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Nutrition Diet, supplements, weightloss, health & longevity

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Old 05-10-2004, 02:15 AM   #1
Paul Kayley
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Does Fruit Make You Fat?

By Patrick Gamboa B.S.

The old adage that your body is a temple is well known and still has relevance today. Without a solid foundation a temple can not be successfully constructed and will eventually collapse. The same holds true for the human body. We at ISSA strive to educate our trainers regarding the synergism between proper exercise, nutrition and behavior modification to effectively draw their clientele into not just a good lifestyle, or even a better lifestyle, but the best way of life; a fitness lifestyle.

The importance of nutrition is imperative as the foundation of any successful fitness program. The core of this foundation should be based around food. Just as certain compounds are necessary to build a solid foundation in a building, specific foods are necessary to build a solid nutritional foundation. Since we have already discussed which foods aid in building this foundation through past articles, we will focus our attention on why certain foods that are considered healthy, actually may not aid in fat reduction.

With the advent of so many nutritional approaches to achieve the ideal look, numerous inquiries regarding the practice of omitting fruit, fruit juices or any of its derivatives from a diet have surfaced. Fruit is a healthy food, full of nutrients, high in fiber, vitamins, minerals, and low in fat and calories. It is a common practice for bodybuilders during pre-contest preparation to omit fruit from their diets, as it should be for anyone looking to minimize bodyfat. We will discuss the chemistry behind the efficacy of this practice.

Our bodies can only absorb monosaccharides (glucose, galactose, or fructose), the single units of sugars and starches. Once absorbed through the small intestines into the portal vein, and then circulated into the bloodstream through the liver as blood glucose, our bodies can put glucose to work in three ways:
1. It can burn the glucose immediately for energy if blood glucose levels are not at a stable level of 20 grams blood borne glucose circulating per hour.
2. If it is not needed for energy immediately, then it is converted into glycogen in the liver or muscles. The liver has the capacity to store 100 grams of glycogen. The muscles have the capacity to store between 250-400 grams of glycogen, depending on muscle mass and physical condition. Liver glycogen supplies energy for the entire body. Muscle glycogen only supplies energy to muscles.
3. If the body has an excess of glucose, and all of the glycogen stores are full, the surplus glucose is converted to fat by the liver and stored as adipose tissue (bodyfat) around the body. If needed, fatty acids can be burned as fuel (BUT the fat cannot be converted back to glucose).
Now that we have outlined how our bodies use glucose, we will discuss why fruit (fructose or fruit sugar) is detrimental in an attempt to maximize fat loss. Since muscles have the specific purpose of contraction, they have a limited number of enzymes for glycogen synthesis. Muscle only has the necessary enzymes to convert glucose (and nothing else) into glycogen. The liver, however, is able to make glycogen from fructose, lactate, glycerol, alanine, and other three-carbon metabolites. Muscle glycogen, which is similar in structure to starch, is an amylopectin (branched chained polymer containing hundreds of glucose units). Unlike muscles, which can only supply energy to themselves through the stored 250-400 grams of glycogen, the liver is responsible for supplying energy to the entire body.

If you have fruit, fruit juice, or any of its derivatives, the following conditions occur:

Referring to the three ways the body uses glucose, assuming that blood glucose levels are adequate, the glucose will then be stored as glycogen. Muscle does not have the necessary enzymes to synthesize fructose into glycogen; therefore the liver converts this fructose into liver glycogen. It would only take three, 8-ounce glasses of orange juice to fully replenish liver glycogen stores. Since the liver is responsible for supplying energy to the entire body, once its stores are full, a rate limiting enzyme in glucose metabolism which is responsible for signaling the body to store glucose as glycogen or convert it to fat (phosphofructokinase), signals the body that all stores are full. If the glycogen stores are signaled as full, then the third way our body uses excess glucose is to convert it to fatty acids and store as adipose tissue. In essence, fruit sugar is easily converted to fat.

Many may be asking why then is fruit low on the glycemic index? If it does not cause a sudden release of insulin, then how could it ever be a poor food choice? Once the fructose (fruit sugar) enters the liver and liver glycogen is already full, then it can not be used by the muscles for glycogen or energy production. It is converted to fat and released back into the bloodstream to be stored as adipose tissue. The low glycemic response is based on the fact that fructose leaves the liver as fat, and fat does not raise insulin levels.

This is the biochemistry behind the recommendations to limit fruit in your diet. As mentioned, fruit is a very nutritious food full of vitamins, minerals, fiber, and low in calories and fat. If your goal is to exclusively to minimize bodyfat, then it is advisable that you consume more complex carbohydrates, which will go to replenishing muscle glycogen stores rather than fruit, which will only replenish liver glycogen stores, and is useless in muscle glycogen replenishment.
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Old 05-10-2004, 10:57 AM   #2
Larry Cook
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Paul,
Thanks for the easy to follow explanation on this. I've noticed that if my fruit intake is too high, I have problems regulating my bodyfat level ... now I have a better idea why. I now live by the adage "an apple a day ...." and try to limit my fruit intake to that, or its equivalent of some other fruit.
Larry
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Old 05-10-2004, 11:59 AM   #3
Brian Hand
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Paul, great article, thanks. Does anyone know of a reference that lists the fructose content of various fruits?
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Old 05-10-2004, 01:07 PM   #4
Paul Kayley
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My next question is... if galactose is treated in the same way as fructose, then dairy products will have the same results.

Is it right that the galactose element of lactose can only be processed by the liver, and is therefore going to be converted into fat when liver glycogen is topped off?
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Old 05-10-2004, 09:56 PM   #5
Paul M
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I've seen this article before, as well as heard the argument before. But its a little hard for me to believe and I think I might've found the flaw in the argument.

First off, liver glycogen is used essentially as a buffer for blood glucose levels. If its been a long time since you've eaten, or if you are exercising or post-exercise, your liver steps up and releases glucose into your bloodstream in order to keep a reasonable level.

The flawed assumption in that article, I believe, is the statement, "assuming that blood glucose levels are adequate" OK, if you're talking about building muscle glycogen, you're likely to be talking about a post-exercise state, in which case I don't think this is a good assumption. If your muscles are very depleted of glycogen, I would think that they would take blood glucose (if it was available) and convert it into glycogen.

The other thing that he doesn't mention is that the liver can convert fructose into glucose directly.

So, what I'm proposing is that, post exercise, eating fructose would result in the following: the liver would convert the fructose into glucose and put it in the bloodstream. The circulating glucose would be absorbed by the muscles and converted into glycogen.

Since there's that extra step in there, where the liver does the conversion, this is not a fast process. That's also why the glycemic index of fructose is low - because it has to be converted into glucose by the liver. Fructose, then, is not a good source of carbs for _quickly_ rebuilding either blood sugar levels or muscle glycogen. But, that doesn't mean that it isn't possible. And, since the liver can convert it into glucose, anyways, it can be used as an energy source aside from glycogen issues.

Another valid point is that, even if you eat too much of any kind of sugar than you can currently use and that sugar is converted into fat -- that doesn't mean that you'll get fat. If you aren't eating more than a maintenence level of calories, you'll need to tap into bodyfat at some point during the day for energy anyways.

-Paul M
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Old 05-11-2004, 08:04 AM   #6
Brian Hand
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Paul M, I think I follow, but do you think this invalidates or only blunts the potential problem with fructose? This is just speculation, but I would think because the fructose / galactose has to go through the liver, it has more of a chance to be stored as fat, and less of a chance to contribute to high energy levels.

Some foods have a low GI because they take longer to leave the stomach / intestine; that is a good thing, because those sugars can't get into trouble waiting in the stomach. However, low GI foods that are waiting in the liver - well, that might be another story.

Also, if someone eats a ton of fructose, isn't there a danger they will exceed what the liver can convert over the course of the day, leaving the liver with a lot of sugar and no way to burn it at bedtime? I have heard this is a potential problem, I don't know.

I am mainly curious just how much fructose is in various fruits. I remember reading grapes were particularly high; I think an apple has only 7 grams. It's going to take a lot of apples to cause trouble, even if fructose does pose a problem.

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Old 05-18-2004, 02:42 PM   #7
Jay Edvardz
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Brian,

http://www.nal.usda.gov/fnic/foodcomp/search/

For many fruits, the database offers a breakdown of what type(s) of sugar(s) is in the fruit.

-Jay
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