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Is a Calorie a Calorie?

Nutrition and nutritional science is often controversial in the sense that there may be vast disagreements over individual aspects of it.   And perhaps one of the biggest is the question “Is a calorie a calorie?”   While this seems obvious at first glance, the simple question hides a bit more nuanced debate.

Is a Calorie a Calorie?

The basic debate over whether or not a calorie is a calorie comes down to the following question: is all that matters to the body calorie/energy balance (i.e. calories in versus calories out) or do the source of those calories matter.

How you answer that question sort of depends on what you are talking about.  Are we talking about weight loss?  Fat loss?  Health?  Fullness?  Something else?    This matters but few will define their terms ahead of time.  And this just leads to people arguing across one another, making points about different things.

Now in the simplest sense, a purely energetic one, a calorie is a calorie.  This is because a calorie is simply a measure of energy.  100 calories from an apply is identical to 100 calories from jelly beans because both are the identical amount of energy.

But does that mean that the apple and the jelly beans do the same thing in the body?  Would 100 calories of apple work differently than 100 calories of protein?    More importantly in the context of what I want to discuss today, does the composition of a fat loss diet impact on the amount of fat lost?

This tends to be the issue with various groups claiming that a specific dietary approach has some sort of metabolic advantage or that its composition somehow sidesteps energy balance.  That is, they argue that a calorie is not just a calorie. Not everyone agrees.

And as usual it comes down to which data set and what you’re looking at.  So let’s look at the different ways that this question tends to get parsed.

Studies with Varying Protein Intake

Most commonly, when folks want to argue that “a calorie is not a calorie”, they will use studies comparing higher and lower protein intakes, pointing out that the higher protein intakes yield superior results.  And there’s no question that this is absolutely true.   Protein requirements go up when someone is dieting and studies that provide sufficient protein always win out over those that don’t.

So at an identical calorie intake, the group that got sufficient protein will show more fat loss with less muscle mass loss than the lower protein group.  Other studies show that protein blunts hunger better (meaning it’s easier to reduce calories) than carbs or fats and a recent study showed better blood glucose maintenance in the diet containing higher protein. Aha, folks say, the source of the calories do matter!

And in this sense they aren’t wrong.   Sufficient protein always beats out insufficient protein and I doubt many or any would argue that point.  I certainly wouldn’t but I’ve been writing about the critical importance of protein intakes when dieting for over two decades.  Honestly, if people only get one thing about their diet correct, it should be “getting enough protein”.  Add resistance training to that and everything else is details.

Low-Carb Diet Superiority?

In this vein, I suspect that dieters who report vastly superior results from low-carb diets compared to higher-carb diets is related to change in protein intake.  In studies, when they compare subjects on self-selected low- or high-carb diets, the low-carb diets always end up eating more protein.

With the high reliance on animal proteins you can’t avoid this.  In contrast, on high-carb diets, many eat so many carbs that they short protein intake.  But while people like to claim that it’s the low-carb nature of the diet that is superior, that’s wrong.  It’s the HIGHER PROTEIN intakes driving the bus.

Back to Protein Intake

For that reason I sort of don’t care about this data set.  Yes, it’s true, if you compare insufficient protein to sufficient protein, sufficient protein always wins.  Well…duh.   When I’m looking at differences between diets, I assume a sufficient protein intake.

This can range from 0.8-1.5 g/lb lean body mass depending on the specifics.   Arguments about studies comparing the inadequate RDA/DRI to sufficient proteins don’t matter to me.

But given that built-in assumption, the question about whether a calorie is a calorie changes slightly.  Because the question now becomes: Given sufficient protein, does the source of the other calories (i.e. carbohydrates versus fat) affect anything or does it simply come down to calories in versus calories out.

To examine this, I want to look at two other data sets.

Studies Where Calories are Rigorously Controlled

The first set of studies, which tend to be in the minority are those studies where subject’s caloric intakes are strictly controlled. These are usually the studies that the “a calorie is a calorie” folks use to support their argument.

In these type of studies, subjects are typically kept in some sort of in-patient setting or a metabolic ward.  At the extremes, this may be done with hospital patients being fed with a feeding tube.  In less extreme cases, subjects are provided with all of the food they are supposed to eat but allowed to eat at home.  This last situation does raise the spectre of people eating additional food.

The benefit of these studies is that they are meticulously controlled.  Literally every calorie the individual is given is recorded and controlled while the exact macronutrient composition of the diet can be controlled to the highest degree.   This allows for certain factors to be set identically while others are varied.  Within the context of this article, that means studies where protein intakes in both groups are the same but carbohydrates and fats are being shifted around.

The drawback is that they are hellishly expensive to do.  Even short studies of a few weeks cost an absurd amount of money.  To control someone’s diet to this level for much longer, or for months, is usually unrealistic.

There is also the question of whether or not these studies have what is called ecological validity.  That is, do they accurately represent the real world.   For some populations they arguably do.  Athletes are often utterly meticulous with every calorie and nutrient that they consume and could realistically replicate this type of study.   In contrast, the average person on the street is unlikely to strictly control their calories with exactly macronutrient composition.

We might also question if what are invariably short-term studies, often only lasting a few weeks necessarily translate to longer time frames.  A small change that occurs over 1-2 weeks might (or might not) become a large change extended over 12 weeks or longer.

Irrespective of that, in general when these studies are done, matching protein and varying carbohydrate and fat intake, there is little to no difference in the overall results.   Studies have compare high- to low-carbohydrate diets or even varying percentages of carbohydrate.

And when protein intakes are fixed and adequate, there is rarely any significant difference in the results.  At most there might be a 1-2 lb difference in either direction but, without fail, 90%+ of the actual changes are due to the calorie deficit and nothing else.  The macronutrient composition simply doesn’t matter much.  Even metabolically there doesn’t seem to be much advantage of one diet over another.

Water Loss

I should mention that studies comparing high to low-carbohydrate diets typically show greater weight losses in the low-carbohydrate group but this can generally be attributed to greater water losses.  This is certainly true of the early studies suggesting that ketogenic diets had a metabolic advantage.

Done over as few as 4-7 days, they measured significant weight losses with very low carbohydrates compared to higher carbohydrate diets.  But this is a known effect of removing carbohydrates from the diet and represents nothing more than water weight loss which comes back immediately when carbohydrates are introduced.

Since water contains no calories, its gain or loss does nothing to change energy balance so this is ultimately irrelevant.    Certainly, arguing for the superiority of one diet over another based on a slight increase in water weight loss seems rather silly, even if I have seen people do exactly that.

Back to Energetic or Fat Loss Advantages

In one of the most well controlled (but admittedly short) studies, Kevin Hall compared a high carb very low-fat diet to a ketogenic (low-carb/high-fat) diet in terms of fat metabolism and energy expenditure.   The high-carb group showed better fat loss although the ketogenic diet group did show a small increase in energy expenditure of 100 cal/day.  However, this only lasted 10 days, a 1000 calorie total metabolic advantage.  This is irrelevant in the big picture.

Certainly there is no systematic advantage of any given diet.   There is always some noise in the equation and people get varying results even on the identical diet.  At that point you’re looking at the interaction of the diet with individual physiology.  And yes that is important as it’s completely conceivable that specific diets might better work for any given individual physiology to one degree or another.

At least some recent research suggests that the optimal diet interacts with insulin sensitivity.  Specifically it found that higher carbohydrate diets were superior for individuals with good insulin sensitivity/low insulin response and vice versa.  But this only suggests that the diet should be matched to the individual rather than supporting the de facto superiority of any given diet.

Differences in Macronutrient Sources

In some cases, different sources of macronutrients are compared.  So one diet might replace some of the complex carbohydrates with sucrose (table sugar).  Here there is either no effect of the sucrose group actually loses more weight.  Similar work has been done with fructose (fruit sugar) intake.  In at least one study, the group consuming more fructose (from fruit) lost more weight than the group consuming less.  Some will keep protein fixed but change the source of that protein t0 see what happens.

By and large the differences are fairly small approaching insignificant.  At least within the context of controlled calories, the deficit explains almost the entirety of the weight and fat loss, at least assuming that protein intake is sufficient to begin with and the calories are controlled.  You can shuffle carbs and fats around or look at different macronutrient sources and it all kind of doesn’t matter in the big picture.

Lean vs. Obese Individuals

Lean athletes and bodybuilders will point out that almost none of the research I’ve mentioned above is done on lean athletes.  Given the changes in physiology that occur with both training and lower body fat levels, it’s completely reasonable to assume that there might very well be differences between diets in terms of fat loss or muscle retained.

There is little to no research available here as, by and large, obesity researchers don’t bother to study lean individuals.  What’s the point?

So all we have here to rely on is anecdotes.  Now, I don’t like anecdotes because it’s too easy to cherry pick.   However, if there is a group that is likely to provide at least useful anecdotal data in this regard, it’s bodybuilders.  Because no other group is so obsessively anal retentive about tracking every nutrient going into their body.  If they tell you they shifted 10% carbs for 10% fat, you can usually believe them.

And factually we have seen that bodybuilders have gotten into contest shape on all sorts of diets.  In the 80’s they did it with high-carb diets, then they did it with low-carb diets.  Now I’d say the biggest change to contest dieting is more protein than was used previously.  And this tends to put people into a moderate carb/moderate fat intake by default. But they all worked.  And they all will work.  Because at the end of the day, the deficit and sufficient protein is what matters.

Even more anecdotally, people who have utilized different dietary approaches for cutting have tried the different combinations.  And for the most part, the differences in anything between macronutrient intakes were minimal approaching nil.

Occasionally someone might report a very slight few pound difference in fat loss or muscle loss in one direction or another.  This likely has to do with other variables, or simply the individual response to a specific diet.   But here this is more about individual response than a specific dietary approach being superior in an absolute sense.

But big picture, it’s all about sufficient protein, a consistent dietary deficit and being able to sustain the diet long enough.


However, in those studies, you generally see minimal (if any) differences in terms of the amount or composition of the weight lost when you vary the different nutrients.

But recall that the above is within the context of a metabolic ward where everything can be controlled strictly.  And outside of obsessive athletes, that doesn’t tend to represent the real world.  As the authors of one review on the topic put it:

Although “a calorie is a calorie” under the controlled conditions of a metabolic unit (i.e., only the level of calorie intake matters and not the source of calories), we conclude that these interrelationships are far more complex in the free-living situation.

The different diet-related factors that condition energy balance, including total energy intake, satiety and hunger sensory triggers, and palatability, must be considered when assessing the efficacy of weight-reducing diets of different macronutrient composition.



I want to mention that relatively fewer studies have been done comparing different sources of carbohdyrates or fat. There are studies looking at the impact of sucrose (table sugar) vs. starch within the context of strictly controlled caloric intakes and they usually show no difference in fat loss or anything else. That is, given an identical caloric intake, the source of the carbohydrates shows minimal differences. Similar studies have been done with dietary fat, typically showing similarly small differences. This is especially true when calories are restricted.

Unfortunately, overfeeding hasn’t been examined in as great a detail in humans. There are studies comparing overfeeding of fat to carbohydrates (in the form of glucose, sucrose, or fructose) and, over the long-term gain in bodyfat are pretty much identical. The mechanism of the fat gain is different but, when the same number of calories are overfed, the same amount of fat is gained. I’d note that these studies rarely if ever include activity and that has the potential to impact on where calories ‘go’ quite significantly. Calorie Partitioning is its own complicated topic.

Studies looking at overfeeding of medium chain triglycerides (MCTs) or some newly developed dietary fats (diglycerols) also show some benefits in terms of decreased fat gain but the difference is typically small as well.

It’s also conceivable that at the extremes of obesity, where all manners of metabolic problems are occurring, a difference might be seen for different macronutrient composition diets. Even there, studies where calories are rigorously controlled generally show little to no difference for varying macronutrient composition in terms of weight loss or body composition. I’d note again that recent research is suggesting an interaction with diet and insulin sensitivity; given that insulin sensitivity often accompanies obesity, differential results might be seen.

I should probably mention that, in studies of weight changes, there is quite frequently a large degree of variance in weight loss or weight gain given an identical number of calories. As it turns out, and I discuss in Metabolic Rate: Overview, this ends up being more an issue of individual metabolism and how it adapts than the diet itself. That is, some people’s metabolic rate goes up (or down) more in response to over (or under) feeding. There is no evidence that the composition of the diet affects this to any significant degree; rather it’s a genetically based metabolic effect.

Summing up this mini-section: for the most part, studies where protein is adequate (or at least close to it), varying carbs and fats within the context of an identical caloric intake tends to have a minimal overall effect. What effect is occasionally seen tends to be small and highly variable (some subjects do better with one diet than another but there’s no consistent advantage). With the possible exception of extreme conditions (folks looking for super-leanness or folks who are super-obese), caloric intake is the greater determinant of results than the macronutrient composition.


Studies Where Calories are Not Rigorously Controlled

In contrast to studies where dietary intake is rigorously controlled, are the ones where it is not.  In this case the study subjects are given a set of dietary guidelines (or even a popular diet book) and left to their own devices.  In some cases there may be nutritional counseling but it doesn’t change the free-living nature of the study.  Researchers attempt to induce control by utilizing food records but this are brutally inaccurate.

But it is in these studies that you will see certain dietary patterns showing slight superiority.   Specifically low-carbohydrate diets usually win out by some margin depending on the duration of the study at least in the short-term.  Mind you, in the long term, any benefits are usually lost and the differences in end weights tend to be very small.   In one review, while low-carb diets showed a 3.3 kg (7+ lbs) greater weight loss than high-carb diets, this had dropped to 1 kg (2.2 lbs) at the 12 month mark.  This is hardly worth pinning the superiority of a specific diet one.


As you might have guessed, these are generally the studies that the ‘a calorie is NOT a calorie’ folks refer to. In actuality, there are two different sets of studies in this group. The first is studies which are looking at nutrient intake on various diets. In such studies, subjects are simply given dietary guidelines (such as reduce fat to below 30% or reduce carbohydrates to 50 g/day or less) and intakes are examined.

Another data set of relevance to this discussion is studies comparing different diets (for example, recent studies have compared low-carbohydrate diets to the American Heart Association diet) under more real-world free-living conditions. Generally, in those studies, the subjects are given recommendations for the diet and let go. They typically report back to the researchers at some interval and frequently food intake is determined by means of self-reporting (which I’ve mentioned can be notoriously misleading). I want to look at each since both are illuminating to this discussion, as well as to how to choose a given diet.

The studies that look at average intakes given various recommendations are important because they often point to the real reason that a given diet works. For example, in studies where folks are told to reduce fat intake below 30% (or some other value), there is frequently an initial reduction in total caloric intake. That is, when they reduce dietary fat, their total energy intake generally goes down (at least initially). This is accompanied by weight loss. But this is not because of some magical effect of dietary fat, it’s simply because they are eating less calories. Of course, longer term studies show that most people end up compensating, eating more of other foods, so the result is pretty short lived.

Studies of low-carbohydrate diets tend to show similar results. Tell folks to reduce (or even remove) all of the carbohydrates from their diet and they tend to eat less automatically without thinking about it. Usually a lot less. What typically happens in such studies is that folks keep their protein and fat intakes roughly the same. So, by removing a food category that might make up 50% or more of total calories, they can’t help but eat less. Of course, this causes weight loss. But it’s not because of the carbs (or lack thereof) per se; rather it’s because they are eating less. There are other reasons, of course, such as decreased hunger (which not everyone experiences) and stable blood glucose that contribute to the reduction in calories but the removal of an entire food group is the main effect.

As I discuss in detail in Introduction to Dieting, many, many, many diet books rely on the rather simple prescription of ‘reduce or remove food X’ to lose weight. With X being something that contributes a lot of calories to the body, such as fat, sugars or highly refined carbohdyrates. But while such diet books typically use all kinds of pseudo-physiology to explain the effect, it’s really quite simple: if food X contributes a lot of calories to your diet and you remove food X, you’ll eat less total calories and lose weight. No magic, simple caloric restriction.

So let’s look at the second set of studies within the context. As I mentioned above, typically such studies look at the effect of different diets under free-living conditions. Similar to the results above, such studies frequently find that a given diet approach generates greater weight (or fat loss) but the effect is almost always due to differences in caloric intake.

For example, a study comparing a low-fat (but calorie uncontrolled) diet to a higher fat (calorie uncontrolled) diet will frequently see more weight/fat loss in the low-fat trial becuase the subjects ate less calories. The same goes for other comparisons. And while a few studies have shown drastically differential effects (such as greater weight loss at higher caloric intakes for a given type of diet), the methodology leaves a good bit to be desired. As I mentioned above, most use self-reporting of food intakes which tend to be notoriously inaccurate.

I should mention that, very frequently, the variance in weight loss tends to be humongous, as do reported caloric intakes. What this would tend to suggest is that, sometimes a certain diet type will reduce (or increase) food intake and sometimes it won’t. Individuals variance and food preferences can play a role as much as anything else.

Explaining the Discrepancy

So now, perhaps, we have a little bit better handle on why two totally different arguments about whether or not ‘a calorie is a calorie’ can come out of the resarch. The problem is that, most commonly, folks are referring to different data sets in making their argument.

As mentioned in the first section, there’s no doubt that studies comparing varying protein intakes almost always find better results with the higher protein intake. As you’ll see next chapter, all of the diets described in this book are based on adequate protein intake so these studies, have no relevance here. From the standpoint of anything I’ll ever write about or talk about, the real debate comes out of studies which keep protein constant and vary carbohydrates and fat and there are two data sets in this regards.

On the one hand are the studies were calories are rigorously controlled, where the subjects are provided their daily food intake. In those studies, differences in weight loss or body composition changes tend to be small and highly variable (some people do slightly better on one diet versus another but there’s no consistent pattern). This is the pattern I’ve observed in the real-world as well: some people do report slightly better results on one diet versus another but there’s no consistent superiority of a given approach.

Simply put, different diets are better under circumstances as discussed in Comparing the Diets.

On the other hand are studies examining spontaneous food intakes on various diets, typically examining a single diet such as low-fat or low-carbohydrate. Such studies frequently find that spontaneous food intake goes down or up given certain macronutrient intakes. For example, when fat intake is reduced below a certain point, claoric intake frequently goes down. The same occurs when carbohydrate intake goes below a certain point. Diets high in both fat (40% of total calories) and carbs frequently show higher spontaneous caloric intakes.

There is also a set of studies looking at changes comparing different diets to one another, using self-reported intakes to estimate caloric intake. While such studies frequently show differences in terms of weight loss, it’s generally related to caloric intake: if a given diet causes people to reduce calories more than another (through whatever mechanism), those people lose weight.

Is A Calorie a Calorie?

So is a calorie a calorie? Yes and no. Based on the data, my general feeling is this:

  1. A sufficient protein intake will always beat out an insufficient protein intake, no matter what you do. Since all of the diets described in this book are based around sufficient protein, this is a non-issue.
  2. Assuming caloric intake can be controlled (and protein is adequate of course), shuffling of carbs and fats tends to have a minor, approaching negligble effect.
  3. There might be exceptions at the extremes (folks going to single digit bodyfat or extreme obesity) but that doesn’t apply to the majority of folks.

In this respect, given adequate protein, it seems to matter very little what diet is chosen. From a weight or bodyfat standpoint, high carb should be as good as low-carb. Right? Well, no. The problem is that there’s a HUGE assumption built into statement #2 above: that calories can be controlled under a given set of conditions. As has been found repeatedly in the real-world, this simply isn’t a safe assumption.

Put a little bit differently, it might very well be possible to lose all the weight/fat you wanted on a calorie controlled junk-food diet with some high quality protein source. The problem that would probably arise is that most people wouldn’t be able to control their hunger or appetite on such a diet and they’d probably end up eating more in the long run. In eating more, they’d either lose less weight/fat or even gain it. Even if a given dietary approach appears optimal for some reason, if you can’t control your caloric intake, and end up eating more because of it, it won’t produce results.

Meaning this:
you’ll frequently see folks make comparisons along the lines of ‘well, it’s easier to eat 300 calories from food X than from food Y, therefore a calorie isn’t a calorie’. They may be generally correct but this criticism is tangential to the main issue. This is why I divided the data sets into studies where calories are controlled (usually in a highly artificial fashion) and where they are not (having more real world application).

It’s obviously easier to overconsume calories from jelly beans or candy than from vegetable just as it’s easier to eat 3000 calories from butter than from celery (no human alive could eat enough celery to get 3000 digestible calories). That matters hugely under conditions where folks are allowed to eat whatever they want. Quite in fact, many many diets are based around this simple fact: make people eat less of the foods that are easy to overconsume and/or make them eat lots of those foods that are tough to overeat and they will lose weight because they automatically reduce their caloric intake. I’ll discuss that topic more in the next chapter.

But that only applies to the situation where calories aren’t being monitored. When calories are being controlled rigidly, the source of calories (whether you’re comparing carbs to fat, or even different sources of carbs and fat) matters to a much smaller degree.

Once again, my point is that if calories are being strictly controlled, the source doesn’t appear to make a humongous difference in terms of body composition changes. As well, once you get protein intake to proper levels, fooling around with carbohydate and fat ratios (within the context of identical caloric intakes) don’t seem to make a huge amount of difference either. The bottom line still comes down to calories in versus calories out; it’s simply that it may be easier to affect calories in (food intake) or calories out (through activity) with different macronutrient breakdowns.

As well, the source of calories can affect other aspects of physiology beyond body composition. Health, energy levels, hunger/appetite and all the rest interact here. So while a calorie controlled diet of jelly beans, butter and protein powder might very well work to lose weight/fat, it probably wouldn’t be as healthy compared to a diet of low GI carbohydrates, healthier oils and lean protein sources.

Understand me here? Issues such as hunger control, long-term adherence, individual variance, athletic performance, and a few others all go into the determination of what food might or might not be a better choice under a given set of circumstances. So while a calorie might be more or less a calorie under somewhat artificial conditions (where calories are or can be strictly controlled), it’s a little more complex than that in the real world. Other issues interact. The next few chapters will adress those other issues.

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