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IMHO, the source of the calories IN has no bearing on the issue (barring disease processes, such as diabetes). You CAN lose weight on 3 chocolate bars a day. And if you eat one apple a day over your maintenance caloric intake, you WILL gain weight.
I'll agree with this as well, with an added thing to consider - you're absolutely right that no matter what you're eating, 1500 calories in is 1500 calories, period....if your calories out are more, you'll lose weight, and if your calories out are less, you'll gain weight. But what people tend to ignore is the OUT of the equation...it's not like this number is static. In addition to varying based on your activity during the day, it also varies based on what you eat. If you're not fueling your body well (protein, vitamins and minerals, etc), that does have a (scientifically proven) effect on the calories out half.
I can give 2 examples of this. The first has to do with fiber digestion. Fibrous foods take a little more energy for your body to digest, so you burn more calories on days when you eat more fibrous foods than on days when you eat processed low fiber junk. The second example is caffeine - caffeine has a boosting effect on resting metabolic rate - you burn more calories if you had a cup of regular coffee than if you had decaf. There are compounds that effect the rate at which your body burns found in foods. And this is just talking immediate impact...not considering the muscle loss that can result from insufficient protein intake and a myriad of other nutritional deficiencies that can result in muscle loss, which of course, ultimately lowers your calories out as well.
So yes, if you eat 1200 calories of Snickers bars, and you burn 1500 calories that day, you'll be at a 300 calorie deficit, which will result in weight loss over time. But that kind of diet may also, over time, reduce the calories you burn so it's harder to create a deficit. I think choosing healthy foods, most of the time, helps to keep metabolic fires burning so you don't experience so much of that slowdown, which is why I eat a healthy diet (plus, it just makes you feel good).