Is a calorie a calorie?

I read a lot of stuff in the popular press about obesity and weight gain and some of it can make me cringe.  However, here is a piece by Mark Bittman, the New York Times food columnist, that I can agree with.  One of the vexing things about this field is that a little bit of knowledge is a very dangerous thing.  When I talk to people about my research, I get various responses.  For those that have never thought about the problem, when I tell them that the obesity epidemic is caused by too much food they, will simply say  “You needed to do five years of research to figure that out?”  However, for those that have thought about it some, I can get more skepticism:  “What about insulin?  What about lack of physical activity?”  What about perturbed microflora?”  “What about fibre?”  “What about high fructose corn syrup?”   The answer is that all those things may matter but they’re second order.  The data show that the increase in food supply more than explains the increase in weight holding everything else constant.  That means if you include these other effects, it would have been even worse!

Now to the question of a “Is a calorie a calorie?”  This causes a lot of confusion because some people believe that low carb diets will lead to weight loss independent of the reduction in calories.  This might be true but the reasoning is often incorrect.  The wrong answer is that carbs cause insulin to be secreted and insulin suppresses the release of fat (i.e. lipolysis).  Your body stores fat in fat cells or adipocytes.  When insulin is low, adipocytes release fat into the blood stream for you to use as fuel.  When insulin is high, lipolysis is suppressed so the body can burn the carbs or more specifically glucose.  The faulty reasoning then goes that if you eat lots of carbs, you’ll suppress lipolysis so that your fat stays trapped in your adipocytes.  It’s only if you eat low carbs will the fat be free to escape.  On the surface this sounds fairly reasonable.  Where it goes wrong is that if you are in steady state then at the end of the day, it didn’t matter what happened to your insulin levels because your body is the same as it started.

Let me be more specific.  Let’s consider a person in steady state, but eating two different diets.  In the first, she eats carbs all day.  Insulin will be high, lipolysis will be suppressed and she will burn all the carbs she ate.  At the end of the day if she ate 2000 Calories of carbs, she’ll have burned all 2000 of it (ignoring for now the small inefficiencies that won’t change the argument).  Now, let’s say she eats very low carbs for a day.  In this case, insulin stays low all day so she burns fat all day.   However, you can’t ignore the fact that she also ate 2000 Calories of food.  If it was all fat, then what she did was to burn 2000 Calories of fat that was some combination of the fat she ate and the fat that was released from the fat cells.  The leftover fat was then repackaged into the fat cells and at the end of the day she stays the same.  (If she ate no carbs at all, then she would have depleted her small stores of glycogen and then her brain would have started using ketones as fuel but that is another story).

This doesn’t mean that low carb diets won’t work.   However, if  low carbs do cause you to lose weight, then the only reason they work was because you either a) burn more energy or b) eat less.  It has been argued that the drop in insulin after you eat carbs makes you hungry and can cause you to eat too much.  This might be true but there is no solid evidence for it as far as I know.  It is also plausible that eating low carbs could cause you to use more energy.  For one, if you ate lots of protein, it could take the body more energy to process it although there is no clear evidence yet.  If you eat lots of fat, there will be excess futile cycling of fat between the blood stream and fat cells and this could cause excess energy to be used but again there is no evidence yet.  Kevin Hall is actually doing some precise experiments to address these questions and will publish some results soon.

Finally, let’s get back to “Is a calorie a calorie?”.   The answer, as Bittman writes, is that it is probably not since the body does process foods differently but we don’t yet know how much it will matter.  The question of how much energy we burn on different diets could be answered shortly with ongoing experiments.  However, the question of how different diets affect appetite is a question we won’t have an answer to in the near future.