I think it is time to describe how to lose weight as simply as possible, and to examine how to apply these results to everyday life. From a biophysical viewpoint, if the energy input (calories) is less than the energy expended in daily motion and metabolism, we will lose weight. It is also important to remember that it takes metabolic energy to digest, process, and burn or store food, so that although fat contains 9 calories/gm, carbohydrates contain 5 cal/gm, and the average protein 4.5 cal/gm, the energy yield of each is less than this, because of the metabolic work that must be performed to make it useful to the body. Don't forget that, for instance, swallowing and peristalsis both use muscles, and these muscles require energy to function.
Again, if you ingest fewer calories than your daily expenditure, you will lose weight (and probably feel hungry throughout the process). The only painless way to lose weight is by cigarette smoking, and we deduce this from inverse data: Everyone who stops smoking gains about 10 to 15 pounds the first year, and women usually gain more than men. From this we infer that smoking not only assuages hunger, but raises our metabolic rate. And it has to be smoking, because the use of nicotine gum or patches does not lead to weight loss.
I might here mention that as far as I can see, antidepressants do not invariably lead to weight gain. Rather, depressed patients usually become vegetative, and lose interest in eating, sleeping, exercise and sex. As they become less depressed by the use of medicine, all their appetites increase.
I should also mention that most women find it impossible to lose the last 5 pounds they put on after they become pregnant and give birth. I am convinced (without any supportive evidence) that there is a permanent endocrine/metabolic change in their bodies.
Now, how can we ensure losing weight? The only method that always works (and cures adult onset diabetes as well, secondary to its enforced weight loss) is gastric banding or gastric bypass. The operation works by severely restricting the part of the stomach that can receive food at a given feeding. The patient's eating habits do not change, but if too much food is eaten at one sitting, the stomach pain is excruciating, so that patients learn to eat much, much less, and typically lose 100-150 pounds. On rare occasions, liver failure occurs 6 months to 2 years after the bypass surgery, and the failure is reversed by reversing the surgery. I have had 3 such patient, and since their fundamental eating habits were never changed, they quickly ballooned up to their previous weights.
I should also mention that between the ages of 25 and 75, your metabolic rate decreases by the equivalent of 3 to 4 pounds per year, so to paraphrase what the Red Queen told Alice in Wonderland you have to run faster or eat less each year just to maintain your current weight.
There is no evidence that posting the calories next to a food choice enables people to lose weight. All it does is spoil the pleasure of eating a hot dog at a ballpark, because you really don't want to know how many calories you are eating when you are eating for fun. It requires a lot of mental discipline to lose weight, and the human brain is not geared to avoid problems 10 years down the road, but only the dangers that are 10 seconds away. (Just as a cigarette smoker knows that cigarettes are dangerous, but feels confident that the particular cigarette he is about to smoke will not kill him.) I have told countless adult-onset diabetic patients that if they lost 10 to 20 pounds they would no longer have elevated sugars and I could stop their diabetic medicine. In all my years of practice not one has ever achieved this, even after being referred to an endocrinologist and a nutritionist.
Insofar as carbohydrates are concerned, it is wise to avoid foods with a high glycemic index. If you are dieting you should NEVER eat white potatoes or any bread (bread is really composed of "useless" calories, and white bread is the worst). It is OK to eat pasta, but never have seconds. Rice is the safest carbohydrate to eat to lose weight. In fact, many civilian captives of the Japanese in their prison camps during WWII lost a goodly amount of weight, and their diabetes went away. Before insulin, the standard nutritional treatment for diabetes was to limit the carbohydrates eaten to just rice. I should also mention here that in paleontologic excavations, skeletons from 9000B.C. when man was a hunter-gatherer show no evidence of tooth decay, but as soon as man started to cultivate carbohydrates, the later skeletons showed caries.
So how do we lose weight? I am ignoring exercise for the time being, except to note that most college graduates gain weight the first year they are working, because instead of running all over the campus and up and down stairs, they are sitting behind a desk for 8 to 10 hours each day. Again, we have to eat less, and exercise portion control. I don't believe in special diets, because few patients can keep them up forever, and it is easier to lose weight on diets that you like. I tell my patients no white potatoes or white bread, to weigh themselves on Sunday, and to re-weigh themselves the next Sunday. If no weight has been lost, I tell them to eat smaller portions any way they choose to, and to weigh themselves again the following Sunday. Repeat the process until weight is lost, and then maintain that diet with those portions. The weight loss always plateaus, because the brain is persnickety and loves the status quo, so as you lose weight, the body's metabolic rate decreases, so it becomes harder and harder to continue the weight loss. (Don't worry, the same thing happens in reverse as well: if you gain weight, the body's metabolic rate increases.) The main fact to remember is that you burn fat most readily when you feel hungry, and your liver usually has a 6 hour supply of glycogen, or animal starch. So if you feel hungry after dinner, you will start to burn fat in 6 hours, but if you don't feel hungry you start to burn fat 6 hours after you do feel hungry. So the best way to ensure fat loss is to feel hungry all the time, which most people cannot do.
The most successful at losing weight are marathoners and other long-distance runners, and anorectics. So at the extremes, both hard exercise and not eating will help you to lose weight. This shows that weight loss is definitely achievable, but no one knows how to motivate people to lose weight, any more than we know how to motivate them to stop smoking or drinking. I don't know if the dieters or the smoke-enders know themselves how or why they changed their habits.
I do think that daily exercise is part of weight loss. When I was young, we all ate "junk food" Bonbons, Dots, Chuckles, Banana splits Milky Way, Twizzlers, etc, but we ran around playing games in the school yard for 3 hours each day until dinner, and all day Saturday and Sunday. Today's children have after-school studies, practice, scheduled play dates, etc, so they have less of a chance to burn off the calories they ate. I assume they have the same food appetites that we did, so how can they lose weight without exercise? I should also mention that weight loss is especially problematic for poorer people, because protein is always more expensive than carbohydrates.
One final point: Why don't adults say to themselves "Hey, I should lose some weight" the first time their pants waists increase from 30 to 32 inches or their dress size increases from 6 to 8? Until we can answer that question I don't think we will ever solve the problem of how to help people lose weight.
And we all like ice cream because it's components are that of mother's milk: sugar and fat!