Individualisation of nutrition

Have you ever heard someone say if I look at a carbohydrate I get fat, or similarly that they can eat anything and not put on weight.  I bet you have.  This fascinates me, how the same people can eat essentially the same diet and have such as different response to it.  The idea of an individualised diet is key.  I think sometimes nutrition can be set up as a one size fits all approach, this worries me and if you don't listen to how your body reacts to food you are missing a trick. 

Lets try and put some science into this.  The first peice of evidence I want to look at is the idea of predicting energy expenditure whether it be from heart rate, accelerometry or some other form.  You tend to get a graph which looks something like figure 1.

Figure 1 - Predicted energy expenditure

Now, science likes graphs like that in figure 1 because there seems to be a positive and pretty good correlation between the actual energy expenditure and the predicted energy expenditure.  This means you can get a nice equation which predicts energy expenditure from something simple like heart rate.  You can then use this to calculate how many calories you burned during exercise and subsequently how many calories you need to eat.

However, not everyone sits nicely on the line.  At an actual energy expenditure of 200 kcal per hour one indivdiual has a predicted energy expenditure of 100 kcal/hour and another 300 kcal/hour.  This means that over one hour of exercise the prediction has over estimated or underestimated energy expenditure by 100 kcal.  Now this might not seem a lot, but for an endurance athlete doing lots of training this could add up.  You could either end up putting on weight if your energy expenditure is over extimated or maybe not train properly if your energy expenditure is under estimated.

What about how you react to different nutrients.  A recent study by Pittas et al. 2006 looked at how healthy men and women reacted to either a high glycemic load diet (basically a high carbohydrate diet) versus a low glycemic load diet.  What was interesting was that one group of individuals lost more weight on a high glycemic load diet and another group lost more on the low glycemic load diet.  The difference between the group was how the hormone insulin responds to a carbohydrate load (called an Oral Glucose Tolerance Test (OGTT)).  If there was a high insulin response in the OGTT then the individuals tended to lose more weight on a low glycemic load, if there was a low insulin response then individuals tended to lose more weight on the high glycemic load diet.

So if everyone has the same diet they are going to respond differently and therefore what you eat should be individual to you.  General guidelines are a great starting place but don't forget to listen to your body