Weight loss can be accomplished by any number of means including surgery, jaw-wiring, eating only one food (I've seen bananas, hot dogs, Twinkies and ice cream all used as single-food diets that resulted in short term weight loss), or significant calorie-restriction. The goal of most people who are trying to lose weight isn't solely to weigh less, but to get a normal physiology and recapture lost health and vitality that became worse with increased weight. So any study that establishes simply that a strategy creates weight loss is interesting but not compelling unless one can establish in addition that 1) the weight loss is sustained, 2) the patients health improves by some objective measures related to outcomes and not just lab test results (for example, fewer heart attacks, strokes, cancers, cases of diabetes or osteoporosis) 3) the diet is nutritionally replete and doesn't cause unwanted side effects.
While the Atkins diet does seem to cause a weight loss of ten to fifteen pounds in most patients, it hasn't been shown in long-term studies to be superior to other calorie-restricted diets.
The American Heart Association is pretty clear cut:
The American Heart Association doesn't recommend high-protein diets for weight loss. Some of these diets restrict healthful foods that provide essential nutrients and don't provide the variety of foods needed to adequately meet nutritional needs. People who stay on these diets very long may not get enough vitamins and minerals and face other potential health risks. ...
A healthy diet that includes a variety of foods and is rich in fresh fruits and vegetables along with regular physical activity can help most people manage and maintain weight loss for both cardiovascular health and appearance. The American Heart Association urges people to take a safe and proven route to losing and maintaining weight. By paying attention to portion size and calories and following our guidelines, you can enjoy healthy, nutritionally balanced weight loss for a lifetime of good health.
So is the American Cancer Society, their web page emphasizes limiting red meat and eating at least 2 1/2 cups of fruit and vegetables daily.
A healthy diet will increase energy and vitality. This alone is a very good reason to adopt it, that it can help you lose weight is simply a by-product of the quality of the food.