In recent years, a carbohydrate-restricted diet (very low-carbohydrate diet) has been recognized as effective for preventing and alleviating lifestyle-related diseases such as diabetes. The antidiabetic effects of a carbohydrate-restricted diet are based on suppressing postprandial hyperglycemia, leading to glycation and oxidative stress, risk factors for cardiovascular diseases. On the contrary, several studies have reported that carbohydrate-restricted diets reduce glucose tolerance. However, these studies have examined glucose tolerance after a relatively short duration of carbohydrate restriction of several weeks to several months. Dietary therapy for diabetes is long-term, lasting for years and even for the rest of the life. Therefore, the effects of carbohydrate restriction on glucose tolerance over a longer period must be investigated. In this study, we investigated the effect of long-term moderate and severe carbohydrate-restricted diets on glucose tolerance using Goto-Kakizaki rats, an insulin-deficient diabetic model. The results reconfirmed that early carbohydrate restriction worsens glucose tolerance. Long-term carbohydrate restriction appears to improve glucose tolerance; however, this may be more influenced by aging. Considering the enormous age-related weight loss and the high number of rat deaths in the severe carbohydrate-restricted group, severe long-term carbohydrate restriction should be carefully carried out.