Maintenance of muscle mass and protein intake are closely related. Insufficient protein intake in each meal or a total of three meals causes a decrease in muscle mass. For the elderly, protein intake has been insufficient at breakfast and then a large amount of protein is necessary for stimulating muscle protein synthesis. Consequently, there is a need to more actively and consciously take protein in older age. There have been conflicting results concerning the effect of protein restriction diet on glomerular filtration rate (GFR) in patients with chronic kidney disease (CKD) from the data of various meta-analyses. A beneficial effect and also no significant effect was found. One of the perspectives suggested that protein restriction diet may make slower CKD progression in T1DM and non-DM subjects, but not for T2DM patients. However, further studies will be necessary in the future.