Abstract: The invention provides the simultaneous, separate or sequential feeding or administration of a carnitine substance (e.g. L-carnitine) with an agent for increasing the blood/plasma insulin concentration. The agent may be protein, carbohydrate or one or more amino acids or a combination of such nutrients. The co-administration or co-feeding of the carnitine substance and the agent provide for the reduction of whole body fat mass, particularly associated with excess dietary energy intake. The disclosed co-administration may also provide for the treatment of insulin resistance, fat oxidation disorders or obesity, and for increasing energy expenditure and fat metabolism. Compositions and kits containing the carnitine substance and the agent are provided, as are methods of treatment employing the co-administration or feeding of the carnitine substance and the agent.