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The 5:2 Lab

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Sucrose / fructose question
18 Jun 2014, 14:41
I was surprised to learn this:
Sucrose is made up of one molecule of glucose joined to one of fructose so maple syrup, technically, is 49% fructose, 51% glucose. Sucrose is split into its two components in the gut.
(from this post)

Since white table sugar is sucrose, does this mean that all the fructose in (sucrose) sugars goes directly to fat?

- In the same way there is no other option for fructose, except maybe with fibers. See here:
It's a video called "Sugar the bitter truth"
http://www.youtube.com/watch?v=dBnniua6-oM
It's long, but it breaks it down to the molecular level of what happens in the liver. It turns out, that the fiber in fruit buffers the way insulin responds to the fructose in fruit
Re: Sucrose / fructose question
18 Jun 2014, 14:54
Yes, this is what I learnt from Dr Robert Lustig' s book "Fat Chance". Made me give up fruit juice.
Re: Sucrose / fructose question
18 Jun 2014, 16:06
Fructose does not all get turned into fat, but the problem is that its metabolism occurs in the liver whereas glucose can be metabolized all over the body. In the liver fructose can be converted to glucose, glycogen (the storage form of glucose) or to fat. As the liver's capacity to store glycogen is limited, if we eat a diet high in sugar (sucrose, which is half glucose and half fructose) or other fructose sources it may be that this results in a greater accumulation of fat in the liver than a diet low in sugars. However, because we rarely eat fructose without glucose in some proportion (even high fructose corn syrup is only slightly higher in fructose and lower in glucose than standard sugar) it is not clear whether fructose is more dangerous than any sugar.

As to the question about fibre in fruit buffering insulin response, it is known that pure fructose has the lowest glycemic index (GI = 19) of all the natural sugars, thus I am not sure that the fibre in the fruit has much to do with anything, it is more likely to do with the fact that fructose goes straight to the liver for processing and so does not result in much glucose being released into the bloodstream.
Re: Sucrose / fructose question
18 Jun 2014, 18:27
carorees wrote: ... that fructose goes straight to the liver for processing and so does not result in much glucose being released into the bloodstream.



And this from Volek/Phinney:

"Fructose - a sugar that partitions like fat

Most of the fructose we eat, whether as sucrose (table sugar), high
fructose corn sweetener, or in natural fruits and fruit juice gets made
into fat by our liver. This is because our body can't convert fructose
to glucose, and the first step in cellular fructose metabolism diverts it
away from the primary pathway of glucose metabolism (the Myerhoff-
Embden pathway). Thus these two 6-carbon sugars, fructose and
glucose, follow separate metabolic paths. In the case of fructose, it
is cleaved into two 3-carbon fragments, both of which primarily con-
tribute to fat production (lipogenesis) in the liver.

Twisted Logic? Most energy drinks and sports beverages use su-
crose or high fructose corn sweetener as their primary energy source.
Given that the average exhausted athlete still has tens of thousands
of fat calories in body energy reserves but is running out of carbohy-
drate (glycogen), why would one want to add a sugar that cannot be
used for quick energy, with most of it eventually ending up as fat?"
Re: Sucrose / fructose question
19 Jun 2014, 14:39
Thanks for the replies! :like:
Re: Sucrose / fructose question
19 Jun 2014, 16:24
carorees wrote: Fructose does not all get turned into fat, but the problem is that its metabolism occurs in the liver whereas glucose can be metabolized all over the body. In the liver fructose can be converted to glucose, glycogen (the storage form of glucose) or to fat. As the liver's capacity to store glycogen is limited, if we eat a diet high in sugar (sucrose, which is half glucose and half fructose) or other fructose sources it may be that this results in a greater accumulation of fat in the liver than a diet low in sugars. However, because we rarely eat fructose without glucose in some proportion (even high fructose corn syrup is only slightly higher in fructose and lower in glucose than standard sugar) it is not clear whether fructose is more dangerous than any sugar.

As to the question about fibre in fruit buffering insulin response, it is known that pure fructose has the lowest glycemic index (GI = 19) of all the natural sugars, thus I am not sure that the fibre in the fruit has much to do with anything, it is more likely to do with the fact that fructose goes straight to the liver for processing and so does not result in much glucose being released into the bloodstream.


Very nice answer! :)

The main issue that probably exists is indeed related to the limited capacity of our liver when it comes to glycogen storage and fructose processing. Excessive fructose consumption is in that sense, logically correlated with increased rate of fat storage.

However, one has to realize the simple fact that as long as the daily energy balance is properly set up (energy deficit or maintenance for all those looking to lose or maintain weight), you consume a balanced diet in terms of the main macro-nutrients, and you are properly active - it really isn't that possible to store more fat in the body in the long term or suffer from fructose related health issues.

There are some nutrient partitioning aspects at play, but unless you basically totally over-consume fructose on a daily basis, don't exercise and over-consume calories as-well - I wouldn't even think twice about consuming fructose. It is simply unlikely to have any negative consequences when it comes to body weight or body fat numbers.

People simply want a scapegoat to blame for health or weight issues. And fructose is easy to blame.

There's a study where people consumed 43% of their daily calories from sucrose and they still managed to lose weight and fat mass just fine and remained fairly healthy as-well. (http://www.ncbi.nlm.nih.gov/pubmed/9094871)

The high-sucrose diet contained 43% of the total daily energy intake as sucrose; the low-sucrose diet contained 4% of the total daily energy intake as sucrose


Results showed that a high sucrose content in a hypoenergetic, low-fat diet did not adversely affect weight loss, metabolism, plasma lipids, or emotional affect.


Which goes to show that fructose - while being a potential aggravator of obesity and health issues - isn't in reality much of a problem at all by itself. People should just realize that they aren't going to find that "one thing" that makes them fat or unhealthy. They just need to practice moderation, stay active and stress-free and just watch the fat melt away and health markers stay strong.
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