For a long time, sugar (carbohydrates) has had a bad rap.
Supposedly, it is one of the main culprits for:
- Diabetes type 2
- Increased blood pressure
- Increased uric acid levels
This is why we often hear that sugar is best avoided.
And yet, top-level athletes consume large amounts of sugar during exercise without suffering from any of the above. As a matter of fact, we actually promote a high carbohydrate intake to athletes.
Sugars and carbohydrates
Sugars are the basic unit of carbohydrates.
There are three basic sugars (i.e., monosaccharides):
Carbohydrates are chains of these three monosaccharides, differing in length, branching, and distribution of these sugars.
Carbohydrates should represent the majority of your daily energy intake.
Carbohydrates are present in many foods, such as fruits, pasta, bread, rice, and potato.
The longer and more branched the molecule is, the longer it takes for sugars to end up in your bloodstream.
Since usually you don't want to raise your blood sugar levels quickly, you are advised not to consume many free sugars and stick to branched carbohydrates. This is why food labels include the value of free sugars next to the carbohydrate content.
The most common sugar in food is glucose. Galactose is found in dairy products, and fructose is found in fruit.
The sugar most negatively commented is fructose. Some even call it "alcohol without the buzz". This is due to its unique metabolism, that is what happens in your body once you ingest fructose.
Fructose vs glucose
Even though fructose has the same chemical formula as glucose (C6H12O6), due to their different structure they act differently in your body.
While glucose can be directly used by pretty much any tissue in the body (brain, muscle, heart), fructose is useless for most cells, as they don't posses the key enzyme to break it down. In simple terms, these cells don't know what to do with fructose.
The fructose you ingest ends up in your liver, as the liver is the only part of your body that contains the necessary enzymes.
Even though fructose is a sugar, unlike glucose it has a low glycemic index.
The glycemic index is a value that tells you how quickly glucose levels rise in the blood after ingestion.
Usually, complex carbohydrates have a low glycemic index, while most sports drinks have a very high one, as they contain high amounts of glucose.
Ingestion of fructose raises blood sugar very slowly because it must be first metabolized into glucose in the liver.
For this reason, fructose was for a long time recommended to people with diabetes, and it was called "sugar for diabetes patients".
Today, fructose consumption is often linked to the formation of fatty liver, as a high amount of fructose is supposed to turn into fat and stay in the liver where it causes health issues.
Long term, this is supposed to lead to diabetes type 2, increased risk of cardiovascular disease, and some other diseases.
The public image of fructose thus changed substantially over the years - for the worse.
On the other hand, fructose-rich fruits have always been a part of people's healthy diet, and we have been consuming fruit since forever. This might make it obvious that fructose's bad rap is unjustified.
Furthermore, sports nutrition scientists have been discovering for the last 20 years that ingesting fructose in certain cases is even recommended for athletes.
Why is fructose supposed to be bad for your health?
The main reason some people claim fructose is unhealthy is due to its strong connection with the use of high-fructose syrup in the United States and the development of type 2 diabetes.
High-fructose syrup is rarely seen on the European market, as manufacturers in Europe mostly use table sugar (sucrose) to sweeten their products.
The composition of sucrose and high-fructose syrup is actually pretty much the same though, as the latter mostly consists of half fructose and half glucose, the exact composition of sucrose.
Fructose is not found only in foods that contain added sugars, it is also found in vegetal food, mostly fruit. This is why fructose if also known as fruit sugar.
And this leads to the first and extremely important paradox.
Fructose is supposed to be bad for your health, but fruit, rich in fructose, is supposed to have a positive influence on your health.
This might lead to the conclusion that fructose by itself is not a problem. The problem might arise from the form and quantity that enters your body.
This is the main reason why observational studies are not always to be trusted. Or, to put it differently, we shouldn't draw conclusion too quickly from studies that seek connections between individual things.
High quality clinical studies that actually research the influence of fructose on health must be performed. Fortunately, there have been quite a few of those.
Fructose in experimental studies
Most studies that researched the influence of fructose on human health fed extremely high doses to its participants.
Often, participants were given fructose in bags with instructions to drink it dissolved in water.
Furthermore, most of these studies surpassed the recommended daily energy intake, as the researchers wanted to emphasize the influence of fructose. In other words, the participants were following a weight-gain diet.
These studies concluded that fructose can be harmful, and that it is more harmful than branched carbohydrates and also glucose.
But such high amounts of fructose and excessive energy intake are not ordinary, which is why the findings of these studies cannot be simply applied to practice.
On the contrary, studies which researched the influence of fructose in conditions where the participants consumed as much energy as they actually used in a day did not find any negative influence of fructose on human health. Especially when coupled with exercise.
Moreover, lately an increasing number of scientists are coming to the conclusion that fructose is in certain cases recommended for athletes.
Metabolism of carbohydrates
Once they enter your body, carbohydrates are broken down in the body into simple sugars - glucose, fructose, and galactose.
The difference between consuming branched carbohydrates and simple sugars is actually only in how quickly they enter your bloodstream, which is also affected by other nutrients in your food, such as fats and fiber.
Afterwards, these basic sugars, assisted by cell transporters, are transported from the digestive system into your bloodstream, and from there into your tissues.
Carbohydrate stores in the body are very limited. Some carbohydrates are stored in the liver and some in the muscles.
Depletion of either of these stores leads to fatigue, often described among endurance athletes as "hitting the wall".
In order to prolong its occurrence or avoid it completely, athletes consume carbohydrates during exercise, usually in the form of energy gels, energy bars, or carbohydrate drinks.
These supplements mostly consist of simple sugars in a combination of glucose and fructose.
Research shows that the body can only absorb up to 60 grams of glucose per hour, which represents around 240 calories of energy.
Glucose and galactose use different cell transporters than fructose.
In theory, this means that the absorption of glucose and fructose is higher than glucose alone.
This was confirmed by scientists, who discovered that the human body can absorb 60 grams of glucose and 30 grams of fructose per hour.
This leads to increased energy availability during exercise, resulting in better performance.
This lead to the development of Nduranz Carbohydrate Ratio.
Fructose and lactate
Glucose in the bloodstream can be used by muscles, brain, and other organs, while fructose must be transformed in the liver into glucose or lactate before other tissues can use it.
Yes, you read that correctly. Fructose is also transformed into lactate, which many people relate to fatigue.
In fact, lactate is being treated unfairly. Similarly to glucose, lactate can be used for energy and is definitely not a redundant byproduct.
When stationary, a large part of fructose is used to form liver glycogen. During exercise, most fructose is turned into glucose and lactate.
Glucose and lactate are then oxidized in the cells and energy that is released along the way can fuel the cellular processes (e.g., physical work).
Only a low percentage of fructose ends up in the form of fat, which clearly shows the silliness of the hypothesis that consumption of fructose leads to a fatty liver when the energy intake matches its daily consumption.
More research in favor of fructose
Scientists found two more examples when fructose is recommended for athletes.
As mentioned, during inactivity fructose is transformed in the liver into liver glycogen, while part of it is transported into the other cells in your body in the form of glucose and lactate.
After intense exercise or when you are fasted, liver glycogen stores are greatly depleted.
New research shows that in this case fructose replenished liver glycogen stores much faster than glucose, which results in faster recovery after intense exercise.
Nutritionists used to recommend an intake of glucose-based carbohydrates to stimulate recovery after exercise, but today we know that a combination of glucose and fructose leads to better results.
Fruit products are an excellent source of nutrition during the recovery period and also an efficient breakfast before an exhausting day.
All things considered, fructose is not as problematic for your health as some people claim.
This doesn't mean that you should consume it in its pure form. On the contrary, if you are not active enough, you are much better off consuming complex carbohydrates, as the food that contains them also contains other important nutrients.
But talking about endurance athletes, whose daily carbohydrate intake can exceed 12 grams of per kilogram of body weight, fructose can be recommended as a source of carbohydrates, as it improves endurance and recovery.