| Summary: | Abstract The semi-domesticated reindeer is a northern species of Cervidae that is exposed to extreme seasonal changes in temperature and nutrition in its living environment. The objective of this study was to examine the significance of thermogenic brown adipose tissue (BAT) for the survival of newborn reindeer in the cold during the critical perinatal period. The other main objective was to study the effect of wintertime undernutrition on serum and bone marrow fatty acid composition in yearling reindeer, with particular attention on the proportions of unsaturated and polyunsaturated fatty acids (PUFAs) and their feasibility as indicators of nutritional status. The results showed that the most of the adipose tissues in newborn reindeer were functionally active BAT. The tissue had specific anatomical locations, specialized cell morphological structure, high aerobic capacity, and tissue-specific mitochondrial 32000 Mr-uncoupling protein (UCP1) that is considered a rate-limiting factor for thermogenesis. The most readily mobilized fatty acids from BAT triacylglycerols were arachidonic, linoleic, and α-linolenic acids. BAT was most active at birth and during the close perinatal period but its aerobic capacity declined during the first month of life while UCP1 disappeared and the tissue gradually adopted the histological characteristics of white adipose tissue. The newborn reindeer had very low proportions of the principal C18-PUFAs, linoleic and α-linolenic acids, in serum lipids. However, the proportions of C18-PUFAs increased during the first few days of life by a rate that suggests a selective retention of these fatty acids from milk lipids. A prolonged restricted feeding of reindeer with lichen during winter and spring induced significant reductions in the proportions of linoleic and α-linolenic acids in serum cholesteryl esters and phospholipids, while proportion of arachidonic acid and serum prostaglandin PGF2α metabolite concentration increased. Plasma leptin and insulin levels decreased in parallel with decreases in feed intake and body weight. In freely ranging reindeer, the proportions of oleic acid and principal C18-PUFAs were significantly reduced in femur bone marrow triacylglycerols as a result of a wintertime undernutrition. In conclusion, active BAT is the dominant adipose tissue type in the newborn reindeer and it is likely to have a major significance on the thermoregulatory heat production and cold resistance of reindeer during the perinatal period. The changes in the specific PUFA proportions of serum and bone marrow lipids refleict the changes in the nutritional status and suggest that these fatty acids are preferentially utilized during prolonged wintertime undernutrition.
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