When people hear term Unified Theory, some times called Grand Unified Theory, or even "Theory of Everything," they probably think of it in terms of physics, where a Unified Theory, or single theory capable of defining nature of interrelationships among nuclear, electromagnetic, and gravitational forces, would reconcile seemingly incompatible aspects of various field theories to create a single comprehensive set of equations.

Such a theory could potentially unlock all secrets of nature and universe itself, or as theoretical physicist Michio Katu, puts it "an equation an inch long that would allow us to read mind of God." That's how important unified theories can be. However, unified theories don't have to deal with such heady topics as physics or nature of universe itself, but can be applied to far more mundane topics, in this case nutrition.

Regardless of topic, a unified theory, as sated above, seeks to explain seemingly incompatible aspects of various theories. In this article I attempt to unify seemingly incompatible or opposing views regarding nutrition, namely, what is probably longest running debate in nutritional sciences: calories vs. macro nutrients.

One school, I would say 'old school' of nutrition, maintains weight loss or weight gain is all about calories, and "a calorie is a calorie," no matter source (e.g., carbs, fats, or proteins). They base their position on various lines of evidence to come to that conclusion.

The other school, I would call more 'new school' of thought on issue, would state that gaining or losing weight is really about where calories come from (e.g., carbs, fats, and proteins), and that dictates weight loss or weight gain. Meaning, they feel, "calorie is a calorie" mantra of old school is wrong. They too come to this conclusion using various lines of evidence.

This has been an ongoing debate between people in field of nutrition, biology, physiology, and many other disciplines, for decades. The result of which has led to conflicting advice and a great deal of confusion by general public, not to mention many medical professionals and other groups.

Before I go any further, two key points that are essential to understand about any unified theory:

A good unified theory is simple, concise, and understandable even to lay people. However, underneath, or behind that theory, is often a great deal of information that can take up many volumes of books. So, for me to outline all information I have used to come to these conclusions, would take a large book, if not several and is far beyond scope of this article. A unified theory is often proposed by some theorist before it can even be proven or fully supported by physical evidence. Over time, different lines of evidence, whether it be mathematical, physical, etc., supports theory and thus solidifies that theory as being correct, or continued lines of evidence shows theory needs to be revised or is simply incorrect. I feel there is now more than enough evidence at this point to give a unified theory of nutrition and continuing lines of evidence will continue (with some possible revisions) to solidify theory as fact. "A calorie is a calorie"

The old school of nutrition, which often includes most nutritionists, is a calorie is a calorie when it comes to gaining or losing weight. That weight loss or weight gain is strictly a matter of "calories in, calories out." Translated, if you "burn" more calories than you take in, you will lose weight regardless of calorie source and if you eat more calories than you burn off each day, you will gain weight, regardless of calorie source.

This long held and accepted view of nutrition is based on fact that protein and carbs contain approx 4 calories per gram and fat approximately 9 calories per gram and source of those calories matters not. They base this on many studies that finds if one reduces calories by X number each day, weight loss is result and so it goes if you add X number of calories above what you use each day for gaining weight.

However, "calories in calories out" mantra fails to take into account modern research that finds that fats, carbs, and proteins have very different effects on metabolism via countless pathways, such as their effects on hormones (e.g., insulin, leptin, glucagon, etc), effects on hunger and appetite, thermic effects (heat production), effects on uncoupling proteins (UCPs), and 1000 other effects that could be mentioned.

Even worse, this school of thought fails to take into account fact that even within a macro nutrient, they too can have different effects on metabolism. This school of thought ignores ever mounting volume of studies that have found diets with different macro nutrient ratios with identical calorie intakes have different effects on body composition, cholesterol levels, oxidative stress, etc.

Translated, not only is mantra "a calorie us a calorie" proven to be false, "all fats are created equal" or "protein is protein" is also incorrect. For example, we no know different fats (e.g. fish oils vs. saturated fats) have vastly different effects on metabolism and health in general, as we now know different carbohydrates have their own effects (e.g. high GI vs. low GI), as we know different proteins can have unique effects.

The "calories don't matter" school of thought

This school of thought will typically tell you that if you eat large amounts of some particular macro nutrient in their magic ratios, calories don't matter. For example, followers of ketogenic style diets that consist of high fat intakes and very low carbohydrate intakes (i.e., Atkins, etc.) often maintain calories don't matter in such a diet.

Others maintain if you eat very high protein intakes with very low fat and carbohydrate intakes, calories don't matter. Like old school, this school fails to take into account effects such diets have on various pathways and ignore simple realities of human physiology, not to mention laws of thermodynamics!

The reality is, although it's clear different macro nutrients in different amounts and ratios have different effects on weight loss, fat loss, and other metabolic effects, calories do matter. They always have and they always will. The data, and real world experience of millions of dieters, is quite clear on that reality.

The truth behind such diets is that they are often quite good at suppressing appetite and thus person simply ends up eating fewer calories and losing weight. Also, weight loss from such diets is often from water vs. fat, at least in first few weeks. That's not to say people can't experience meaningful weight loss with some of these diets, but effect comes from a reduction in calories vs. any magical effects often claimed by proponents of such diets.