Background: Abnormal fatty acid metabolism is an important feature in the mechanisms of insulin resistance and β-cell dysfunction. Carnitine palmitoyltransferase-1a (CPT-1a, liver isoform) has a pivotal role in the regulation of mitochondrial fatty acid oxidation. We investigated the role of CPT-1a in the development of impaired glucose tolerance using a mouse model for CPT-1a deficiency when challenged by either a high-carbohydrate (HCD) or a high-fat diet (HFD) for a total duration of up to 46 weeks.Methods: Insulin sensitivity and glucose tolerance were assessed in heterozygous CPT-1a-deficient (CPT-1a+/-) male mice after being fed either a HCD or a HFD for durations of 28 weeks and 46 weeks. Both glucose and insulin tolerance tests were used to investigate β-cell function and insulin sensitivity. Differences in islet insulin content and hepatic steatosis were evaluated by morphological analysis.Results: CPT-1a+/-mice were more insulin-sensitive than CPT-1a+/+ mice when fed either HCD or HFD. The increased insulin sensitivity was associated with an increased expression of Cpt-1b (muscle isoform) in liver, as well as increased microvesicular hepatic steatosis compared with CPT-1a+/+ mice. CPT-1a+/-mice were more glucose tolerant than CPT-1a+/+ mice when fed the HCD, but there was no significant difference when fed HFD. Moreover, CPT-1a+/-mice fed HFD or HCD had fewer and smaller pancreatic islets than CPT-1a+/+ mice. Conclusions: CPT-1a deficiency preserved insulin sensitivity when challenged by long-term feeding of either diet. Furthermore, CPT-1a-deficient mice had distinct phenotypes dependent on the diet fed demonstrating that both diet and genetics collectively have a role in the development of impaired glucose tolerance. © 2011 Macmillan Publishers Limited All rights reserved.