I think anything from the lemon--- as long as it is from a non-chemically grown, naturally ripened lemon-- will have benefits. Even if it is in concentrated form, it probably would be okay-- As long as the lemon concentrate is processed in a way where they don't use factory solvents and also ruin the nutrients from lemon by the factory processing-- and use fresh organic lemons.
Also-- don't forget that many processed juice or concentrate companies use almost spoiled fruits and fruits sprayed with pesticides for juices, because it saves them $$$ instead of having to totally toss out moldy fruit devoid of nutrients. Pastuerization is their excuse to solving the spoilage problem-- so what you are left with is a fruit juice or concentrate taken from over-ripe or almost spoiled fruit parts that are pastuerized so noone gets e.coli from using the concentrate.
Even with orange juice-- they use the refuse garbage oranges for the juice and add a little orange coloring to make it look fresh.
Buyer Beware!-- most of the concentrated lemon juice you see in the typical every day grocery store for " the masses" is garbage. It may be less convenient, but the simpler, the better. If you can get your hands on an organic orange-- make your own concentrate. At least you know what's in it.
Some small organic companies do make far better lemon juice concentrate-- that you probably can only find in a healthfood store, or Alternative grocer before the FDA eventually finds an excuse to kick them out of business in the future.
The zest part taken from the rind, even the pulp has benefits, depending on what you are using them for. From what I read along time ago, every part of the lemon has some type of nutrient that works with the other nutrients in the other parts. Bioflavonoids , I think, are in the rind-- and they work with the Vitamins in the juice. Trouble is, the oil based pesticide sprays these days make using the rind dangerous for your health. Using an organic veggie cleanser to clean them before cutting them would help a little, but youre better off with an organic whole lemon washed with veggie wash.
In the mornings, I squeeze lemon from a whole lemon into water to drink.
As for carb concern-- I'd not worry about carb content in low glycemic fruits like lemons if you cut out white flour, refined sugars and starches from the diet.