January 25, 2011

Why junk food makes you fat

Posted in Uncategorized at 5:00 pm by Chris

This a dry run for part of a lecture on food production. Disclaimer: I’m an ecologist, and this is based on my understanding of nutritional ecology and animal foraging behaviour. It should be taken as, at best, dilettante commentary on population health, and definitely isn’t medical advice. Especially if your doctor is telling you something to the contrary about your particular clinical circumstances!

Over the last ten or fifteen years there’s been a quiet shift in the way we think about foraging behaviour. In other words, our understanding of how and why animals choose both their overall diet and individual food items is better and more powerful now than it was 25 years ago. Key to this shift has been throwing out the (always faintly ridiculous) idea that a foraging animal is aiming for some specific target of calorie intake and nutrient balance. Instead, it’s more accurate to say that, given the choice, animals choose food items over the course of days or weeks that maintain a certain ratio of various nutrients in their diet. This happens within the constraints of energy requirements that vary wildly between different animal species, and even between individuals and over seasons. In effect they’re chasing a “best line” of nutrient intake over time; this is sometimes called a “nutritional rail”. Staying on the rails no matter what? We’ll get to that in a second.

I used an esoteric version of this concept to try and explain the mechanisms of chemical defenses in my PhD thesis, but in a broad concept we’re normally interested inprotein macronutrient ratios. Macronutrients are the different classes of food that you’re probably familiar with: specifically protein, carbohydrates, and fats. To the great joy of reductionist types everywhere, fats and carbohydrates can be basically lumped together in a nutritional sense over any time frame greater than a few hours: animals, including us, can happily convert one to the other. The major difference is that fats represent a much more concentrated energy source, and that complex carbohydrates are harder to digest (and both have their own implications for the health of individual humans, see “I am not a doctor”, above).

That leaves us in a lovely simple nutritional world where there’s “protein” and “everything else”. Or nitrogen and carbon, if you’re of a botanical and horticultural bent. And, having made ourselves a lovely simple world, this is where things get interesting.

It turns out that humans (and indeed, other animals) prefer a certain percentage of protein in their diet: the figure is about 15%, although you’ll want more carbohydrate than that (probably in the form of fat) if you live in a cold climate or if you’re very active. But we don’t regulate our food intake based on the overall ratio: we regulate based almost entirely on how much protein we’ve eaten recently. Let that sink in for a minute, as we turn to ponder this implications…

I should say at this point that I’m mostly parroting the work of other people, and particularly the Behaviour and Physiology Research Lab at the University of Sydney. That lab is led by Steve Simpson, who co-authored a paper called “Obesity: the protein leverage hypothesis“, from which I’ve borrowed some figures, and you can probably see where this is going. The key word there is “leverage”. Because we need (relative to other stuff), a fairly small proportion of protein in our diet, but that proportion is what we regulate our food intake on, things start to go haywire if we’re confronted with a low-protein diet. Even a few percent less protein than we really want leads to us eating substantially more of the available food than we want or need: 10% comes up pretty quickly and 20% isn’t hard. This is represented visually in the figure below; although the difference looks extreme, it’s been experimentally demonstrated in humans. Some other animals either use a less extreme method of balancing their diet, or they burn off excess energy by keeping warm. We tend to eat the difference, and store the excess as fat. This seems to be a fundamental factor in human physiology, and if we don’t fully understand the mechanism it reflects on our understanding of physiology, rather than our powers of observation.

Nutrional rail for protein versus carbohydrate

Human intake of fats and carbohydrates versus protein intake, from Simpson and Raubenheimer 2005

Now you may be thinking, so what? Don’t eat a protein-poor diet, only got yourself to blame, get off the couch, etc, etc. But here’s the rub: we’re not good at selecting foods for protein based on their taste. We can only “kind of” taste protein, and we tend to use a lot of proximate clues, things that we associate with protein-rich foods: tastes like salt and umami, plus smells and textures and such. That works fine in the wild (as it were), but in the era of processed and manufactured foods, it’s a complete disaster.

Manufacturers of foods, and although I’ve used the term junk food in the title this encompasses a whole range of things, are well aware that people like the taste of protein. They’re also well aware that this taste is easy to fake, and that the fake is way cheaper than real protein. There’s plenty of cheap sources of starches and fats, but protein is relatively expensive: meat for all the obvious reasons, but vegetable protein isn’t actually much better (and may even be less efficient to produce than meat, for a given land area).

Barbeque sauce, the quintessential “meaty” taste: in its simplest form* it’s not only a vegan product, but the only protein it contains is whatever’s in the soy sauce. The same is true of most “meaty” or otherwise protein-ish flavours: they’re very easy (and very cheap) to fake. That leaves the consumer of large amounts of processed foods (who, statistically, tends to be a person on a lower income) with very little information available about what they’re actually eating. The result is a diet with, initially, too little protein; the longer-term, behavioural and physiological response to which is to eat too much carbohydrate and fat. This is stunningly (and depressingly) illustrated in the figure below.

Percent obesity versus change in % protein intake for 13 countries

Percent adult obesity versus change in % protein intake, from Simpson and Raubenheimer 2005

Not only does the USA (point 13 on the graph) lead the world in obesity, it also leads the world in the proportion of carbohydrates and fats to protein in its diet. Presumably this is at least partly related to the monolithic and many-tentacled corn-reprocessing industry, but I suspect that’s a huge topic and not one I feel like dealing with right here and now.

Incidentally, the Atkins and other high-protein diets work by applying the protein “lever” in the other direction: forcing you to eat too much protein, so that you drop your carbohydrate intake disproportionately and start breaking down stored body fat to stay alive. I believe the jury is still out on the long-term health consequences of said diets.

A larger implication for this protein leverage concept (and the reason I’m putting it in a food-production lecture) is that the ultimate limit on a person’s food intake is their protein intake. That means, should push come to shove, that the limits on our ability to feed the teeming billions relate not to food production but to protein production. Which is a very attention-focussing idea, I think.

(*: pepper, soy sauce, paprika, sugar, vinegar)

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