Tuesday, 22 July 2008

Another Appetite Rant

I was thinking about that old piece of nutritional advice:

Calories In = Calories Out.

This is the standard advice that is fed (pun intended) to us to explain obesity. The idea is that if you are obese then it MUST be because you 'stuff your fat face'. Such an argument can be addressed in a single word. Appetite.

Why Can't We Trust Our Appetite?

This is a theme I keep coming back to. Time and again in clinical trials, epidemiological trials and the like effort is expended on how much we eat and what we eat. To me this misses the point. We have evolved as a highly developed and successful species over the past two million years. We can eat a range of food, and inhabit a range of habitats. We are dynamic.
Our energy expenditure varies with respect to the habitat we are in and in the effort required to obtain new energy (food).

Habitat

Now think about our habitat - and lets look at one variable such as temperature. We use energy to keep warm in cold environments. Cold can come from long term environmental factors lasting decades, habitation variation (living at extreme latitudes such as near the poles), seasonal variance (which in itself can more noticeable at, say, altitude), or simply the transition from day to night. Jumping in to cold water on a warm day elicits a similar effect. Clearly we have to develop a dynamic response to temperature changes. These changes can be long term (climatic) or very short term (a jump in to a mountain stream). When cold, we shiver. this is the body trying to make us move as movement generates heat.

This movement takes energy. Given variety of sources from which we can experience 'cold' our bodies need to adapt to short term and long term changes in energy requirements.

Food Sources

When considering food sources, from an evolutionary point of view you have to realise you are 'a meal looking for a meal'! Just as you would love to trap a bear and end the day snuggling down with a new bear-fur loin cloth and matching bear fur duvet, there were (and in some countries still are), myriad other predators who view you as a potential 'low-carb' snack.

In a savannah type environment you would need speed. In fact, did you ever wonder why you freeze when in front of an audience? Well, imagine you are on the savannah and there were lots of eyes on you....not a problem unless they were the eyes of a pride of lions. If you keep still then you have a chance that they will not spot you (although they might still smell you), but if they get too close or start to circle you then a burst of speed, jinking from side to side and then climbing a tree, might well offer the greatest chance of escape.

Now if it is you and your tribe doing the hunting, you will use your creativity and ingenuity to kill game. You may come up with an elaborate trap involving a trip wire or snare (classic problem solving). You may chose to use the brute strength of the exhaustion hunt (which, far from involving marathon style steady state exercise, often involved a variety of speeds, dictated by the difficulty of the obstacles in the terrain around you). If you are tracking animals you will draw upon a wealth of knowledge and experience. Memory function will be tested as will visualisation. You have to remember the route back to camp and, as the game is dictating the route, you'd do well to have remembered water holes and wild food available en-route.
Big game would require greater strength to bring down, but being generally more fatty than that of smaller animals, would overcome any problems associated with the consumption of excess protein.

Energy Requirements Vary

So there you have it! I have tried to paint a picture of how our energy demands can vary rapidly in both how we expend our energy and how we obtain it.

You may wonder where I am going with this! There is a minimum amount of energy you require to simply function as YOU. This is your basal metabolic rate (BMR). This is the amount of energy you will expend simply moving around, beating your heart and working hour lungs. If you throw in a run or the lifting of some weight, then this is energy expenditure over and above your BMR.

What is obvious is that looking at the variables that can affect us, our energy demands can vary wildly. The environmental factor of cold might kick in more at night, or simply for a few minutes (swimming across a river), or it might last months (such as surviving over winter). The 'meal factor' is also incredibly variable; if a rhino is chasing you and you climb a tree, the chances are you are safe. If a big cat is after you, your escape may be more drawn out - the lion may follow you up the tree so you might have to spend some time wielding your spear or other such weapon).

So given all these variables on energy expenditure, what should regulate our energy requirements. APPETITE!

Appetite

There is no evolutionary advantage to allowing ourselves to eat ourselves in to a state of immobility. There is no evolutionary advantage to us being unable to function due to a lack of energy.

How did we overcome this problem? Well one look on your local high street will tell you all you need to know. We developed the ability to store fat. Fat storage is a survival mechanism to keep us going between meals. Fat is a dynamic energy source. You should NEVER be weak with hunger, ONLY with starvation. Fasting for one or two days DOES NOT AND SHOULD NOT constitute a state of starvation. If you are hungry, your body should be recycling fat stores and bringing the energy 'online'. If you run down your fat reserves, then your body will look to rebuild them. That is the 'deal' that means you will always have the energy to get yourself out of trouble and over a lean period. Simple really.

In relation to this mobile energy store, the other part the of the deal is that the body should not allow you to pack on so much weight in fat that your athletic ability is compromised. This much should be obvious.

So to ensure we 'eat enough' to replenish our fat stores, 'but not too much' such that we impair mobility, we need a tightly controlled mechanism. Such a precision mechanism must have evolved over millennia. The exacting requirements over which it has control mean that it must be highly tuned and an optimal piece of biological, evolutionary engineering. And indeed it is.
It is called our appetite.

Now somebody tell me why such a refined tool as our appetite seems to fail so many on a Western diet? While you are at it, tell me why so little research is dedicated to appetite. Simply focusing on calorific intake and calorific expenditure seems to me to be, in effect, shutting the stable door after the proverbial horse has bolted. Our appetite is being misled by a change only visible in non-hunter gatherers. Hmmmmm I wonder what THAT could be?

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