Background: Low fat high carbohydrate (LF) diets increase plasma triglyceride (TG) levels, but the role of hepatic de novo fatty acid (DNFA) synthesis is uncertain. We previously found a relationship between TG level and DNFA synthesis. Objectives: To use isotopic methods to examine the effect of dietary fat on plasma TG FAs. We hypothesized that eucaloric higher fat (HF) intake would result in lower TG levels and VLDL-TG saturated FA composition and DNFA synthesis changes. Methods: Six subjects were fed 2 diets differing in fat energy (LF<25%, HF>35%) for 3d (crossover, 1-mo washout). Blood samples were drawn before and 24h after deuterium-labeled water consumption. Results: Plasma and VLDL TG were lower following HF intake. Composition of VLDL-TG FAs showed a higher amount of saturated FAs after LF intake. No significant change was found in total DNFA synthesis between diets, but LF resulted in more palmitic and stearic acid synthesis. A relationship was found between FA synthesis and plasma TG only for LF. Conclusion: When compared to LF, HF lowered plasma TG and resulted in important differences in VLDL-TG FA composition that may have been influenced by DNFA synthesis. Funded by the CDA. Nutrition & Metabolism Biochemistry of Vitamins and Minerals (400-ASN) Energy and Nutrient Metabolism (401-ASN) Human and Clinical Nutrition (402-ASN) Metabolic and Disease Processes (403-ASN) Late Breaking Category - Nutrition & Metabolism 400-ASN Biochemistry of Vitamins and Minerals 401-ASN Energy and Nutrient Metabolism 402-ASN Human and Clinical Nutrition 403-ASN Metabolic and Disease Processes [ABSTRACT FROM AUTHOR]