As much as it pains me to say this, I find myself agreeing in part with Taco. My only saving grace here is that I'm only partially agreeing with him .
This is a very interesting thread because it cuts to the core of the body composition debate, such as it is. Further, the thread touches on almost all of the fundamental misconceptions regarding diet out there that have made this process so confusing to so many people.
Calories Count, Always Have, Always Will
Calories are simply a unit of measure for energy, so all calories are equivalent by definition. To deny or debate this is tantamount to not understanding the children's riddle : "Which is heavier? A ton of feathers or a ton of bricks?" If you don't understand the riddle, find a kid and have them explain it to you.
The problem with calories is that nobody eats calories, we all eat nutrients, and then we metabolize them. The amount of energy that you extract from a steak may be markedly different from the amount of energy that I would derive from it, assuming of course that we could somehow get exactly identical steaks on our plates. Incidentally, you would also have to force us both to chew the steak an identical number of times to ensure that the surface area presented to digestive enzymes were identical. And while we're on the subject of digestive enzymes, you'd better find a way to ensure that we both secrete identical amounts of hydrochloric acid, among other things. It would also be very helpful if you could somehow control the amount of time that steak took to transit each of our respective digestive tracts to ensure that it was exposed to equivalent digestion and absorption processes.
Unless you are some minor deity, you have no hope of controlling the myriad of variables that affect how any two individuals process nutrients. This means that in the end, it is virtually impossible to ascertain the caloric content of nutrients in isolation because we need to consider every nutrient within the context of a specific individual's metabolism. We can generalize, but specific instances need to be taken on their own merits. You are not what you eat ... you are what you assimilate.
All is not lost, however. If you were willing to subject yourself to living in a metabolic ward, we could precisely measure your caloric expenditure and substrate utilization using calorimetry. We could then pretty much arbitrarily modify your body weight and composition by tightly controlling the calories we administered to you, more or less independent of the nutrient composition of your diet, with the proviso that we didn't induce any deficiencies.
I put the blame squarely on Gary Taubes for this needless debate, as he's actually put forward the notion that he doesn't believe that thermodynamics applies to biological systems. While for the most part I respect the man and his writings, this is one area where the wheels fall off his train of thought. What Taubes is probably trying to say is that equilibrium thermodynamics don't apply to biological systems. And if that is indeed what he is meaning to say, then I agree with him. Biological systems are not systems at equilibrium, far from it, they actually fall under a relatively new area of thermodynamics that is being actively explored and elaborated : dissipative systems. This is a fascinating area of study, but it is far too involved to get into it in a forum post! If this sort of thing is your cup of tea, you should definitely consider doing some readings in this area. Two particularly good books on the topic: Biological Thermodynamics, which is the more technical read, and Into the Cool: Energy Flow, Thermodynamics, and Life which is more suitable for casual reading. In the interest of full disclosure, those are Amazon affiliate links, so if you do wind up buying a book, I get a nominal amount to spend my on my coffee.
Excess Carbohydrates Are Not Preferentially Converted to Fat ( De Novo Lipogenesis - DNL )
Excess carbohydrates _can_ participate in DNL, but you would have to eat significantly in excess of your caloric requirements in order for this to happen. Rather than being stored as adipose tissue, excess carbohydrates go into hepatic glycogen storage. Once the liver's capacity to store glycogen has been exceeded, then DNL will occur. Your liver has abundant excess capacity for absorbing glucose, something on the order of an excess intake of 500g before starting to synthesize additional fatty acids:
Practically speaking, however, the above doesn't do you too much good, since the resulting "high carbohydrate-oxidation rates" displaces oxidation of other energy substrates, such as protein ( yayiii! ) and lipids ( boo!! ). Since you are left with greater levels of circulating lipids, this gets you increased lipid re-esterification ( i.e. your adipose tissue reabsorbs the fatty acids that it previously released, so net net, you "gain" fat ).
Originally Posted by Am JClin Nutr 1988;48:240-7.
This same principle, substrate availability, works for all macro nutrients. If you have high levels of circulating amino acids, for example, guess what your metabolism will oxidize? In oxidizing more protein, however, the body will be oxidizing less of the other substrates, which means that again, there will be more circulating fatty acids to be re-esterified in your adipose tissues.
The exact same thing happens with high levels of exogenously sourced fatty acids. I don't care how ketogenic your diet may be, if you are eating 5000 kcal. of fat a day, you will gain adipose tissue mass. Period.
All Diets Eventually Culminate at an Equilbrium, Even If They Kill You
The body has the capacity to convert mass into energy, and energy into mass. In situations of energy surplus, two things broadly happen : 1) surplus energy is converted to mass, and 2) energy expenditure increases. The converse is true in cases of deficits, surplus mass is converted to energy, and energy expenditure is curtailed. Neither of these situation can be carried on indefinitely. Chronic energy shortages culminate in the organism starving to death, whereas chronic surpluses get you metabolic syndrome. At both ends of the spectrum, one can be arguably said to have achieved weight stability. Between these two extremes lie an infinity of non-pathological equilibria that reflect a particular organism's energy context at a given point in time. The only question is whether you will be satisfied with what you see in the mirror at the particular equilibrium point that you achieve on any given dietary intervention.
If you were to take a sample of individuals with type II non insulin-dependent diabetes, you would find that in general, they are weight stable. They have rather poor body composition metrics, even poorer metabolic health metrics, but they have achieved a weight gain plateau. In fact, full blown metabolic syndrome develops precisely around the point where they achieve the weight equilibrium, because that is precisely the point when the body stops responding to insulin, and fat accretes at the same rate as it leaks out of adipose tissue. Net net, zero change in weight, but a metabolic train wreck.
On the flipside of this, when you are losing weight, your metabolism is actively changing to accommodate its energy context reality, and since there is a shortfall, a number of things happen. As I've already mentioned, first and foremost, the response to an acute energy shortfall is for the body turn to converting mass into energy ... this is what weight loss means. Generally, the body converts fat to energy, that's precisely what it's there for, but it will also utilize expendable protein stores eventually. Incidentally, the way you indicate to your body which protein stores are not expendable is to use them ( i.e. work the muscles you intend to keep ), which is why you find so many skinny-fat people out there who have tried to achieve a "beach body" via diet alone.
If the energy shortfall is chronic, then the secondary metabolic response is to attempt to utilize less energy overall. This entails an endocrinological response, and the principal actor there is a reduction in T3 levels, which in turn down regulates many catabolic processes in the body, most notably thermogenesis. Eventually, your metabolism either succeeds in curtailing energy use, at which point you are again weight stable, or it does not, and you are on an E ticket ride to starvation city.
Ok, enough for now. It's getting late, I'm sleep deprived, and I find myself ranting ...