To determine the transport and utilization of dietary saturated, monounsaturated, and n‐6 and n‐3 polyunsaturated fatty acids for the developing brain and other organs, artificially reared rat pups were fed a rat milk substitute containing the perdeuterated (each 97 atom% deuterium) fatty acids, i.e., palmitic, stearic, oleic, linoleic, and linolenic, from day 7 after birth to day 14 as previously described. Fatty acids in lipid extracts of the liver, lung, kidney, and brain were analyzed by gas chromatography‐mass spectrometry to determine their content of each of the deuterated fatty acids. The uptake and metabolism of perdeuterated fatty acid lead to the appearance of three distinct groups of isotopomers: the intact perdeuterated, the newly synthesized (with recycled deuterium), and the natural unlabeled fatty acid. The quantification of these isotopomers permits the estimation of uptake and de novo synthesis of these fatty acids. Intact perdeuterated palmitic, stearic, and oleic acids from the diet were found in liver, lung, and kidney, but not in brain. By contrast, perdeuterated linoleic acid was found in all these organs. Isotopomers of fatty acid from de novo synthesis were observed in palmitic, oleic, and stearic acids in all tissues. The highest enrichment of isotopomers with recycled deuterium was found in the brain. The data indicate that, during the brain growth spurt and the prelude to myelination, the major saturated and monounsaturated fatty acids in brain lipids are exclusively produced locally by de novo biosynthesis. Consequently, the n‐6 and n‐3 polyunsaturated fatty acids must be transported and delivered to the brain by highly specific mechanisms.

Fatty acid cycling by chain shortening/elongation in the peroxisomes is an important source of fatty acids for membrane lipid synthesis. Its role in the homeostasis of nonessential fatty acids is poorly understood. We report here a study on the cycling of saturated fatty acids and the effects of troglitazone in HepG2 cells in culture using [U- 13 Fatty acids extracted from cell pellets after saponification were analyzed by gas chromatography/mass spectrometry. Peroxisomal ␤-oxidation of uniformly 13 C-labeled stearate (C18:0) and oleate (C18:1) resulted in chain shortening and produced uniformly labeled palmitate (C16:0) and palmitoleate (C16:1). In untreated cells, 16% of C16:0 was derived from C18:0 and 26% of C16:1 from C18:1 by chain shortening. Such contributions were significantly increased by troglitazone to 23.6 and 36.6%, respectively (p < 0.001). Desaturation of stearate contributed 67% of the oleate, while reduction of oleate contributed little to stearate (2%). The desaturation of C18:0 to C18:1 was not affected by troglitazone. Our results demonstrated a high degree of recycling of C18:0 and C18:1 to C16:0 and C16:1 through chain shortening and desaturation. Chain shortening was accompanied by chain elongation in the synthesis of other long chain fatty acids. Troglitazone specifically increased recycling by peroxisomal ␤-oxidation of C18 to C16 fatty acids, and the interconversion of long chain fatty acids was associated with reduced de novo lipogenesis.The peroxisomes and the mitochondria are two separate fatty acid ␤-oxidation systems having distinct roles in fatty acids catabolism, energy production, and substrate cycling within the cell. The ␤-oxidation system of the peroxisomes, unlike that of the mitochondria, is not coupled to oxidative phosphorylation and is an important source of acetyl (2-carbon) units for the synthesis of long chain fatty acids by chain elongation (1). Fatty acid cycling of polyunsaturated fatty acids in the peroxisomes has been shown to play an important role in the metabolism of essential fatty acids (2). The role of fatty acid cycling by chain shortening/elongation of saturated fatty acids is not well known. Because of recycling of label and the lack of proper isotopic methods, the study of chain shortening/elongation of nonessential fatty acids has been difficult.Recently, we have developed stable isotope methods for the study of essential and nonessential fatty acid metabolism using uniformly labeled compounds and mass spectrometry (3, 4). For example, chain shortening of [U-13 C]stearate produces palmitate with a mass shift of 16 daltons due to 13 C carbons, and the elongation of [U-13 C]stearate produces arachidate (C20:0) and behenate (C22:0) with a characteristic mass shift of 18 daltons. Thus, chain shortening and elongation can be measured by the formation of these unique isotopomer species. We report here a study of chain shortening and elongation of stearate (C18:0) and the role of activation of peroxisome oxidation with troglitazone, a peroxisome proliferat...

The study demonstrates that a gas chromatograph-mass spectrometer can be used to analyse [13C]urea breath tests, thus potentially lowering the cost of the test and increasing the number of laboratories that can perform the test.

We hypothesized that whole body glucose uptake (Rd) during exercise is not related in a simple, linear manner to O2 uptake (VO2). To test this, seven healthy male subjects (age range 23-34 yr) were studied in the postabsorptive but not glycogen-depleted state. Three conditions were examined: 1) rest, 2) 40 min of constant exercise in which the work rates were carefully chosen to consist of low-intensity exercise (no elevated blood lactate, a mean of 40% maximal VO2), and 3) 40 min of high-intensity exercise (markedly elevated blood lactate, 79% maximal VO2). Gas exchange was measured breath by breath, and glucose uptake and production were measured using [6,6-2H2]glucose. Low-intensity exercise (n = 7) resulted in a small but not statistically significant increase in mean Rd [3.06 +/- 0.37 (SE) mg.min-1.kg-1] compared with resting values (2.87 +/- 0.39 mg.min-1.kg-1) despite a fourfold increase in the production of CO2 and VO2. By contrast, the high-intensity exercise Rd (n = 5, 6.98 +/- 0.67 mg.min-1.kg-1) was significantly greater than the resting value (3.03 +/- 0.56 mg.min-1.kg-1). Results of glucose production were virtually the same. Similarly, mean levels of epinephrine and norepinephrine increased significantly above resting values during high- but not low-intensity exercise. Our data demonstrate that whole body glucose dynamics and regulation during 40 min of exercise do not change in a simple linear manner with respect to metabolic rate.