What is so good about vegetables and weight loss?
Many of them have a lot of water and fibers. Fibers are those indigestible carbohydrates that make our stomach full and help to produce the sense of satiety. It is a poor man (and now it is actually rather a rich and wise man) form of stomach reduction surgery. Just fill it up with low calorie high fiber high water food, and you will not have too much room for other high calorie food and, what is more importantly, you will have less food cravings.
Below is a simplified guide how to navigate between different products of vegetable origin.
Stems and leaves - usually high in fibers and good carbohydrates and are the most desirable to eat for weight loss purposes.
Seed bearing parts of the plants - grains and legumes (see separate section), fruits which are treated as vegetables, like tomatoes, cucumbers, squash, or fruits, also nuts which are different from other seed bearing parts in the way the preserve most energy. Most of them would store energy as carbohydrates (in vegetables) or oils (in nuts) for seed development, many of them may be high in calories. If they are processed/prepared/cooked in the wrong way, you might end up eating pure starchy part with high glycemic index, with all good nutritious components removed. Avoid/control/watch/learn healthy ways for weight loss purposes. As a rule if they can be consumed in the raw/unprocessed form, it would be the healthiest.
Nuts belong to the same category as above seed bearing parts of plants. The difference is that the energy preserving part is comprised of mostly oils and some carbs. Most of the oils from nuts appear to be healthy. The major concern about nuts from weight loss prospective is that oils are extremely calorie dense (9 calories/gram which is more than twice comparative to 4 calories/gram for carbohydrates and proteins)
Grains and legumes see separate link.
Roots Root vegetables (beets, carrots, daikon etc.) store their winter energy reserves as complex starches in their roots. Most of the raw root vegetables have relatively “good” carbohydrates for weight loss purposes, but when cooked, they often degrade to “bad” carbohydrates similar to potatoes with high glycemic index.
Fruits are another seed bearing food. Usually sweet, most vegetables have a low glycemic index, with some exceptions (like bananas). Fruits in a raw/frozen state are the best. Any fruit processing potentially removes the healthiest part of the fruit and often results in added sugar for preservation purposes. Fructose is not necessarily the most desirable or the healthiest part of the fruit.
Cooking
In general, any processing (including cooking, preserving etc.) of fruits, vegetables, fruits, grains or legumes is more likely than not to increase chances of transforming carbohydrates into “bad” ones. Also, any processing usually results in concentration of one or another components or fruits and vegetables. And guess which component gets usually concentrated and preserved and which usually is removed? Don’t ask me why, but it is almost always that starches, carbohydrates and fats are concentrated and preserved. The appetite supressing and fulfilling fibers and water, microelements and often vitamins are removed. Any thermal processing reduces the water soluble vitamins, often in half or more. Do it twice and there is only a quarter or less of vitamins left.
I am not an advocate of a raw food diet, but I think when the choice exist, a raw form of fruits and vegetable is the healthiest one.
We can not consume unprocessed grains or legumes and it is very difficult to predict how healthy are the products resulting from their processing, very often such products are commercially made with a mystery manufacturing process and unclear added additives along the way. This is one of the reasons I do not like and avoid them.