Naturally occurring monosaccarides

What monosaccharides occur naturally (in humans or human diet)? This has been surprisingly difficult to Google. Also, what monosaccharides are essential for human survival? A friend asked me to read about a questionable medical supplement called “glyconutrients,” and I want to know just how much of what its proponents say is crap.

Right off the top of my head: glucose, fructose, galactose, ribose all occur naturally. Glucose is the primary (if not the sole) energy source for the brain as well as the muscles and most of the other parts of the body; it’s the monosaccharide transported by the blood. Fructose is found in fruits and many other foods. Galactose is found in milk, in combination with glucose as the disaccharide lactose. Ribose is a component of RNA; its derivative deoxyribose is a component of DNA.

A quick trip to Wikipedia tells me these are naturally occurring monosaccharides:
[li]aldohexoses: glucose, galactose, mannose[/li][li]aldoketoses: fructose, psicose, sorbose, tagatose[/li][li]pentoses: ribose, arabinose, xylose, ribulose, xylulose[/li][li]tetroses: erythrose, threose, erythrulose[/li][li]trioses: glyceraldehyde, dihydroxyacetone[/li][/ul]

As to which ones are essential, I believe none of them are, as the body is capable of synthesizing them from other substances, like fats or proteins, or extracting them by breakdown of complex carbohydrates like amylose.

This article, A Friendly Skeptic Looks at Glyconutrients and Ambrotose®, may give some insights. While glycobiology is a legitimate field of medical research, there seems to be little solid evidence of many of the claims made by its lay proponents.

You could make a really good argument for glucose being essential. In order for your body to digest proteins and fats, you need some of the compounds made when your body breaks down glucose. There is also an efficiency issue. When one glucose molecule goes through the metabolic process, you get a bunch of ATP (ATP is what your cells use as energy), when you break down protein or fat without glucose, you get a few ATP. I don’t have the actual numbers, but a biochemistry textbook might help you out with the details. I have Stryer, but not here with me.

None. Glucose is essential because it is the part of basic energy pathway of the body, but all disaccharides and polysaccharides eventually break down into glucose in the body.

In fact, fructose is the only naturally occurring monosaccharide that is at all common. The others are found only rarely: galactose and glucose separately are found in a few plants; mannose can be processed from the manna ash tree; and glucose and fructose mixed together make what is sometimes called “invert sugar” but is naturally known as honey.

Lactose is glucose + galactose and is found only in milk. It’s then broken down in the small intestine by the lactase enzyme to the monosaccharides. The galactose is then converted to glucose:

Other reaction paths are also given on that page.

Similar reactions turn sucrose (glucose + fructose) into glucose. Maltose (glucose + glucose) and starches, which are polysaccharides consisting of longer chains of glucose, are whittled down to their component glucose molecules.

Let me borrow from their “science” and say that 157.928275462521936% of it, exactly, is crap.