Look around the web for information on dieting and nutrition and you're likely to come across a lot of conflicting advice and even conflicting studies. In fact you'll probably stumble onto a fair few vicious debates as well, where strangers angrily defend their points of view with long lists of references and a few choice words. It's a battleground out there!
One of the biggest causes of disagreement is the question of whether a 'calorie is a calorie'. In other words, can you effectively control your weight and body composition simply by counting the number of calories going in and the number of calories going out? Or are there other important factors to consider here?
The Two Stances
If you ask many people how you should go about losing weight, they will recommend that you maintain a 'calorie deficit'. This simply means that you are burning off more calories than you are taking in in a single day. We lose a certain number of calories simply through breathing and pumping blood around our body (our 'basal metabolic rate') and we can increase this amount of calories by exercising. People like this strategy because it's simple, it's easy to understand and it certainly appears scientific.
On the other side of the fence are those who believe other important factors play a role. These include things like meal timing, the type of nutrients you are getting your calories from (carbs vs saturated fats for instance) and the thermic effect of different foods.
Unfortunately this latter camp gets something of a bad rep thanks to all the 'fad diets' that recommend unusual ways of burning calories. Things like intermittent fasting, the Atkins diet, carb backloading, the paleo diet etc. all focus on these sorts of techniques. Not all these diets are bad necessarily (there is a case to be made for all of them except Atkins), but they do tend to inspire some rather militant attitudes and extreme views. Subscribing to any 'set' diet is always going to be potentially problematic as it can cloud your judgement which only provides the 'calorie is a calorie' camp with more ammunition.
So those are the contenders… who wins?
Reasons a Calorie Might Not Be a Calorie
If you're being strictly literal here, then yes a calorie is a calorie – in much the same way that a donkey is a donkey. Specifically a calorie is 4,184 joules of energy which can be precisely measured for every piece of food.
And no one would argue that calories don't correlate with weight, or that they aren't directly responsible for weight gain.
But the issue is that your body doesn't treat all calories as equal. Apart from anything else, the calorie actually has to reach your bloodstream if it's going to stand any chance of getting turned into fat – and that's not always what happens with them.
Likewise if you find you are burning calories through the simple act of eating a food, then it's important to subtract those calories when you assess that piece of food's overall 'value'.
Thus there are a number of very valid arguments for considering not just the calories of a food, but also how the body uses those calories.
Some examples of things that might be worth considering include:
The Thermic Effect
That last point – about the body using calories of its own in order to digest and metabolise different foods is important. More specifically, the thermic effect of protein is about 25-30% compared with 2-3% for fat and 6-8% for carbs. That means that if you eat 100 calories of protein, this will only 'count' as 75 calories, whereas 100 calories of fat will count as 98%.
The reason for this is that different foods go through different metabolic pathways and are processed differently. The process for protein is less 'efficient' compared with carbs and fats, which means your body actually heats up during it and that's where the calories are lost. Here's the study.
This paper discusses the laws of thermodynamics and shows how dissipation ensures that some energy is lost as far as your body is concerned whenever you eat any kind of calorie.
Whole grains also have more of a thermic effect, which is why in one study it was found that eating a sandwich made from whole grains and cheddar burned twice the number of calories as one made from processed cheese and refined grains (here's the link).
Calorie Restriction and Metabolism
Another important point to consider is that eating less will slow down your metabolism. Some people try to refute this, but while you probably won't enter a 'starvation mode' you're still going to see a reduction in metabolism that means you subsequently burn fewer calories. Sometimes it's worth taking in a few calories just to keep your metabolism up and it's all part of a more fully thought out weight loss 'strategy'.
Different foods affect hunger differently and if you want to be smart about the way you're restricting calories, then you need to ensure that you will feel full for longer periods of time. Fat takes longer to digest as compared with bread, as do 'slow carbs' like sweet potatoes. This means you feel fuller for longer.
Additionally, other changes to your diet can also impact on hunger. For example, fructose has been shown in studies to increase ghrelin – our hunger hormone – more than glucose.
Protein has also been shown to reduce snacking. This study shows that simply asking someone to eat more protein will immediately lead to them eating fewer calories from other sources.
Carbohydrates and especially 'fast' carbohydrates like bread and cake are more damaging when it comes to gaining weight than fats, even though they contain fewer calories.
How is this possible? It's because it takes longer for the body to utilise the sugar available in fats compared with the carbohydrates. This means that carbohydrates create more of an insulin spike, which in turn means more energy gets stored as fat (insulin signals this process in the body). If you can avoid the insulin spike, then you can provide your body with a steady supply of glucose instead.
Here is a systematic review of 13 studies comparing low carbohydrate and low fat diets. Here is another one. This study compares a low fat diet with a low carbohydrate diet in a randomised trial. Another one.
Some fans of the 'calorie is a calorie' approach will argue that this is a moot point seeing as most meals don't consist of just fats, proteins or carbohydrates. The point is though that if you want to lose weight you should be avoiding the fast carbs during some meals. This can be as easy as having a chicken salad for lunch, or choosing pumpernickel bread over regular bread.
Different Calories Trigger Different Hormone Responses
When you eat protein this triggers a release of IGF1 and testosterone which in turn puts you in a more anabolic state leading to more muscle building. When you eat carbs this triggers the release of insulin leading to more fat storage and suppressing anabolic hormones.
This paper demonstrates that postprandial thermogenesis increases 100% in high protein diets.
Timing Has an Effect
When you sleep, your body is an anabolic state meaning that it is more likely to burn fat and build muscle. Bodybuilders have known this for a long time which is why they will often consume a 'slow release' casein protein shake just before they hit the sack.
Similarly timing is important when you consider that sugar will be used to restock your muscle glucose after a workout, making snacks less damaging for weight loss, or that we are in our most 'fat burning' mode early in the morning which can be preserved if you want to burn more calories.
So from all of these studies what conclusion can we draw?
Well, what you have to remember on the one hand is that all these effects are relatively minor. You're not going to completely transform your body overnight by eating more protein and doing so won't enable you to then gorge on cake. But that said, there clearly are other things at play when it comes to weight management, and if you combine all these effects then they might add up to quite a notable difference.
The fact is that maintaining a caloric deficit is the very best way to ensure you lose weight. But if you can further enhance that effect by eating a little more protein, by using whole grains for breakfast, or by choosing saturated fats over carbs… then surely that's a good thing!
What Is the Best Practical Advice?
Despite all this mounting evidence that a calories is not just a calorie, many people will still refuse to believe that any method other than maintaining a caloric deficit could be useful. One of the arguments you'll still commonly hear is that telling a beginner anything other than to eat fewer carbs is only going to lead to confusion and failure.
My main issue with this is that it's condescending and severely underestimating the intelligence of… well everyone. More to the point though, it underestimates just how hard and joyless it is to count calories which many people also fail at for those reasons. Surely if you could use a few smart 'tricks' to get more from your metabolism and see quicker results this would be a useful thing for a beginner?
It also forgets that it's actually impossible to be all precise when counting calories. BMR changes from day to day, not every apple contains the exact same number of calories (or even every beer), you lose calories when you shiver in the shower or bob your leg… point is, your best estimates are going to be inaccurate at best. And probably just flat out wrong.
There's also a danger if you tell someone to eat 'anything' as long as they stick within a certain number of calories, they could end up eating nothing but sugary sweets and not getting any of their essential micronutrients. And when you're restricting calories, there's a good chance that you'll want to do just that and continue to eat just the things you enjoy the most.
Even if you still only care about calories (how stubborn are you??) then you can look simply to the studies demonstrating that eating fewer carbs leads automatically to fewer calories – moreso than restricting any other macronutrient. Here is the review looking at over 20 randomized trials.
This is especially true when you focus on refined carbohydrates that have a high GI (meaning they release sugar very quickly). In one study participants were given milkshakes that differed only in their GI, the high GI group were hungrier and had more cravings shortly after.
So in other words, the logical conclusion is that the 'a calorie is a calorie' stance is neither accurate nor practical advice for beginners. Sure 'eating less' is a good piece of advice, but the very best diet will be one that takes into account multiple factors including timing, hunger, lifestyle, macros and more. And it will be one that is designed to work specifically for the individual using it.