Dr Giles Yeo, MRC Metabolic Disease Unit, University of Cambridge
Once you swallow a sugary snack, it travels down your esophagus and into your stomach where itís digested but how is it broken down and converted into fat? And is sugar digested differently to other foods? Dr Giles Yeo gave Chris Smith a pit stop tour of our stomach...
Giles - So what the stomach does is to begin as you say ďkick off the digestion process,Ē so it begins by putting in gastric acids and all kinds of other really quite nasty stuff to begin breaking down the food but, what then happens is the food then passes from the stomach and into the small intestine, and itís within the small intestine that that digestion continues, but where all the absorption of the foods then occur.
Chris - So the food has been dismantled into its chemical components and at that point it then starts to go through the wall of the small intestine, it gets distributed round the body and the blood streamÖ
Giles - Thatís absolutely right. So what you donít absorb, is you donít absorb starch, you donít absorb protein like in a big chunk of steak, you donít absorb fat. What you do is you have to break it down, so for example for proteins youíd be breaking it down to amino acids, for fats youíd be getting down to something like lipids and triglycerides, and for sugar youíd be actually be absorbing it as - sugar.
Chris - Right. So how does it compare then. If I eat say 500 calories and I eat them as broccoli or brussel sprouts, some vegetable matter, fruit, whatever compared with 500 calories of sugar, how does that differ,because itís the same number of calories?
Giles - Itís the same number of calories if you can actually break it down molecularly. The issue is however, it takes energy to break calories down so, the problem with sugar is 100 calories of sugar, or 500 calories of sugar is 500 calories of sugar. Whereas 500 calories of broccoli or celery maybe even is probably Ė I donít know the number Ė but it would be like far less than 500 actual calories and that is the problem, itís energy availability.
Chris - Is there also timing effect in the sense that when I eat something like sugar, digestion can begin and end almost instantly because itís all in a form that can be instantly absorbed. Whereas, if Iíve got something thatís a more complicated sort of food, so thereís therefore a longer time as that process is happening. Therefore Iím going to sort of control the rate at which the calories get into me. Does that make a difference as well?
Giles - Absolutely correct.
Chris - Does that mean then that I less hungry because Iíve got a full stomach and intestine for longer so Iím less tempted by other treats while Iím processing the stuff Iíve already eaten?
Giles - I think there is evidence for that to actually be the case, yes.
Chris - Now are you very thirsty and hungry, or have you just brought in a whole load of fruit juice and a pile of apples for no reason?
Giles - I am trying to demonstrate one thing because people - Iím not a sugar nazi you know, but I want to demonstrate something about availability. So here I have, Iím thirsty and I have my apple juice, about half a glass of apple juiceÖ.
Chris - About half a pint or so.
Giles - About half a pint of apple juice. What I want to demonstrate how many apples it went into making that half a pint of apple juice, so Iím going to pass you the apples over here and weíre going to have a race. Iím going to drink the apple juice and Iím going to see how long it takes for you to have these, I donít know, eight apples.
Chris - Eight. Is that really eight apples?
Giles - It is eight apples. It depends on how large the apples are I guess, but yes really eight apples.
Chris - Okay pass the over. Do I have to eat all the apple including the pips, because you know thereís cyanide in apple pipsÖ
Giles - Just the non-cyanide pips.
Chris - We did calculate that 54 apples, I think, is potentially lethal dose of cyanide. Iíve got to eat that lot. Right, Iíve got to start Ė 7 apples thenÖ
Giles - Absolutely, Iím going to start drinking now.
Chris - Okay so heís starting drinking his apple juice and Iím going to take my first bite. While you digest your apple juice and I eat my apple, when the calories come into me as sugar, or anything. Sugar's carbohydrate so, why is it people who have a lot of energy actually end up fat?
Giles - Okay so sugar is, glucose anyway, is your base energy units almost, so you need to maintain a level of sugar within your blood Ė a level of glucose in your blood to not feel faint to not be ill. So, a) youíve got to have enough glucose to keep it at that level, b) what happens is that you need a quick release source as storage and thatís called glycogen. That tends to be stored in your liver and your muscle, but only, I would say, 100-150 calories of glycogen are in you at any one point. Everything else which is not required at that time, itís broken down and then reassembled as fat.
Chris - Ah, so you tend to preferentially store things as fat. Why is that? Because itís capable of storing a lot more energy than glycogen?
Giles - Absolutely. In fact fat is the professional energy storing unit because itís so dense. Itís the densest amount of calories you can fit into a given weight, given gram for example. So thatís why you store it as fat.
Chris - Would it be worth then trying to adopt a diet thatís completely sugar free?
Giles - Thatís a very interesting question. I donít know if it technically would be possible because you need fruits and vegetables and they actually have sugars within their cells. Now can you have foods that are completely lacking in additional sugars? Plausibly but then it probably wouldnít be very palatable.
Chris - So the bottom line is that we need to adopt what Ian was suggesting, about 5% of our calories in the form of these extrinsic free sugars. The rest of the carbohydrates up to about 50% of calories in the form of this apple Iím eating, complicated locked away calories and then the rest from fats and proteins in order to receive the right balance of nutrients we need.
Giles - I think thatís probably very wise. I mean we need to reduce the amount of food we eat, period, but thatís probably wise.