Your bodyweight depends on your total caloric intake more than on your macronutrient ratios (how many of your calories come from carbohydrates, proteins, fats, and Alcohol). Increased caloric intake as an independent variable is more than sufficient to explain the current obesity epidemic, without the need to find a scapegoat, such as high-fructose corn syrup.
A trial in a controlled setting (a metabolic ward) compared several isocaloric diets composed of 15% protein, 15-85% carbohydrate, and 0-70% fat. It concluded that caloric restriction, not macronutrient ratios, determined weight loss. Comparing low- and high-carbohydrate diets over 6 weeks and 12 weeks led to the same conclusion, as did comparing a low-fat/high-protein diet with a high-fat/standard-protein diet.
Another trial in a metabolic ward noted that, in healthy individuals overeating for 8 weeks, caloric intake alone accounted for the increase in body fat. However, caloric expenditure, total weight, and lean mass increased with protein as a percentage of caloric intake. In contrast, a previous study on the impact of protein on weight loss had noted that women lost as much weight on a high-protein diet as on a high-carb diet, but that subjects with high triglycerides lost more fat on the high-protein diet.
In people suffering from hyperinsulinemia, insulin resistance, or type-2 diabetes, the results are mostly the same: Caloric restriction, not macronutrient ratios, leads to weight loss. Two studies noted, however, that lean mass was better preserved in women (but not men) on a high-protein diet, and one study did find a greater weight loss (nearly entirely from fat) in the high-protein group (men and women).
Independent of your diet’s macronutrient ratios, a negative energy balance (consuming fewer calories than your body needs) is responsible for weight loss.
Many diets, fad or not, do work. This is mainly because they reduce calories.
Several diets restrict your carbohydrate intake. The ketogenic diet is very high in fats, low in proteins, and very low in carbohydrates. The Atkins diet is high in fats, high in proteins, and very low in carbohydrates. The “paleo diet” (hunter-gatherer diet) is high in fats, high in proteins, and low in carbohydrates.
Fats and proteins digest more slowly than carbohydrates, so are more satiating. In addition, most diets (including the three already mentioned) recommend the consumption of foods that are less calorie-dense (more fibers and a higher water content: a pound of broccoli packs less calories than a pound of grains). Finally, carbohydrates participate in the synthesis of serotonin, which can cause cravings in some obese individuals.
Therefore, people on a low-carb diets lose weight because they naturally eat less and avoid the large binges caused by carbohydrate cravings. People on very-low-carb diets can also lose weight very quickly on the short term because the depletion of their glycogen stores leads to the excretion of bound water. That explains why two trials found that people on a low-carb diet had lost more weight than people on a low-fat diet after 6 months but not 12.
Consuming your macronutrients together (balanced diet) or separately (dissociated diet, also known as “foot combining”) makes no difference with regard to weight or fat loss.
Since prolonged fasting might increase heat expenditure, diets that manipulate fasting (Intermittent Fasting, Alternate Day Fasting) may have some benefits on the “calories out” side of things. Yet, even here, weight lost is mostly due to the fact that you control eating: It is much harder to overeat in 8 hours than in 16.