Hello Cheggars25,
I’m not familiar with shakes so I looked them up and they are quite high in carbs at 22 grabs net per 100 grams. If you’re drinking 600 calories worth, that would put your carb intake at 36 grams per day. That’s not a bad rate, but you want to keep an eye on your totals. Everyone is different in terms of what daily carb intake works for them. If you have blood sugar issues of any kind, you might need to go very low, 20-30 grams a day, to see significant weight loss and improvements in BG levels. Others do very well at 50 grams a day.
The phd shakes have oats (the source of most of the carbs) and the artificial sweetener sucralose in the ingredients. Sucralose is one of the worst sweeteners; it can spike blood sugar and is damaging to beneficial flora in the gut. Having said that, when I started out on the Fast800 I made the occasional zero carb protein shake that had sucralose in it and I lived to tell the tale. Your plan to use them as a three-week introduction to the calorie restriction seems reasonable to me. That way you won’t become dependent on them and can move on to whole foods without sucralose and if you chose to eat oats you can enjoy (very small) portions of pinhead/steel-cut oats that still have their fiber intact instead of the steamed, rolled and powdered kind.
Good luck and welcome to the forum!