All diets that produce results are based on a simple principle: if you eat less than you burn, you will lose weight. It's creating this calorie deficit that causes weight loss. As far as losing weight is concerned, it doesn't matter whether or not the weight loss is done in a healthy matter, as long as your body burns more calories than you eat, the weight will come off.
A calorie deficit is what fuels weight loss. A calorie deficit is the difference between what you burn and what you eat. If you burn 2,500 calories each day and eat only 2,000, your calorie deficit is 500 calories per day. Since your body needs 2,500 calories but you're only giving it 2,000, those extra 500 calories need to come from a non-food source, body fat. Each day your body will burn 500 calories of stored body fat to make up for the calorie deficit that you've created. Overtime, this will cause weight loss.
Healthy and Unhealthy Weight Loss
Creating a calorie deficit is all about increasing your physical activity level and eating less. It doesn't matter whether your diet is filled with healthy food or junk, as long as you don't cheat on your daily calorie allowance, you will lose weight. Diets such as Atkins succeed only in cutting calories and causing weight loss, not improving your overall health.
The Atkins Diet
The Atkins Diet is based on the principle that carbohydrates are bad and should be eliminated from the diet. The reason that the Atkins Diet causes weight loss is because carbs are a huge part of a normal diet. Carbs typically account for 50-60% of your daily caloric intake. Eliminating them from your diet will cause a drastic reduction in your daily calorie intake, even if you do try and compensate by eating more fats and proteins. Eliminating all carbohydrates is a bad idea but there are some carbs that you should stay away from to improve your overall health.
Good vs. Bad Carbohydrates
Carbs aren't all bad. There are good and bad ones. Eating the good ones can improve your overall health while eating too much of the bad ones can hurt you. Carbohydrates are all made up of sugar molecules that are linked together. The more sugar molecules a carb is made up of, the more complex it is. Carbs can be made up of anywhere from 1 or 2 sugar molecules to thousands of them linked together.
The simple carbs are the ones that you want to stay away from. Because their structure is small, the body can digest and absorb them very easily in a relatively short period of time. This quick digestion time causes you to get hungry soon after eating them which leads to hunger and cravings. Complex carbs take the body much longer to digest which helps you eat less throughout the day. Simple carbs are found in soda, fruit juices, doughnuts and anything that's high in sugar. Complex carbs are found in 100% whole wheat products (bread and pasta), brown rice, vegetables and beans.
Healthy Weight Loss
Weight loss can be achieved in both healthy and unhealthy ways. The Atkins Diet is an example of unhealthy weight loss. Healthy weight loss can be achieved by cutting out the junk food you eat and replacing the bad calories with healthy ones. Calories you should avoid include saturated fat (sources: pork, dairy, meat), trans fat (sources: anything that includes hydrogenated vegetable oil as an ingredient) and simple carbs.
Foods that can improve your overall health and help you with your weight loss goals include complex carbs, unsaturated fats (sources: olive oil, canola oil, seeds, nuts, avocados, seafood) and lean proteins (sources: chicken, turkey, some cuts of beef, low fat dairy).
The Bottom Line
All diets work off of the same principle: increased physical activity coupled with a decreased calorie intake. Any diet claiming to have a shortcut to weight loss is lying. By skipping the shortcuts and fad diets, you'll reach your weight goals a lot easier, quicker and most importantly, healthier.