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Therefore, the purpose of this study was to determine whether low muscle glycogen availability increases R<jats:sub>d<\/jats:sub>glucose under conditions of impaired fat oxidation. Six cyclists exercised at 50% peak O<jats:sub>2<\/jats:sub>consumption (V\u0307o<jats:sub>2 peak<\/jats:sub>) for 1 h after 2 days on either a high-fat (HF, 60% fat, 24% carbohydrate) or control (CON, 22% fat, 65% carbohydrate) diet to manipulate muscle glycogen to low and normal levels, respectively. Two hours before the start of exercise, subjects ingested 80 mg of propanolol (\u03b2B), a nonselective \u03b2-adrenergic receptor blocker, to impair fat oxidation during exercise. HF significantly decreased calculated muscle glycogen oxidation ( P &lt; 0.05), and this decrease was partly compensated for by an increase in fat oxidation ( P &lt; 0.05), accompanied by an increase in whole body lipolysis ( P &lt; 0.05), despite the presence of \u03b2B. Although HF increased fat oxidation, plasma glucose appearance rate, R<jats:sub>d<\/jats:sub>glucose, and glucose clearance rate were also significantly increased by 13, 15, and 26%, respectively (all P &lt; 0.05). In conclusion, when lipolysis and fat oxidation are impaired, in this case by \u03b2B, fat oxidation cannot completely compensate for a reduction in muscle glycogen utilization, and consequently plasma glucose turnover increases. These findings suggest that there is a hierarchy of substrate compensation for reduced muscle glycogen availability after a high-fat, low-carbohydrate diet, with fat being the primary and plasma glucose the secondary compensatory substrate. This apparent hierarchy likely serves to protect against hypoglycemia when endogenous glucose availability is low.<\/jats:p>","DOI":"10.1152\/ajpendo.00302.2004","type":"journal-article","created":{"date-parts":[[2004,8,18]],"date-time":"2004-08-18T01:08:11Z","timestamp":1092791291000},"page":"E1195-E1201","source":"Crossref","is-referenced-by-count":13,"title":["Manipulation of dietary carbohydrate and muscle glycogen affects glucose uptake during exercise when fat oxidation is impaired by \u03b2-adrenergic blockade"],"prefix":"10.1152","volume":"287","author":[{"given":"Theodore W.","family":"Zderic","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Simon","family":"Schenk","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Christopher J.","family":"Davidson","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lauri O.","family":"Byerley","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Edward F.","family":"Coyle","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"24","reference":[{"key":"R1","doi-asserted-by":"publisher","DOI":"10.1152\/jappl.1999.87.1.111"},{"key":"R2","doi-asserted-by":"publisher","DOI":"10.1007\/s001250100005"},{"key":"R3","doi-asserted-by":"crossref","unstructured":"Blaak EE, van Baak MA, and Saris WH.Beta-adrenergically stimulated fat oxidation is diminished in middle-aged compared with young subjects.J Clin Endocrinol Metab84: 3764\u20133769, 1999.","DOI":"10.1210\/jc.84.10.3764"},{"key":"R4","doi-asserted-by":"publisher","DOI":"10.1111\/j.1469-7793.1999.293af.x"},{"key":"R5","doi-asserted-by":"publisher","DOI":"10.1152\/jappl.1989.66.2.548"},{"key":"R6","doi-asserted-by":"publisher","DOI":"10.2337\/diabetes.51.5.1310"},{"key":"R7","doi-asserted-by":"publisher","DOI":"10.2337\/diabetes.49.8.1281"},{"key":"R8","doi-asserted-by":"publisher","DOI":"10.1152\/ajpendo.1996.270.1.E116"},{"key":"R9","unstructured":"Eggstein Mand Kuhlmann E.Triglycerides and glycerol determination after alkaline hydrolysis. 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