Some are meal replacements (like high protein and fiber ones) while others are meal supplements. Check the calories and protein and all the rest, if it's not about 30% of the DV then it isn't a 'meal'. I've lived off a few of them for a few days (intestinal surgery sucks: liquid only for a few days, clear liquid for 2 days with the nastiest antibiotics...) but I really don't recommend it. I suppose that with the right gelatin & oil based multivitamin you could get by on them. But it's cheaper, if you don't worry about sodium, to grab a $0.99 microwaveable meat+starch+vegi frozen food (pot pie, sandwich, pizza, whatever) and then use the meal replacement shakes as a supplement to round out what the other food doesn't get you.