Here's what http://www.breakingtheviciouscycle.info/ says about juice from concentrate:
Juice from concentrate is normally illegal because when it is reconstituted they often add other things (like sugar). Often additives are not listed on the label.
Do you have an Aldis around you? I use Aldis tomato juice in a can since it is not from concentrate. It boils down really well, and has a nice smooth consistency. I season it with salt, basil, oregano, garlic (McCormick brand since they say that all ingredients are declared on the package. Some garlic powders are not legal, as companies sometimes add free-flowing agents to the spice, and don't state it on the label), honey, and dried onions to make a delicious spaghetti sauce.
Isaiah 40:31 "But those who wait on the Lord shall renew their strength; They shall mount up with wings like eagles, they shall run and not be weary, They shall walk and not faint." Isaiah 40:31
May God give you strength to persevere with the diet. The results are truly wonderful. I know from experience!