Hello,
Welcome to Just Answer and thanks for your question. You are eating too few calories to get all the nutrition you need. Men need at least 1,500 to 1,800 calories per day. You should add these calories as green vegetables and fruits since your shakes are already supplying all the protein you need. The added bulk and fiber from the veggies will keep you feeling fuller and give you more energy between meals. You should aim for a sensible 1-2 pounds of weight lost per week. Fast weight loss usually results in even faster weight regain. To gain muscle, you really need to add some weight training to your exercise program. Aerobic exercise is healthy for your body, but it takes weights to really put on muscle. Even 20 minutes three times a week will help.
Eating too few calories puts your body into starvation mode, and can change your metabolism, making it still more difficult to lose weight or maintain weight in the future. Slow and steady is the best way to take the excess weight off. After all, it didn't get there at the rate of 15 pounds per month, and it isn't healthy to try to lose it that way either. That is the case whether you have 25 pounds to lose or 250 pounds. It's still lost at 1-2 pounds per week.
Nurse Practitioner
Board Certified Nurse Practitioner, MS, RN. Private practice and hospitalist experience.