[thedoc (OP)]

Adam Waller  asked the Naked Scientists:
   
I'm on a diet and tracking my caloric intake as well as other nutritional information. I notice that I always seem to go over on my sugar intake when the only sugars that I am getting are from fruits. Can the body tell the difference in the sugars from fruits and those from candies and other sweets??

Thanks,
Adam Waller
South Carolina, USA

What do you think?

[Lmnre]

Can the body tell the difference in the sugars from fruits and those from candies and other sweets?

The simple answer is yes. Let's take a closer look at our digestion and sugars.

Of sugars, our digestive system can absorb only simple sugars, called monosaccharides. If you break the word monosaccharide into its pieces, mono-sacchar-ide, you'll see mono (one), sacchar (sugar), and ide (class of compounds). So, monosaccharides are a class of compounds composed of single sugars. This class includes fructose, glucose and galactose. These names indicate they are sugars by their -ose endings. These monosaccharides are C₆H₁₂O₆ molecules of different configurations (thus the different names).

Humans have evolved to identify fructose as the sweetest of all sugars. Fructose is so named because it is found in fruits, and both words derive from the Latin fructus meaning enjoyment or profit. Fruits develop mostly containing glucose, a less sweeter sugar. Part of the ripening process then converts some of the glucose into fructose, and thus, the fruit turns from a not so sweet, unripened fruit into a sweeter, ripe fruit. Thus, a ripe fruit appeals more to humans than unripened fruit.

Our bodies metabolize fructose mostly through the liver. Glucose, for example, stimulates the pancreas to produce insulin, which stimulates cells to absorb glucose from the blood. Brain cells use glucose for energy, so a diabetic with low blood glucose can suffer a diabetic coma because the brain loses its energy source. However, the body can produce glucose, and people don't necessarily need to eat foods containing glucose.

Anyway, there are other sugars called disaccharides and polysaccharides (di- means two, and poly- means many, and I'll let you figure out what class of compounds described by these words). Sucrose is what we commonly called "table sugar", and it's a disaccharide along with lactose found in milk. Polysaccharides is a class of compounds of structures of three or more simple sugars ... what we usually call starches, which we can digest, but also other compounds such as cellulose that we cannot digest.

Basically, if we can't break a disaccharide or polysaccharide into simple sugars, we can't absorb them. This is one way our bodies tell them apart. We break down disaccharides and polysaccharides using enzymes specifically for that job that are called by the complex sugar they break up. For example, sucrase is the enzyme that separates sucrose into its fructose and glucose monosaccharides. We know this name is of an enzyme because of its ending -ase. A problem suffered by some people is often called "lactose intolerance", when more technically, it's "lactase deficiency". If you guess that lactase is the enzyme that breaks lactose (milk sugar) into its glucose and galactose monosaccharides, then you're right. When our bodies don't produce enough lactase to break down the sugar in milk, we do not digest it, and it passes into the large intestine, where bacteria eat it all up, producing gas in the process, and causing cramps, gas, diarrhea, etc. Nasty. When you see milk in the stores for "lactose intolerant" people, the maker has added lactase to it which has "digested" the lactose into its two sugars. It's partly digested milk, which might sound gross, but really isn't.

High fructose corn syrup (HFCS) is interesting. Corn syrup (at least in the US) is mostly glucose, and not so sweet. HFCS is produced by converting some of the glucose into fructose, thus making it sweeter to our tongues AND easier to digest than regular table sugar (which is a fructose-glucose disaccharide). This is because the sugars are already in their simple form and there's no need to break them down. So they're absorbed somewhat faster than, say, sucrose (table sugar). A friend claims he can tell the difference between two versions of a popular brand of soda, one with sucrose and the other with HFCS. I don't know.

[Bored chemist]

The practical answer is not much.
There are not many disaccharides in our diet.
One is sucrose- ordinary table sugar from cane or beet, which  is rapidly split into glucose and fructose.
The body can absorb and use those.
The other is lactose- essentially only produced by animals and only present in milk and dairy products. Most people can digest this, and those who do not are aware of it.

Any monosccharides are readily absorbed and used.

In effect, a hundred grams of any sugar  will be converted more- or -less rapidly to glucose in the body. It will, in due course, provide about 400 Calories.
Some sugars are a bit sweeter than others and some take a bit longer to be absorbed. The starches and cellulose that  LMNRE talks of are not sugars.