PURPOSE: High fat-very low carbohydrate diets (HFLC) have become increasingly popular in the endurance community, but there is little data concerning the effects of HFLC on markers of cardiovascular disease risk. The aim of this study was to examine glucose and lipid marker responses following a prolonged HFLC. METHODS: Eight middle-aged (39.5 ± 9.9 y), trained but non-elite (VO2 peak = 48.5 ± 4.5 ml/kg/min) runners (1.77 ± 0.08 m; 81.7 ± 7.0 kg; 19.3 ± 6% body fat) served as participants. Venous blood was drawn from an antecubital vein after an overnight fast with standardized evening fluid intake on 4 occasions. During the first phase, runners simply consumed their habitual high carbohydrate diet (HC). Blood was collected around 0600 following ~48 h of restriction from any intense exercise. A 50-min run in the heat followed by a 5-km time trial was implemented following blood collection to induce significant heat and exercise stress. A fasted blood sample was acquired the subsequent morning to further delineate the influence of exercise stress. This protocol was repeated after 3 weeks during which time runners continued normal training but consumed <50 g of carbohydrate/day with ~70% of daily calories derived from fat. RESULTS: Diet intervention approached (p = 0.07) but did not reach significance for glucose. Triacylglycerol did not differ between treatments (pre-exercise HC = 65 ± 17; HFLC = 67 ± 35 mg/dL) but decreased (p < 0.05) for both treatments 24-h after exercise (HC = 42 ± 16; HFLC = 35 ± 21 mg/dL). There was a main effect for diet on HDL-C (pre-exercise: HC = 48 ± 10 and 50 ± 11; post-exercise: HFLC = 57 ± 13 and 60 ± 13 mg/dL). There was also a main effect (p = 0.02) for diet on LDL-C with HFLC exceeding HC at both collection points by ~20 mg/dL. Total cholesterol was approximately 30 mg/dL higher for HFLC both before and 24-h after exercise (p < 0.05). There was no change in VLDL-C and Lp(a). CONCLUSION: Implementing a HFLC does not appear to elicit significant negative cardiovascular disease risk in male runners 30-50 years of age with desirable pre-intervention lipid and glucose marker status.