Making Sense of Glucose and Fructose

Posted August 5, 2007 at 11:00 AM by Shannon Clark

Section: Her Health, Her Nutrition, Healthy Eating, Weight Control, Workout Fuel

Everyone is talking about carbohydrates these days - are they good or are they bad? And how much of them should you be consuming?  Which ones are going to cause the most trouble and which ones will actually help you? These are all questions that are running through many peoples’ minds in their pursuit towards achieving their fitness goals.

While there is no doubt that low carbohydrate diets can work great for some people, others will not feel good on them and they will need carbohydrates in order to function properly.  This does not mean that people who consume carbohydrates will not lose weight, however, they just need to learn the types of carbohydrates they should be consuming and when.

The important thing to note is that there are two main types of carbohydrate molecules: glucose and fructose.  Starch, a polymer of glucose (which means many glucose molecules linked together), is another form of carbohydrate you might have heard of before.  Because of its molecular structure, foods higher in starch take longer for the body to digest than foods equal in the number of calories and made up of glucose.

When you consume glucose rich foods, the body uptakes this glucose and stores it in the muscle tissue as muscle glycogen, or else it uses it for muscular energy (depending on your nutrient status at the time).  Therefore, glucose containing foods are great to eat before a workout and right after because this is the prime time when your body is going to be receptive to storing muscle glycogen.  Having a high carbohydrate meal pre and post workout and then making the rest of your meals lower in glucose throughout the day is generally the best approach when trying to maintain a favorable body composition.

You do need to be careful, however, because glucose will affect insulin and a large amount of simple sugar taken in at one period of time with no accompanying fat or protein will cause a blood sugar rush followed by a crash - something most people need to avoid.

Fructose, on the other hand, gets handled by the body differently than glucose (or starch) does. When you eat fructose rich foods, the carbohydrates get transported to the liver where they help maintain liver glycogen stores.  Liver glycogen stores are responsible for signaling the body that it is either in an anabolic or a catabolic state. This is important because you should be trying to stay in as anabolic of a state as possible so that you are not losing large amounts of muscle mass.  Furthermore, when liver glycogen levels become depleted, you may start dealing with issues of hunger more so than if they weren’t, so including some foods in the diet that contain fructose is a smart move.

The downside to fructose is that the body has a limited amount of storage room for it and if you consume excess amounts, it will be turned into triglycerides and released into the blood stream setting you up for numerous negative health effects. Generally you should be trying to limit your intake of fructose to around 50 grams per day. 

Foods that contain fructose include fruits, some vegetables, soda beverages, fruit juices, and then any other pre-packaged foods that contain the ingredient high fructose corn syrup.  Fruit and vegetables contain relatively small amounts of fructose so you can feel safe making them a regular part of your diet (and you should due to the nutrition they provide); that said, anything with high fructose corn syrup should be limited as they have a much higher level of fructose in them and will set you up for an increased risk of fat gain.

It is also important to note that because fructose does not provide any energy for the muscle cell, nor does it get stored as muscle glycogen, this is not the best type of carbohydrate to have before or after a workout. 

Likewise, because of this factor, it will not affect insulin levels so it will be a good type of carbohydrate to have at other times in the day when you do not want to see blood sugar levels spiking and then falling. 

So to sum up, the majority of your carbohydrate intake should come from glucose (or starch) as this is what will provide you with muscular energy. Your body also has a relatively large storage capacity for glucose so there is less of a chance that it will turn into body fat. 

Fructose is important to include in the diet because it will help to prevent muscle loss and hunger as well as help to keep your blood sugar levels stable.  With fructose, though, you should be limiting the amount you consume because excess will readily be converted to fat. If you keep the source of your fructose coming from fruits and vegetables, you shouldn’t have too big of an issue keeping your intake levels where they need to be to prevent fat spill-over.

So the next time you are talking to someone about carbohydrates and they tell you to avoid them at all costs, rethink this. It is important that you understand the types of carbohydrates you are consuming and when you should be including them in your diet.