This is where we get into crux of true debate and why two schools of thought are not actually as far apart from one another as they appear to untrained eye. What has become abundantly clear from studies performed and real world evidence is that to lose weight we need to use more calories than we take in (via reducing calorie intake and or increasing exercise), but we know different diets have different effects on metabolism, appetite, body composition, and other physiological variables...

Brink's Unified Theory of Nutrition

...Thus, this reality has led me to Brink's Unified Theory of Nutrition which states:

"Total calories dictates how much weight a person gains or loses; macro nutrient ratios dictates what a person gains or loses"

This seemingly simple statement allows people to understand differences between two schools of thought. For example, studies often find that two groups of people put on same calorie intakes but very different ratios of carbs, fats, and proteins will lose different amounts of bodyfat and or lean body mass (i.e., muscle, bone, etc.).

Some studies find for example people on a higher protein lower carb diet lose approximately same amount of weight as another group on a high carb lower protein diet, but group on higher protein diet lost more actual fat and less lean body mass (muscle). Or, some studies using same calorie intakes but different macro nutrient intakes often find higher protein diet may lose less actual weight than higher carb lower protein diets, but actual fat loss is higher in higher protein low carb diets. This effect has also been seen in some studies that compared high fat/low carb vs. high carb/low fat diets. The effect is usually amplified if exercise is involved as one might expect.

Of course these effects are not found universally in all studies that examine issue, but bulk of data is clear: diets containing different macro nutrient ratios do have different effects on human physiology even when calorie intakes are identical (1,2,3,4,5,6,7,8,9,10,11).

Or, as authors of one recent study that looked at issue concluded:

"Diets with identical energy contents can have different effects on leptin concentrations, energy expenditure, voluntary food intake, and nitrogen balance, suggesting that physiologic adaptations to energy restriction can be modified by dietary composition."(12)

The point being, there are many studies confirming that actual ratio of carbs, fats, and proteins in a given diet can effect what is actually lost (i.e., fat, muscle, bone, and water) and that total calories has greatest effect on how much total weight is lost. Are you starting to see how my unified theory of nutrition combines "calorie is a calorie" school with "calories don't matter" school to help people make decisions about nutrition?

Knowing this, it becomes much easier for people to understand seemingly conflicting diet and nutrition advice out there (of course this does not account for down right unscientific and dangerous nutrition advice people are subjected to via bad books, TV, 'net, and well meaning friends, but that's another article altogether).

Knowing above information and keeping Unified Theory of Nutrition in mind, leads us to some important and potentially useful conclusions:

An optimal diet designed to make a person lose fat and retain as much LBM as possible is not same as a diet simply designed to lose weight. A nutrition program designed to create fat loss is not simply a reduced calorie version of a nutrition program designed to gain weight, and visa versa. Diets need to be designed with fat loss, NOT just weight loss, as goal, but total calories can't be ignored. This is why diets I design for people-or write about-for gaining or losing weight are not simply higher or lower calorie versions of same diet. In short: diets plans I design for gaining LBM start with total calories and build macro nutrient ratios into number of calories required. However, diets designed for fat loss (vs. weight loss!) start with correct macro nutrient ratios that depend on variables such as amount of LBM person carries vs. bodyfat percent , activity levels, etc., and figure out calories based on proper macro nutrient ratios to achieve fat loss with a minimum loss of LBM. The actual ratio of macro nutrients can be quite different for both diets and even for individuals. Diets that give same macro nutrient ratio to all people (e.g., 40/30/30, or 70,30,10, etc.) regardless of total calories, goals, activity levels, etc., will always be less than optimal. Optimal macro nutrient ratios can change with total calories and other variables. Perhaps most important, unified theory explains why focus on weight loss vs. fat loss by vast majority of people, including most medical professionals, and media, will always fail in long run to deliver results people want. Finally, Universal Theory makes it clear that optimal diet for losing fat, or gaining muscle, or what ever goal, must account not only for total calories, but macro nutrient ratios that optimize metabolic effects and answer questions: what effects will this diet have on appetite? What effects will this diet have on metabolic rate? What effects will this diet have on my lean body mass (LBM)? What effects will this diet have on hormones; both hormones that may improve or impede my goals? What effects will this diet have on (fill in blank)?

Simply asking, "how much weight will I lose?" is wrong question which will lead to wrong answer. To get optimal effects from your next diet, whether looking to gain weight or lose it, you must ask right questions to get meaningful answers.

Asking right questions will also help you avoid pitfalls of unscientific poorly thought out diets which make promises they can't keep and go against what we know about human physiology and very laws of physics!

There are of course many additional questions that can be asked and points that can be raised as it applies to above, but those are some of key issues that come to mind. Bottom line here is, if diet you are following to either gain or loss weight does not address those issues and or questions, then you can count on being among millions of disappointed people who don't receive optimal results they had hoped for and have made yet another nutrition "guru" laugh all way to bank at your expense.

Any diet that claims calories don't matter, forget it. Any diet that tells you they have a magic ratio of foods, ignore it. Any diet that tells you any one food source is evil, it's a scam. Any diet that tells you it will work for all people all time no matter circumstances, throw it out or give it to someone you don't like!