There are a lot of misconceptions when it comes to weight loss, and these misconceptions can lead to a lot of frustration for those trying to lose weight. As a trainer, I came across many such misconceptions, but these three were by far the most common.
Myth 1: With the right combination of diet and exercise, I can look just like [insert actor/model/whatever here]. We like to think that we are in total control of our bodies (this particular logical fallacy is known as attribution bias for those keeping score at home), but in reality, there are significant and permanent limitations on that control.
First, your basic shape (skeletal structure) is determined by genetics. Some people have thin frames, others have heavier frames. In addition, we all tend, regardless of diet or exercise, to carry a certain amount of weight on those frames (exercise physiologists divide people into three categories: ectomorphs, mesomorphs, and endomorphs). This cannot be changed. A lot of people who want to be "thin" simply can't be. (Similarly, many "99-pound weaklings" want to have large muscles, but no amount of training will achieve this.) This is NOT to say that diet and exercise cannot achieve weight loss and muscle development or even, within limits, change the shape of your body. However, the effects of diet and exercise are determined to a large extent by genetics and physiology. Realistic goals are important for weight loss; if you want something that's physiologically impossible, you will only set yourself up for failure.
Secondly, the effects of diet and exercise are not exclusively, nor even primarily in most cases, reflected in changes to structural shape/appearance. In other words, the functional effects may be far more substantial than the structural ones. For example, marathon runners have highly developed but not large muscles, while power lifters often appear fat and out of shape, but they can lift immense loads. Both groups of athletes have highly conditioned muscles, but have them switch roles, and the results would not be very impressive, regardless of their diets and exercise regimens. Training is crucially important, but it doesn't trump genetics.
Lastly, some people gain/lose weight easily, others do not. This is not particularly well understood but likely comes down to genetics as well. The bottom line is that you must learn to embrace the fact that you have only so much control over your body; reasonable weight loss is a very achievable goal, but remember that a healthful diet and routine exercise are intrinsically important for health regardless of how much they affect your weight.
Myth 2: To lose weight, just do lots of repetitions with light weights. This seems to make intuitive sense, but it's quite wrong. Much of the confusion stems from the unfortunate concept of "burning fat". Most people seem to think that when they do more, they "burn" more calories, which will come from fat. Not really. A strenuous, one-hour long workout will probably burn, at most, about 300 calories; that's the equivalent of an apple and a small granola bar. To make things worse, most of those calories will not come from fat. Although fat has a lot more energy per gram than carbohydrates (9 kcal/g vs. 4 kcal/g, respectively), it's a much less readily accessible energy source (i.e., it's a much slower process to release energy from fat than from carbohydrates). So, the bulk of those 300 calories will come from muscle glycogen, not fat.
How, then, do you use fat? You will not burn much fat during a workout, but you will for less pressing energy demands, namely those associated with staying alive. The average adult has a basal metabolic rate of 1200-1400 kcal/day; that's the energy you expend to keep your heart pumping, your organs functioning, and your core temperature stabile. In other words, just being alive for a day takes about 4 times the energy that a strenuous one-hour workout requires. What exercise does is increase (slightly) your metabolic rate and create energy deficits (by depleting glycogen levels, creating minor tissue damage that must be repaired, etc.). Since fat is a slow but rich energy source, it is ideal for these kinds of processes. Therefore, you do burn fat as a result of exercise, but mostly in the 24 hours or so after your workout has ended. Furthermore, this effect has little to do with the kind of workout you do (this isn't strictly true, but I am simplifying a bit for the sake of clarity); generally speaking, the effect is roughly proportional to the intensity of the workout, not the kind of workout. There are a few things you should keep in mind, however.
First, if you are eating more calories than the combination of your basal metabolic rate and your exercise deficit (in this example, 1500-1700 kcal/day), you will likely not see any significant weight loss, as any fat you burn will quickly be replaced. Weight and weight loss are not exclusively determined by the balance between intake and expenditure of calories, but it is a significant component.
Secondly, because fat is such a rich energy source, it goes a long way. No matter what the amount of carbohydrates you burn through in a workout, it take less than half that amount of fat to replace that energy.
Lastly, people tend to burn fat according to the "last stored, first used" principle. Your body will use its most recent fat deposits first, working backward to its earliest deposits. Where your body stores fat (and in what order) is largely determined by genetics.
Myth 3: If I do enough crunches, I'll lose the last of my tummy fat. By now, you probably have a sense for why this is wrong. Muscle glycogen is site specific, fat is not. In other words, crunches will definitely deplete the glycogen in your abdominal muscles, but not necessarily the fat in your abdomen; there is no connection between the location of an exercise and the location from which fat is utilized. Think about those power lifters again; they all have exceptionally strong muscles, but most of them also have a fairly large amount of fat on them as well, even in places where their muscles are most heavily worked.
What should you take from all this? Here are a few general guidelines for losing weight:
(1) Set realistic goals. It is unhealthy to lose more than 2 lbs. per week, but it is also unrealistic to expect major physical changes in a short time. Accept your shape and basic physiological body type, and work with what you've got.
(2) Use yourself as a model, not, well, a model. It's much better to think "I'd like to look like I did when I was 30" than to think "I'd like to look like Brangelina".
(3) Even better, forget how you look--how do you feel? Focusing on how you feel, especially after you've been on a new regimen for at least a few weeks, will often be a better guage of success than a mirror or a scale. For example, are you sleeping better? Do you have more energy? Are you stronger? Has your mood changed? etc.
(4) Input < output. If you eat more than you use, you will probably not lose weight. However, don't starve yourself. You will not only fail to achieve sustained weight loss, but you will also do considerable harm to your body (and mind). Start with small dietary changes and gradually modify your habits until your diet is in line with your needs.
(5) Your diet should be composed of as much natural food (i.e., unprocessed or minimally processed foods) as possible. Fresh meat, eggs, nuts, legumes, vegetables, and fruits and minimally processed foods, such as yogurt or juice, are generally better for you than heavily processed foods like chips, granola bars, or breakfast cereal.
(6) Combine regular, strenuous exercise (3-6 days/week) with everyday exercise (e.g., use stairs instead of escalators/elevators, use a push mower instead of an electric/gas mower, rake instead of using a leaf blower, walk/bike instead of driving when feasible, play with your dogs/kids/whatever, etc.).
