urinary N-telopeptide (NTx). We hypothesize that dietary AA and DHA will influence the circadian rhythm of NTx and OC. Piglets were randomized to receive one of four formulas for 15 days: control or control with AA:DHA (0.5:0.1, 1.0:0.2 or 2.0:0.4 g/100 g of fat). Measurements included polyunsaturated fatty acids (PUFA) and plasma OC (sampled at 0900, 1500 and 2100 h on day 15) and urinary NTx:creatinine (collected from 2100 h on day 14–0900 h, 0900–1500 h and 1500–2100 h on day 15). Main effects (litter, diet, time) were identified by mixed model repeated measures ANOVA. In those fed AA and DHA, regression identified relationships among plasma PUFA and NTx. There was a diet (P=.0467) and time (P<.0001) effect on urinary NTx:creatinine, whereby those receiving 1.0:0.2 g/100 g of fat as AA:DHA had the lowest values and values were lowest at 2100 h. Likewise, diet (P=.0001) and time (P<.0001) affected plasma AA and DHA; higher dietary AA and DHA elevated values and time reduced values. There was a diet by time interaction on eicosapentaenoic acid and DHA proportions, suggesting dietary AA and DHA altered their circadian rhythm. In regression, plasma AA and DHA were not associated with urinary NTx:creatinine. Dietary AA and DHA at amounts similar to that found in breast milk reduce bone resorption, but do not alter its circadian rhythm.