Science of weightloss

Conventional wisdom states that the only way to lose weight is to burn more calories than you consume, which makes sense. But I've always heard that diabetics have a harder time losing weight and was wondering if there's something more complex going on in our bodies (like maybe we don't actually burn as many calories as a non-diabetic doing the same work would, just as an example). I know that we lack a certain digestive enzyme that slows stomach emptying, which means we get hungrier faster. Is this all that's behind the claim that it's harder to lose weight when you're diabetic--since we get hungrier faster, we're more likely to eat/consume more calories and hence have a harder time burning more than we consume? (On a similar note, why/how does taking certain medications cause a person to gain weight or make losing weight more difficult?)

Sorry for the onslaught of questions recently. I'm (finally) on one of my health kicks and am really trying to understand all this (diabetic) stuff...

Oh boy, what a question!  I believe there are many factors involved.  First off, regardless of diabetes, there are more overweight and obese people today than there were 20 or 30 years ago.  According to statistics cited Gary Taubes' book Good Calories, Bad Calories (p. 232) , there is a lot of evidence that American men increased their calorie consumption from the 1970's to the 1990's by 150 calories a day, whereas women increased theirs by 350 calories a day.  At the same time, low-fat diets came into popularity and consumption of carbs increased.  Meal sizes increased and super-sized orders became popular.  Lots more snacks were marketed and there was a general decrease in physical activity.  That is, we drove more and didn't walk or ride bikes to go shopping.  So, not only were many people eating more but they were not as active.  Soft drinks switched from cane sugar to high fructose corn syrup and corn syrup began appearing as an ingredient in cereals and other products.  Carbs more than protein or fat in the food prompt the body to produce insulin for metabolism for energy but when too many carbs are consumed and so much insulin is produced, the person gains weight if that person can't use up the energy or doesn't work it off.  Insulin is a fat-building hormone.  Type1 diabetics who don't produce their own insulin may become emaciated if they're not diagnosed and treated because their cells are literally starving since there's no insulin to help get nutrients into the cells. 

I don't know about diabetics missing a digestive enzyme.  I have not heard of that.  I've heard of some people having gastroparesis - which is delayed stomach emptying.  I know that a high carb diet will make you hungrier later and so you might eat more.  And of course carbs do increase blood sugar which is one reason to cut down on them.

There are many more facets to this and I don't understand it all.  And then there are the medications that add weight!  Like prednisone and other steroids.  And insulin...  It's puzzling!