This has without a doubt been the most hotly debated question in the world of nutrition for at least the last two decades, and many billions of dollars have been made by capitalizing on the ongoing controversy. And it’s interesting, because this question is also one of the few in the field, which is quite difficult to effectively study, for which an answer has been extensively proven.
There’s no point beating around the bush:
On a physiological level, yes, calories, and only calories in fact, are what ultimately matter for weight loss.
A quick high school science recap: in a theoretically closed system energy in must equal energy out because energy can’t be created or destroyed. Weight loss occurs when the human body burns its own tissue, usually stored fat, and the only way for the in equals out equation to remain true in that scenario is if there’s a deficit in required calorie intake which the burned energy stores have to compensate for.
This rather banal explanation has been shown to be true in tens of tightly controlled scientific studies, and there are hundreds more studies that corroborate it with varying degrees of directness. I’m going to do my best to summarize this research succinctly so that this post doesn’t completely go off the rails.
The most decisive way to see what does or doesn’t cause weight loss is what’s called a ward study – take two groups and feed one more calories than the other and see what happens, or feed them the same amount of calories and isolate some other variable like carbohydrates, fat, meal size, meal timing, etc. I’ve managed to come across about 40 such studies and the results are unanimous: if calorie intake is different, weight loss (or gain) is affected; or if calorie intake over time is the same, no other controlled variables influence weight change. The one quazi-exception is higher protein intake does slightly influence weight loss, but that’s explainable by the fact that protein is digested around 15-20% less efficiently than carbs or fat, which have roughly the same efficiency.
If you look for any studies that result in weight change when calories are controlled for, and many people have searched far and wide, you won’t find any.
This meta analysis by Hall et all and this page on Examine.com summarizes said research well. I’ll put my own list together here if I ever have it in me, but for now I don’t see much point in reinventing the wheel.
But what about all those studies where people ate less, and they thus should have lost weight, but didn’t? Or what about so and so who claimed they ate less on [insert fad diet] but didn’t lose weight?
Well, it’s notoriously difficult if not impossible to glean definitive conclusions from free living self reported studies – or personal anecdotes – because they don’t effectively control variables. We simply can’t know if self reported food intake is accurate or not, and research indeed shows that people tend to underestimate and/or underreport their food intake.
Now let’s talk about corroborating statistics. Pretty much everyone knows obesity has been on the rise in America for the last half century, and if the calorie hypothesis is true, we’d expect to see a correlating increase in calorie intake. And indeed there is:
This is a graph put together by Dr. Stephan Guyenet (source) who says “the [correlational] fit [between calorie intake and obesity rates over time] is extraordinary.” his longitudinal data is corroborated by an analysis published in the American Journal of Clinical Nutrition which concludes that “increased [overall calorie] supply is more than sufficient to explain the US epidemic of obesity.” And it isn’t all that surprising when you consider that, according to the USDA, the US food industry is producing around 4000 calories per person (source*, source), which is double the standard 2000 calorie/day allowance.
*See “nutrient availability” spreadsheet dated 2/1/15
Consider also that in the US occupational related calorie expenditure has decreased by around 15% since the 1960s, which is explainable by the fact that “sedentary” jobs have risen by ~30% while non sedentary “manual” jobs have concurrently fallen by ~30%. The fact that people are expending less calories at their jobs in conjunction with consuming more calories overtime makes the strongly correlating increase in obesity over time all the more ironclad.
There is actually an underdiscussed silver lining to all this that’s very liberating: though calories do matter for weight/fat loss, nothing else really does. Many people think that weight loss is prohibitively complicated or involved because they need to adhere to a rigid and extensive rule-set for diet and exercise. And it doesn’t help that all these espoused directives seem to constantly contradict one another! But as it turns out, weight loss is possible on pretty much any diet or exercise regimen so long as it creates a calorie deficit over time. And this was indeed shown in Stanford’s Comparative Weight Loss Study, where over the course of a year participants were able to lose weight on every of the popular diets they compared. That same study also showed participants failing to lose weight on every of those diets as well. Point being: it’s ultimately important that whatever system you use is something that you can realistically stick to, which per the title of my website will usually be the simplest and easiest one for you.