I hate this commercial. As a science journalist, and a generally curious person, I always want to hear and understand the complicated explanation. It’s hard for me to hear the tone of the commercial as reassuring, instead of condescending.
I don’t think people need partial, glossy truths. I believe that more often than not oversimplifying a topic just creates confusion. But too much information is equally confusing. It’s a tricky thing to balance clarity, accuracy, and simplicity when explaining science. And I think that when discussing dietary sugar, the simple explanations aren’t doing us any favors.
One of the challenges of writing about food and food science is that nutrition is such a big part of both topics. Last year an editorial in the journal Nature talked about the power of simple messages and the issues with loss of detail for the sake of sound bites in nutrition:
“It is risky to oversimplify science for the sake of a clear public-health message. . . . [S]imple messages and themes are seductive. . . . Black-and-white messages can cause confusion of their own.”
I’m not a nutritionist, or a dietician, nor do I want my writing to be prescriptive. But I do want it to be clear. So, with that very large caveat, here is some of what we do and don’t know about the difference between how our bodies process added sugar versus intrinsic sugars.
Added sugars vs. intrinsic sugars
Sugar is important. Sugar provides fuel for cellular processes and body functions. Sugar is delicious and we are programmed to enjoy it.
However, like other carbohydrates, sugars aren’t considered essential nutrients. (Essential nutrients are molecules that the body can’t create by itself or are needed in large amounts to maintain health.)
Americans get the bulk of their added dietary sugar in two different forms; sucrose (table sugar) or high fructose corn syrup (HFCS). Processing sugar cane and, more recently, sugar beets yields sucrose. Corn, naturally, is processed into HFCS.
High fructose corn syrup first emerged in the 1960s, thanks to technological advances creating the capacity to convert corn-derived glucose into fructose. Despite it’s name, the resulting syrup is composed of almost equal portions of glucose and fructose.
Sucrose consists of the two simple sugars glucose and fructose. Glucose and fructose have same chemical formula but they differ slightly in structure and greatly in sweetness. Fructose is twice as sweet as glucose.
Both sucrose and HFCS are composed of the same approximate 1:1 ratio of glucose and fructose. But, again, the significant difference between the two is structural. Sucrose: glucose and fructose are chained together. HFCS: glucose and fructose are unconnected and free floating.
During digestion sucrase, an enzyme in the intestines, breaks sucrose into glucose and fructose for processing.
When we digest sucrose the enzymes in our small intestine must break the sucrose molecules into glucose and fructose to be used for energy. At first glance, it looks like the sugars in HFCS would be absorbed faster the simple sugars in sucrose (since HFCS basically skips that digestive step). But because sucrose is converted to glucose and fructose in just a few seconds, HFCS and sucrose appear to behave the same way metabolically.
After converting sucrose (and not converting HFCS) the glucose and fructose in the small intestine is then ready to be absorbed into the bloodstream. But first both simple sugars head to the liver and here is where their paths diverge.
Before letting glucose flood the bloodstream, an specific enzyme (phosphofructokinase) checks to make sure that the liver has enough energy. If the liver doesn’t need energy, it sends the glucose into the bloodstream.
Our bodies exert strict control over the amount of glucose in our blood. Glucose is basically our default “fuel”. It used by every cell in body and is especially important for brain function. The brain uses up to 20% of the energy used by the human body, and can only use energy in the form of glucose. When your blood glucose levels drop, so does brain function.
Too much glucose in the bloodstream is also a problem. Over abundant blood glucose signals the pancreas to secrete insulin which removes excess glucose from the blood. Excess glucose also promotes the release of leptin, a hormone which suppresses feelings of hunger.
Fructose’s first stop is also the liver. Fructose is not immediately useful to most cells, and is largely metabolized through the liver, where it is converted into glucose and lactate. But unless you have no energy reserves at all and energy is needed elsewhere in the body, the fuel created from fructose seems to stay in the liver (even when it doesn’t need that extra energy). In the liver, some fructose might be converted into glycogen for short-term storage, and some fructose is changed into fat for long term storage.
Unlike glucose, fructose doesn’t promote insulin production. Instead it promotes the release of grehlin, a hormone that reinforces feelings of hunger. This has led some researchers to suggest that ingesting large amounts of fructose might lead to overeating.
A recent trend has been to say that all sugar is toxic, when in fact most researchers who are pushing that message are only referring to fructose. There are many studies that look at individual harmful aspects of fructose metabolization, and it can seem as though fructose is inherently harmful.
But most experts seem to think that fructose is not inherently harmful and what is more important is the amount of sugar in a person’s diet as a whole and the form the sugar is consumed in.
Drinking a soda is a classic example of ingesting “empty” calories. A coke provides a lot of energy with very little digestive effort, zero fiber, and almost no nutrients. Very shortly after drinking a soda sweetened by either sucrose or HFCS, fructose floods the intestines and is shuttled to the liver to be broken down.
But eating an apple requires significantly more digestive energy than harvesting free sugars from a soda. The fiber (cellulose) structure of the fruit slows down digestion while digestive enzymes break down cell walls to get to the sugars inside. It takes time for the freed fructose from the fruit to reach the liver, and it arrives in more of a trickle than a flood.
So it’s not just about the actual sugar content, but the structure of the food. Added sugars, which aren’t part of the cellular structure of a food, are absorbed almost instantly by our bodies. While intrinsic sugars, like those in fruits, take more time and effort to harvest.
Finally, foods with intrinsic sugars are usually more nutritionally dense and lower in calories, making them a better choice for your daily sugar quota overall.
Header Image Credit: Hello Sugar, Martin Wright
Sucrase: Courtesy Clarie Hollenbeck, PhD and the Department of Nutrition and Food Science at San Jose State University via WikiMedia