I was contemplating this question as I was watching a show on Dr. Phil about morbid obesity recently. The answer may depend on several factors.
People who are several hundred pounds overweight can make huge progress just by limiting the number of calories they eat to a normal amount, even a high "normal", let's say 3000 calories/day. If they also stop eating processed foods than this can make just as big a difference.
Of course most people who are trying to lose weight on their own rather than being at an expensive in-patient weight-loss clinic have had tremendous difficulty giving up processed foods and eating "normal" amounts because processed foods are filled with addictive ingredients, primarily sugar and wheat. For anyone who is not convinced that wheat products (incl. whole wheat) are addictive please take a look at the book "Wheatbelly" by William Davis.
Replacing these addictive foods with other foods that are just as satisfying but not addictive can be critical to success. Sugar and carbs in general are addictive in part because every time we eat them our body responds with releasing insulin from our pancreas so that the carbs, all of which our body turns into sugar, can be moved from our blood stream to our cells. Insulin is good and necessary but if we eat carbs a lot every day as most people do, our cells get gradually less responsive to insulin and our pancreas has to produce more of it, ultimately leading to type 2 diabetes when our pancreas can't keep up any more. The other problem is that every time our insulin levels go up, our sugar levels then go down quickly, leaving us hungry for more carbs. For most people this cycle is the reason they feel hungry every couple of hours. THIS IS NOT NORMAL but a sign that our body has become too reliant on carbohydrates for energy.
Therefore the way to eliminate the constant hunger is to cut down carbohydrates, and get more of our calories from fat.
The other advantage of eating fewer carbohydrates and more fat is that our body is getting used to using fat for energy and is getting better at it the fewer carbohydrates are available to use for energy. Our body does not distinguish between using fat from our diet and body fat as far as using fat as fuel. Once it is used to burning fat it will be better at burning fat, giving us plenty of energy for all our daily activities.
For people who have already lost most of their excess weight, eating fewer carbohydrates is often the only way to improve their body composition, because once their body has gotten used to burning fat there is so much more energy available for working out making working out much more easy and fun.
This is an experience that I share with many people on the low-carb, paleo and primal teams on spark. So, regardless of how much weight you have to lose (and you may at first be able to lose on almost any diet) if you can wrap your mind around giving up sugar and grains you will be working with your body to lose weight and get in shape.
For all those who are convinced they could never give up grains and/or sugar: It is actually much easier than you might think: Xylitol is an excellent low-glycemic sweetener that will not affect insulin levels much and it can be used just like sugar. Stevia is another healthy sweetener that many people enjoy and it is completely calorie and sugar-free.
Grains are easily replaced with nut meals/flours, the most popular ones of these are almond flour and coconut flour, but also flaxseed meal.
A diet that leaves you hungry and without energy will make any exercise an uphill struggle which is very discouraging for someone who does not have a long habit of enjoying exercise. Eating a lower-carb diet (between 50-100grams/day) while consuming more fat and a moderate amount of protein will hugely increase energy levels and joy of exercising and thereby increase muscle mass as body fat is reduced.
At least that has been my experience over this past year.
Now I'm going shopping for our Thanksgiving meal, including meats, eggs, full-fat dairy, different colors of vegetables, nuts, fruit and even some dark chocolate.