Sugars are not all sweet stuff: Part 3
It is somewhat curious as to why the body has elected to use glucose as its primary source of energy. After all, fructose and galactose also have the same chemical composition - but it is glucose which has been selected for glycolysis.
There is, of course, an evolutionary reason for this and although it is a good one, it still leaves room for some possible errors during the processing of glycolysis.
The story starts with the fact that all monosaccharides are reducing sugars. As mentioned earlier, the molecules in these sugars are prone to losing an electron to any other compound capable of accepting it - it's a process called oxidation.
In normal conditions (ie. without the involvement of enzymes), a reducing sugar would give up an electron to bind with an amino acid - this is a process called glycation. However, glycolysis does not involve glycation as the entire process is carefully managed by a series of enzymes.
Not all reducing sugars are the same in terms of reactivity. Fructose is particularly good at reacting with other compounds - this is because the molecular structure of fructose is an open chain which means there are more points to which another compound can bind.
Galactose has a slightly more complex structure than fructose as it can exist in two configurations - one is an open chain and the other is as a cyclic (ring-shaped) molecule. The open chain form of galactose is as reactive as fructose.
Glucose, on the other hand, exists mostly as a cyclic molecule, although a very small proportion of glucose molecules are also open chains. In aqueous solutions, only about 0.25 per cent of glucose molecules exist as open chains and hence it has a much lower tendency to react blindly with other compounds. It is this relative stability which makes glucose much easier to manage during a complex process like glycolysis - hence, glucose is very much the preferred energy source of all mammals today.
As mentioned, glycolysis happens in cells all over the body and once in a while, a cell encounters the rare open chain form of glucose. Processing this open chain glucose molecule may cause damage to the particular cell handling the glycolysis - normally all such damaged cells are then swept up and destroyed by the body's defence mechanisms.
Glycation - the inside story
Although the wider public may not be wholly aware, the scientific community has had evidence since the last century that ingesting large amounts of either fructose or galactose causes accelerated ageing, at least in test rats - though subsequent studies have provided more evidence that the same effect also happens in humans.
This is simply because open chain monosaccharides tend to form strong molecular bonds with amino acids during glycation. Proteins are large bundles of amino acids and are therefore significantly harmed by glycation - and damaged, inelastic or non-functioning proteins are a symptom of ageing. As an example, the skin of an aged person is seldom as smooth, elastic and firm as the skin of a young person.
Glycation within the body is known as endogenous glycation and generally happens rather slowly. Via a process known as the slow Maillard reaction, endogenous glycation eventually leads to the production of Advanced Glycation End-Products (appropriately known as AGEs).
Many AGEs are either removed from the body or simply remain within their incapacitated cells and are relatively benign - but a small subset may actually become more reactive than the open chain sugars that formed them. This can happen when the AGEs mess up the normal functions of some cells, which can cause the damaged cells to produce and release highly oxidising compounds such as hydrogen peroxide and other free radicals. These oxidising compounds can then spread and damage other cells in the body.
AGEs have been implicated in a wide spectrum of diseases ranging from diabetes to heart disease and cancers. The hardening of the lenses in the eyes of old people is also a result of glycation - lenses contain long-lasting proteins called crystallins and over time, they will get damaged by glycation. Other long-lived cells such as nerves, brain cells and the beta-cells (insulin production cells) in the pancreas also tend to be affected by AGEs.
The problem is also compounded by the nature of the AGEs themselves - AGEs which are formed by the binding of a single amino acid to a single sugar molecule can be detected and cleared by the body relatively easily via urine.
However, AGEs formed by the multiple bonding of amino acids and sugars are not so easily removed and may therefore persist in the body for a very long time.
Glycation - the outside story
Glycation and the formation of AGEs are exponentially accelerated by the general Maillard reaction, which happens when food is cooked using dry heat (eg. frying, baking, grilling, etc) at temperatures between 140 deg C to 190 deg C or so.
The type of glycation which occurs in dry heat is known as exogenous glycation. Most of you might recall from an earlier article that the Maillard reaction is what causes the browning of food under heat - and that the flavours of food can be considerably enhanced as a result of the browning. That is the good news.
The general Maillard reaction is a rather complicated chemical process, but at the simplest level, it can be described as the heat-induced binding of reducing sugars with amino acids - and amino acids are large integral components of cells, muscles and other tissues. The reducing sugar involved is a significant factor in the reaction - the more reactive the sugar, the faster glycation will happen with a resulting increase in the number of AGEs.
The quantity of AGEs produced is a function of the rate of glycation which would be determined by (a) the available amount and type of reducing sugars, (b) quantities of amino acids present and (c) degree of heat applied.
The bad news is that fructose is particularly conducive to promoting the rate of glycation. Studies have indicated that the open chain structure of fructose can cause the formation of up to ten times as many AGEs compared to glucose. Galactose is also several times more reactive than glucose in terms of forming AGEs.
However, as flavour is improved by the general Maillard reaction, and AGEs are also part of the resulting flavour molecules, it is not uncommon for fructose to slip into commercial cooking sauces and marinades as this sugar can improve the flavour of the resulting cooked meats.
In general, it is probably true that processed cooked foods can contain more AGEs than home-cooked food. This may be because of the ingredients used and also the higher temperatures used to prepare commercial foods.
Once ingested, AGEs from dry cooking processes can affect the body adversely in more or less the same way as AGEs formed by endogenous glycation, even though dry cooking AGEs are often different compounds.
At this point, it has to be added that the human body has probably evolved to adapt to the AGEs arising from exogenous glycation - this may be a result of several hundred thousand years of cooking with fire during our evolution. An example may be an AGE called acrylamide - this is an established carcinogen for test laboratory animals.
However, the toxicity level for humans is still somewhat unclear - and it is feasible that humans have adapted somewhat to eating cooked foods containing acrylamide while other animals haven't.
If you're wondering about what acrylamide actually is, you can see it yourself in the brown bits on fried potato chips or any starches severely browned by dry heat.
And if you want to avoid acrylamide, then you should avoid cigarette smoke as a study has indicated that tobacco smoke can increase blood acrylamide levels very significantly.
As an aside, whenever possible, I would give my dog good quality raw meat rather than cooked pet food - this is a way to cut down on feeding him AGE compounds which would not have existed in a canine's natural diet. As far as I know, dogs (and all other animals) have not been cooking their meals over fires for several thousand generations - cooking food is a uniquely human activity.
Years ago, I saw a packet of chewing gum blazing the word "Xylitol" across the packaging. Initially, I thought it was some form of breath freshener but on investigation, it turns out that xylitol is actually a sugar alcohol, which is a class of polyols.
This was somewhat intriguing as polyols are primary components of synthetic materials such as polyurethane and I was curious if xylitol was present as a texture ingredient.
It turns out that xylitol is actually a sweetener and as it is not broken down by saliva, the sweet sensation from xylitol can persist longer in the mouth than with gums with normal sugars.
It is also less sweet than sugar so it is often combined with other sweeteners - and because it is also unaffected by bacterial activity, it does not contribute to tooth decay, and in fact might actually improve dental hygiene as a result.
One curious effect of xylitol is that it also appears to promote the sensation of cooling in the mouth, like a cool mint.
There are other sugar alcohols used in food. Some examples are maltitol, sorbitol, erythritol and isomalt. All these are used in commercially-produced foods and may help in diets for diabetics as they have much less impact on GI than sugar. DNA and sugar
It may be curious that meat can brown on a hot pan or open grill when there are no other ingredients added. The browning is clearly due to the Maillard reaction, but where does the reducing sugar come from?
It turns out that meat itself has its own sugar - or rather the DNA of the meat has the sugar. DNA is the acronym for deo xyribonucleic acid and the structure of DNA is a double helix consisting of two strands of chained nucleotides.
Without going into great detail, a nucleotide is basically a sugar-based link holding molecules of either cytosine, guanine, adenine or thymine. The sugar involved in DNA is ribose (or more accurately, deoxyribose) which is a monosaccharide (notated chemically as C5H10O5) - and this sugar is released when the DNA breaks down under heat.
As all monosaccharides are reducing sugars, ribose is used by the general Maillard reaction to bind with the amino acids in the muscle tissues when cooking meat by itself.
Coffee, dimers and sugar
A recent piece of research from the University of York has de-termined the probable reason why many people like to add sugar to coffee, especially bitter coffee.
The bitterness in coffee is mainly due to caffeine and people usually compensate for the bitterness by adding sugar.
However, the York research indicates that the bitterness of caffeine is actually not "masked" by the sugar or something simple like that - it turns out that sugar actively promotes a somewhat unusual chemical process called dimerization on caffeine.
Simply stated, sugar causes caffeine molecules to pair up and bind together as dimers and this has the effect of tricking the taste cells to taste such bound pairs of caffeine molecules as single molecules, therefore significantly reducing the taste of bitter. Natural sweetener?
This last part is about stevia, the new wonder natural sweetener. Normally, stevia is produced by extraction from the plant, Stevia rebaudiana.
The active ingredients are the two steroid glycosides (stevioside and rebaudioside) which are around 150 times sweeter than table sugar.
Despite a ban on stevia imposed by the FDA since 1991, there is no real evidence that it is toxic to humans in normal quantities.
The funny story is that stevia was initially banned probably due to the lobbying of the artificial sugar industry which was horrified about the new competition, and had actually sent an industry complaint to the FDA bemoaning the potential toxicity of stevia. However, when major food companies themselves discovered a cheap method to produce huge quatities of stevia-based sweeteners, they immediately lobbied for a lifting of the ban - and so in 2008, the FDA allowed stevia-based sweeteners to be sold in the United States.
One of the main selling points of stevia is that it is a "natural" sweetener. However, some of the commercially-sold stevia sweeteners actually have relatively small amounts of the steroid glycosides from the plant - mainly these new sweeteners contain other sweeteners not derived from the stevia plant and in some eyes, would hardly qualify as a wholly "natural" product.
I mention this because the price of stevia sweeteners is currently considerably higher than other sweeteners and there may be no justification for this if most of the ingredients are not derived naturally.