When is a calorie NOT a calorie

Sugar has been shown to make us gain weight but how does the sugar we eat turn to fat? And is this different for when we eat other foods?
20 November 2015

Interview with 

Dr Giles Yeo, MRC Metabolic Disease Unit, University of Cambridge


Once you swallow a sugary snack, it travels down your esophagus and into your applesstomach where it's 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 "kick off the digestion process," 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 it's 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 stream...

Giles - That's absolutely right.  So what you don't absorb, is you don't absorb starch, you don't absorb protein like in a big chunk of steak, you don't absorb fat.  What you do is you have to break it down, so for example for proteins you'd be breaking it down to amino acids, for fats you'd be getting down to something like lipids and triglycerides, and for sugar you'd 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 it's the same number of calories?

Giles - It's 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 - I don't know the number - but it would be like far less than 500 actual calories and that is the problem,  it's 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 it's all in a form that can be instantly absorbed.  Whereas, if I've got something that's a more complicated sort of food, so there's therefore a longer time as that process is happening. Therefore I'm 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 I've got a full stomach and intestine for longer so I'm less tempted by other treats while I'm processing the stuff I've 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 - I'm not a sugar nazi you know, but I want to demonstrate something about availability.  So here I have, I'm thirsty and I have my apple juice, about half a glass of apple juice....

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 I'm going to pass you the apples over here and we're going to have a race.  I'm going to drink the apple juice and I'm going to see how long it takes for you to have these, I don't 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 there's cyanide in apple pips...

Giles - Just the non-cyanide pips.

Chris - We did calculate that 54 apples, I think, is potentially lethal dose of cyanide. I've got to eat that lot.  Right, I've got to start - 7 apples then...

Giles - Absolutely, I'm going to start drinking now.

Chris - Okay so he's starting drinking his apple juice and I'm 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 - a level of glucose in your blood to not feel faint to not be ill.  So, a) you've  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 that's 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, it's broken down and then reassembled as fat.

Chris - Ah, so you tend to preferentially store things as fat.  Why is that?  Because it's capable of storing a lot more energy than glycogen?

Giles - Absolutely.  In fact fat is the professional energy storing unit because it's so dense.  It's the densest amount of calories you can fit into a given weight, given gram for example.  So that's why you store it as fat.

Chris - Would it be worth then trying to adopt a diet that's completely sugar free? 

Giles - That's a very interesting question.  I don't 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 wouldn't 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 I'm 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 that's probably very wise.  I mean we need to reduce the amount of food we eat, period, but that's probably wise.


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