This page on the British Nutrition Foundation website was mentioned here recently. It encapsulates almost every incorrect belief in mainstream nutrition, and I thought it would be instructive to pull it apart:
nutrition.org.uk/healthyliv...
I do realise, incidentally, that I come across as a bit of a know-it-all when I do these dissections of mainstream nutrition policy. Physiology is a complex subject and a lot of what goes on inside our bodies is still unknown - and perhaps unknowable. Since even very-qualified, immensely clever people don't know everything, it follows that I know even less. But I probably know more than the average nutritionist - as, I suspect, do many of the members here - because I make the effort to find out how things actually work, as opposed to just making stuff up and attaching some sciency-sounding words to it. With that proviso, here's my critique:
Fat contains 9 kcal (37 kJ) per gram ...
The total energy content of a food can be found by burning it and measuring how much heat is released.
The idea that our bodies derive a constant amount of energy from the macronutrients we eat, equivalent to their heat of combustion, is fundamentally wrong. Chemical energy is an inherent property of the bonds that hold a compound together, so the amount of energy released from any given compound is a function of the reaction(s) performed upon it. While the foods that we eat ultimately end up as carbon dioxide and water, and all of its potential energy ends up as heat, this does not mean that our food has been "burned" as it would have been in a heat engine. Food goes through a whole bunch of conversions, and storage and recovery operations, each reaction having a certain efficiency associated with it.
As a rough handwaving approximation, our bodies extract about 20% of the chemical potential energy from our foods as useful work, the remainder being lost directly as heat. Interestingly, this efficiency figure is much, much higher than it would be for a heat engine operating at body temperature (it'd be 3-4% at best). In other words, "the laws of thermodynamics" do not apply to human beings, whatever the nutritionists might think.
TL;DR: it is broadly true that our bodies can extract more useful energy from a gram of fat than from a gram of carbohydrate, but the statement that "a gram of fat contains 9kCal" is misleading, because we do not eat in order to generate heat. Our bodies use some fraction of food-energy to perform work .. but we have no way of knowing what that amount is, or how efficiently it can be extracted from our food. The idea that we can calculate how much food we should be eating is therefore false.
Basing your diet on foods which are lower in calories (or have a lower energy density), and eating foods which are high in calories (or have a higher energy density) less often and in small amounts, can help to control you overall calorie intake.
There is no obvious reason why this should be true. They just made that up. It could only be true if human bodies were incapable of regulating their energy intake and expenditure. Since our bodies actually have extremely sophisticated methods for maintaining both energy balance and mass balance, we would expect that people who eat larger amounts of fat (relative to carbohydrates) would simply eat less. And experimentally, we know that this is exactly what happens ... at least, in the absence of food-science modifications which are designed to mess with one's appetite.
Carbohydrate is the most important source of energy for the body because it is the main fuel for both your muscles and brain.
No, it isn't. There is nothing inside your body that can use carbohydrates for energy. Certainly, most of your organs can use glucose, and a few of them need a certain minimum amount of it. But "glucose" is not the same thing as "carbohydrates". A fat molecule, for example, will be broken down into three fatty acids and one glycerol molecule; the former can be burned directly or indirectly by almost all of your body's cells, and the latter is generally converted to glucose (by a short and elegant chain of reactions) for those few systems that need it.
At low power output, your muscles actually rely mostly (80%+) on fatty acids for fuel, conserving glycogen/glucose for bursts of high-power activity - the implication is that sedentary adults have very little need for carbohydrates, and would be better off eating dietary fats. Your heart muscle uses fatty acids almost exclusively, and ketones to a lesser extent. It will only use glucose if there's a glut of it relative to fatty acids (that is, while a carb-heavy meal is being digested).
Your brain does need a little glucose, but the amount is considerably less than nutritionists assert. It is physically impossible to deprive your brain of glucose.
Your weight depends on the balance between how much energy you consume from food and drinks, and the total amount of energy that is used by your body. When you eat or drink more energy than you use, you put on weight; if you consume less energy from your diet than you use, you lose weight
There are at least three assumptions here which are demonstrably wrong:
a) Your body does not, or cannot, regulate its own energy balance
b) "Weight" is the same thing as "bodyfat"
c) Bodyfat is merely a dumping ground for excess energy and has no other physiological function
Most fat people are not actually getting fatter, but just "maintaining" at an unhealthy bodyfat ratio. My cousin's wife has been overweight for as long as I've known her (a very long time), but she's been consistently overweight. She's not ballooning. If the nutritionists are correct, she's in caloric balance - and why shouldn't she be? All human bodies are, most of the time. So the question arises: why is she maintaining so much bodyfat? The nutritionists don't know. Their theory doesn't offer any explanation.
The correct way of phrasing the original statement is as follows:
If you are gaining weight, you are eating more than you are expending; if you are losing weight, you are eating less than you are expending.
This is a completely uninteresting statement, with no earth-shattering implications. Why? Because energy-in and energy-out are variables, not constants. Your body can adjust energy-in (via appetite) and energy-out (in a whole bunch of complex ways). Deliberately eating less than your appetite demands is something that you cannot maintain for more than a few weeks or months, in most cases ... and it won't help you anyway. All that happens is that your body will dial back its metabolic rate to match the caloric restriction - which is precisely what you'd expect from a homeostatic system. Of the people who attempt to lose weight this way, 95%+ of them fail within 3 years.
The point here is that bodyfat has a purpose: to store excess food energy, and release it as needed (ie., when you're not eating). In fact, since humans aren't eating all the time, it's clear that "carbohydrates are the main fuel for your body" is incorrect. The average body is running mostly on stored glycogen and fat. Here's the conundrum: why do some bodies store more fat than they (apparently) need? The nutritionists don't know, but they confidently assert that the solution is:
eat less, move more!
There is now a vast mountain of evidence - not to mention personal experience - that shows this doesn't work. And yet they keep repeating it. Why? I can think of only three possible explanations:
1) The nutritionists simply don't have enough scientific background to understand how flawed their theories are.
2) The nutritionists are so attached to their theories that they're unwilling to discard them in the face of overwhelming disproof.
3) The nutritionists are under financial or political pressure to keep this game of musical chairs going for as long as possible, because it's not going to look pretty when the music stops.
So which is it, BNF? Do you have an alternative explanation? I've emailed to you ask. Feel free to post your reply here.
