I am in the same boat...I lost 14 lbs in the last month or so and lost 4lbs in one day and I'm down to 143 which is kinda on the "punny side" lol for a male 5'9".
Boost and Ensure have high levels of corn syrup in them and a powder mentioned above has fructose in it.
I looked into these supplement drinks and checked out lots and lots of weight gain and protien powders and most either have stuff in them you don't want if you are an IBD'er or cannot tolerate artificial sweetners and or just want to cut down on sweeetners period. I make killer smoothies using Pacific Natural Foods Almond Milk in Vanilla flavor (now with 35% less sugar and NO artifical sweetners or fructose etc) I add almond butter, strawberries and some rice based vanilla protien powder (Nutri-Biotic) that is all natural made from whole grain brown rice with natural vanilla and has only 1 gram of sugar w/ no funky sweetners, ice and a nice banana thats not too ripe and some cinnamon (which also helps with the small amount of sugar). Yummm...tastes great plus it's low in sugar, not too high in fiber so they are flare friendly : )