The slow loris (Nycticebus coucang; Lorisidae) is a slow-moving, arboreal mammal with a very low metabolism relative to other eutherian species of its body mass. A slow pace of life has been causally linked to a low intake rate of usable energy due to a diet that is generally low in energy, is unpredictably periodically scarce, and contains high amounts of toxins or digestion inhibitors. To assess whether the slow loris faces any of these limitations, we studied its dietary habits in an area of West Malaysia (Manjung District, Perak State) by direct observations of radiocollared individuals and by fecal analysis. The diet was composed of 5 distinct types of food: floral nectar and nectar-producing parts, phloem sap, fruits, gum (another group of plant exudates), and arthropods. The largest proportion of feeding time was spent on phloem sap (34.9%), floral nectar and nectar-producing parts (31.7%), and fruits (22.5%). These foods should provide high amounts of easily digestible sugars, indicating that slow lorises did not face an energy-poor diet. Dietary habits were indistinguishable between rainy and dry seasons, even though most dry-season data were collected during periods of extreme drought induced by the 1997–1998 El Niño Southern-Oscillation event. However, many genera of food plants contain secondary compounds that are toxic or reduce digestibility. We suggest that low metabolism in slow lorises is at least partly related to the need to detoxify secondary compounds in high-energy plant diet.