Juicing when you have diabetes will give you the full power of plant nutrition. There are just a few vitamins and minerals that a juice diet may not provide.
The fat-soluble vitamins A, D, and E are not abundant in plant foods. Vitamin K is concentrated in leafy greens, but the body needs at least a little fat (about 1 teaspoon, or 5 grams) to absorb vitamin K. Up to 5000 IU of vitamin A per day, up to 2000 IU of vitamin D, up to 400 IU of vitamin E, and 1 mg of vitamin K per day can help prevent deficiencies while you are on a juicing diet.
Vitamin B12 is found in mushrooms and in some fermented foods, but it's not abundant in juice. Taking as little as one 1000-mg capsule or tablet per week will prevent deficiency. Diabetics who take metformin may benefit from up to 1500 mg per day.
Calcium is also short in most plant-food diets. While it is true that ounce for ounce and gram and gram of dry weight plant foods often contain more calcium than cheese or meat, juices are never the dry form of a plant. There is a lot of water in juice that there isn't in cheese, yogurt, milk, fish, or meat, and it's almost impossible to get an adequate amount of calcium just from juicing. Moreover, the calcium in leafy greens is locked in phytate that isn't broken up by juicing.
It's easy to get all the calcium you need from supplements. You just can't get all the calcium you need from a single dose of calcium per day. The small intestine can only absorb about 400 mg of calcium at a time, so you should not take more than 400 mg of calcium at a time. The excess causes constipation. If you are on a budget, take calcium carbonate. If you aren't, take calcium citrate malate or bisglycinocalcium.
Calcium citrate and bisglycinocalcium are slightly better absorbed than calcium carbonate. But calcium carbonate is a lot cheaper. Taking 400 mg of any calcium supplement three times a day is enough. Beware taking too much—a single dose of 1000 mg of calcium can actually lower your bloodstream calcium by triggering shifts in hormones in your kidneys.