The hottest argument in nutrition today:
“a calorie is a calorie”.
Of course, this is completely true.
Of course, this is completely not true.
Well, which is it?
It is an insolvable conundrum because it is a sentence fragment.
Whenever this statement is made, the meaning includes words not said.
Meaning One: ”A calorie is a measurement of the energy content of food and all the total energy content of food contributes to whether a person is in a position to burn stored energy or gain stored energy or stay stable that day.” This is often re-phrased as “there are no magic types of food you can eat where the calories in that food will not count equally with other types of calories from other types of food”. To truly get the meaning of that phrase, you would need to imagine hearing it said with a disdainful, contemptuous tone.
Of note, when this is said, all players understand that:
- in this context, “calories” is a defined term that specifically refers to food calories. Food calories are agreed by all players to be derived from those molecules in food that the body is able to digest from food, absorb into the body and use in the bodies metabolic pathways to derive energy-containing molecules. The four types of molecules that fit that description for humans are fats, proteins, the digestible carbohydrates and alcohol.
- the casual use of the term “calories” is potentially mis-leading to non-insiders because it is another fragment. It is a lazy way of saying what we are really talking about, which is food calories, which, by the way, are actually kilocalories – in other words, 1,000 times more energy than the “calories” referred to in physics. You may have seen the use of the term “Kcal”.
- all players agree and understand that this phrase “a calorie is a calorie” neglects to include a reference to the known, agreed and understood fact that there is some difference in the energy cost of digestion/metabolism of fats, digestible carbohydrates and proteins. The amount of this difference is not significant enough in the over-all picture to be worth constantly mentioning as that would just interfere with discussion of the core principle.
- all players agree that, at base, what we are discussing is always in the context of a diet that is adequate in the essential nutrients – suitable for use by humans at least for some weeks or months.
Meaning Two: ”a calorie of one type or from different food sources has the same energy content, but, varying from one person to another, has a different impact on metabolism, with a different impact on that person’s propensity to gain or lose body fat stores.”
For example, there are a number of studies that suggest that a relatively higher intake of protein results in better satiety (satisfaction of appetite, in this case for a given calorie intake) than a relatively low but still nutritionally adequate protein intake.
There is evidence that liquid calories, usually taken to mean fruit juice, sodas, milkshakes, etc. are not properly “noticed” by the appetite control network. Since they seem to be not fully tallied by the body, the person eats about as much other food in the day whether they had the extra calories in the liquid drink or not.
And how often have you heard the advice that you should choose foods with slower-digesting carbohydrates than fast-digesting carbohydrates? For example, choose lentils instead of white bread. The best attempt at properly determining what are the slower and faster digesting carbohydrate foods resulted in the list we call the glycemic index. This list is useful, but has a number of flaws.**
Strangely, in my experience it is never these examples of calorie-containing molecules of different types (e.g. protein) or of different food sources (e.g. liquid calories) or that reach the blood stream in different patterns (e.g. fast versus slow carbs, also breakfast versus no breakfast) that make the “a calorie-is-a-calorie” people fume, turn purple and foam at the mouth. I haven’t encountered any of them that would dispute these concepts – which all directly contradict “a-calorie-is-a-calorie” in the “meaning one” sense.
No, the “a calorie-is-a-calorie” rant is pretty much reserved for just one topic, low-carbohydrate eating.
At the root of this is the fact that, generally, low-carb eating has been presented as a plan that lets you “eat as much as you like” and still lose weight. This makes it sound like a false offering of something magic. In reality, a person who eats very low carb will go into nutritional ketosis (have some ketones in the blood, at this level a physiologically normal and safe state for someone who has not become unable to make insulin) and have their appetite suppressed. They may eat more in the initial transition, but soon the appetite will diminish, usually but not always to the point where less calories are taken in and body fat stores are burned.
(Some individuals report experiences that they feel suggests they are more resistant to weight gain during over-eating when eating low-carb. This is interesting but has not been studied and, no matter if true for some, would not be predictable for any given person. It has been documented for many decades that there is great variability in individual resistance to weight gain with over-eating. The people who have the best metabolism to burn off extra food don’t care about the whole debate – pretty much they are slim!)
Helping the reduction in appetite are a number of other factors, such as not eating foods that may be ”triggering” or “addictive” for that person (e.g. sugars), breaking of food habits by meals being usually much different than before, getting tired of eating just protein foods and salad vegetables (until they learn the expanding world of low-carb gourmet cooking) but, most importantly, by being able to gain access to stored fat that previously was being walled-off by high blood insulin levels.
Regrettably, if too much fat is eaten, the body will have no need to remove any fat from the fat stores. If even more fat is eaten, the extra that is not burned will have to be deposited in fat stores (unless there is abnormal digestion and the fat is not absorbed – which would quickly be obvious in the toilet as greasy stools). You can’t pee out calories (except with sugar spilling in the urine when people have dangerously high blood sugar levels in diabetes) and the calories wasted by ketones lost in the breath and urine is also, regrettably, a small amount. The same, of course, is true for protein.
Thus we come back to “a-calorie-is-a-calorie, meaning two” – there are different metabolic impacts of differing calorie sources, which, at the same calorie intake, have different impacts on the person’s ability to feel that their appetite has been satisfied (to stop eating) and their ability to access their stored fat. Much has been written and said about this topic, so there is no need to repeat it all here.
So, we have “a-calorie-is-a-calorie, meaning one” in that all food calories, no matter the source, necessarily, by the definition of the word, contain the same amount of food energy. The total absorbed food energy must be dealt with by the body. A certain amount is used for body structure, for energy for digestion, metabolism and tissue repair, for energy for muscle movement. A certain amount is excreted or lost to the body in stool, urine, skin oil, shedding of dead skin and digestive tract cells, hair and nail growth, etc. A certain highly variable amount of food energy is burnt for extra heat generation in brown fat tissue – at any given point in a life, some people have an ability to do this to a meaningful degree and others are not so lucky.
The fact that our assessment tools do not allow us to come to a mathematical calculation predicting the number of calories used/lost in any given day does not mean that there is no such number. It is somewhat like the situation with someone in a coma on intensive life support. It is very difficult to state what the moment of death is, but that in no way means that there is no such thing as death occurring.
Some people have a great talent for resisting body fat gain while eating high amounts of calories – “wasting” the calories as caloricly inefficient heat generation. This has been documented. Unfortunately, not every person’s body has the ability to do this and it is not something one can will or intend.
The true indicator that conclusively shows whether calorie intake has been above, below or equal to total food energy use/loss is whether there is a gain, loss or stability in body composition. This tells the tale. All else is understood to be approximation.
The thing that counts, then, is the total calorie level. (Assuming the use of diets that do not induce frank malnutrition.) This leads to the decision to advise a hard cap on total calorie intake, with that cap adjusted downward until the body “reports back” by showing reduction in body fat. If the body responds by slowing metabolism or fatigue results in a reduction of muscle movement, the need to further lower the calorie cap will be evident by the same feedback signal – the gain/loss/stability of body composition.
But is the total calorie level the only thing that counts?
For this process to be of net medical benefit, the benefits must out way any harms. The most hidden and under-studied potential for harm is the almost universally experienced increase in hunger (when the hunger is not specifically addressed through drugs, ketosis, etc.) and the dissemination of the attitude that the hunger is just a nuisance to be dealt with as best as possible. If hunger is just a nuisance, the target calorie level can be lowered to any level compatible with the adequate intake of all other basic nutrients. In this case, all is right with the world. Just stay the course and someday somebody will publish a research paper documenting that this is a safe and effective medical intervention.
However, if sustained hunger is harmful, and thus if satiety-focused weight health is our best hope, then we had better hope and pray that “a-calorie-is-a-calorie meaning two” is valid.
This would be:
“a-calorie-is-a-calorie in the sense that every absorbed food calorie contains equal food energy and food energy intake must be lower than food energy use/loss in order for there to be a net reduction in body energy stores”
“a calorie of one type of food molecule or from foods with different properties can, varying in individuals and with individual circumstances in time, have different effects on physiology and appetite regulation, which have biologically significant impact in addition to their calorie content”.
It is worth remembering that the “heat” in the argument comes from people who are passionately committed to do their best to offer assistance to those trying to control their body fat content. They just have differing experiences, differing exposure to information and differing views of the best way forward, and all seeing a crisis unfold.
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** The flaw with terrible impact and very dangerous is that fructose, since it is rapidly turned into fat by the liver and doesn’t show up in the blood as glucose, doesn’t register in the glycemic index, which measures changes in the glucose in the blood stream (“blood sugar”). This makes foods with a high content of fructose look good for you, because the glycemic index (rise in blood sugar) is low. Of course, instead of showing up as blood sugar (glucose), the fructose is being turned into fat in the liver. This makes food with high amounts of added sugar, as sucrose (“table sugar”, which is half made up of fructose) or high-fructose corn syrup (which is more than half made up of fructose) or honey falsely look better for you than a potato or a piece of bread (the starch in foods contains no fructose, starch is made up of very long strings of glucose molecules). (Of course, a piece of bread would make me very sick, but that is another topic – gluten. And I am not about to eat a potato any time soon as I (at least currently) require low-carb nutrition to maintain my health.)