Mark Ellwood asked:
Given the ever increasing amount of evidence that sugar (in particular fructose) is largely responsible for the worlds obesity epidemic. A thought occurred to me.
I have read that fructose in humans is metabolised to fat and little else. Fructose is particularly sweet almost addictively so thereby encouraging overeating. Fructose is readily available in most forms of fruit, a form packaging that arrives in a glut in the autumn. Herbivores and Omnivores then have a food source that arrives in huge quantities, encourages overeating and is metabolised straight in to fat, just prior to the winter, when food is scarce and fat reserves are needed.
Could this be the source of the maladaptation to modern sweet foods? Are we are all stuck forever preparing for a time of scarcity that never arrives being shaped by a metabolic pathway that no longer fulfils its intended purpose?
We put this to Giles Yeo, from MRC Metabolic Diseases Unit...
Giles - If I may Mark, it is not true that fructose is converted to fat and little else. Undoubtedly glucose, which is our normal fuel that increases insulin and everything else, that is absorbed by your muscles and your fat and thatís not true for fructose. It goes largely to the liver where itís actually metabolised there. Only about 1% or so, or slightly less, goes directly to fat.
Chris - What happens to the other 99% then?
Giles - Itís actually divided between glycogen and actually being converted to glucose in itself. There is, however, a variation in how people handle fructose. OK, so some people handle fructose slightly better than others but, at the end of the day, all things will be converted into fat if you have to much of it. So, even if you have too much glucose, once your body uses up the glucose it needs to use, it will be converted to fat. The same thing for protein, the same thing or fructose as well.
Chris - So does Mark have a point that you have this big glut of fructose at one time in the year and it does end up as fat because, effectively, animals are taking in more energy that they need so it favours becoming obese?
Giles - But itís not only fructose. I mean fructose is found largely in fruit, but together with glucose, they form sucrose. Itís sugar in itself - too much of it when you have too much of it will be converted into fat. That is the purpose of having that glut where the bears come along having the fruit in order to actually get enough energy to hibernate. But thatís always going to be true if you have too many calories in which sucrose is readily available.
Chris - EleanorÖ
Eleanor - Just quickly from an animal tasty thing. I was ready quite recently about taste perception and why are sweet things tasty. And from looking at birds it seems to be that it evolved to teach them that this is lots of calories therefore itís worth eating, therefore itís sweet, so thatís the kind of evolutionary thing. Is it the same in people do you think or do you think it a bit more complex?
Giles - No, no. I think it is quite that simple. Sugar is one of the most energy dense things we can have per gramme because you get 100% of the calories from it. And interestingly, if you take babies, they can take food that is sweet, that is incredibly sweet that we would find inedible, but babies will go, um delicious, yummy. And then what happens is as they get out of babyhood into childhood, then into our age when our palate becomes adult-like, we canít take food that is as sweet as our kids can. And if you actually taste anything the kids eat - oh my god!. And so I think itís quite that simple - yes.
Chris - Wasnít you who said once Kat - I love babies but I couldnít eat a whole one?
Kat - Yes, yes, I think theyíre sweet. Too sweet for me.