[David Cooper (OP)]

I've often heard that you should chew food thoroughly in order to let saliva start the digestion process, but I don't see how it can have any significant impact in comparison to the battery acid in the stomach which goes to work on the food moments later. Is its real purpose just to dissolve soluble molecules in the food so make it possible to taste them in order to judge the quality and to determine things like how much sugar might be in them?

Also, how long does it take before any sugar in the food can actually be used by the body? Does it get through the wall of the mouth, throat or stomach to any significant degree (in the way cyanide clearly can) or does it have to wait until it's gone beyond the stomach?

[SeanB]

Saliva does contain enzymes that are activated by the stomach acid, and this enhances the digestion of food when it hits the stomach. Chewing also ensures all food particles are wetted with saliva, making digestion faster, as there is mor surface area and more area of wetted surface to be attacked with digestive enzymes.

Some molecules ( like asprin) are also dissolved and absorbed through the mouth and throat, but the majority is done in the small and large intestines, where there is a deliberate large surface area with active molecular transport in the lining for desired nutrients.

[chris]

Saliva contains an enzyme called ptyalin, which is a realtively impotent cellulase (capable of breaking down starch); thus, during the mastication (chewing) process it will begin to release some glucose, which can be absorbed orally - people going into a diabetic hypoglycaemic attack can be effectively revived by massaging glucose or sugar syrup into their gums and oral mucous membranes.

However, your point is well made, that salivary amylase (ptyalin) is a very minor contributor to digestion up against the cocktail awaiting your lunch further down the digestive tract.

However, some people have suggested that its role might be that of a "chemical toothpick", to dissolve out plant matter lodged between the dentition. But if that's the case, one wonders where is the "cellulase" - to degrade tougher, cellulose-rich plant husks - and proteinase - to get rid of the last meat meal - have got to?

[David Cooper (OP)]

It seems odd that saliva contain enzymes that have to be activated by the stomach acid as they might as well just be in the stomach acid to start with, so is the science of that sound? Could it just be a coincidence and that they may perform some other role before that?

Another thing I've heard is that when you eat something sugary and get a boost of energy, most of that energy comes not from absorbing that sugar directly, but from the body releasing more energy (sugar/glucose?) into the bloodstream on the basis that it will be able to replace it from what's being eaten (with taste having a role in informing it as to what's on the way [and which may not be delivered if artificial sweeteners are involved]). How much truth is there in that, and does this also apply to with diabetic people when they eat something sugary to avoid a collapse or do they rely heavily on absorbing the stuff before it's reached the stomach where there could be a long delay before any more can be accessed?

[chris]

No, you can definitely absorb sugar through the oral mucous membranes, which is the reason that I cited the case of a diabetic experiencing a hypo as evidence that this can increase blood sugar rapidly.

Regarding ptyalin - the oral starch-degrading enzyme, this does not require stomach acid to be active; it is active immediately upon release. You are thinking of stomach pepsin, which depends upon the acid environment of the stomach to be active.

[David Cooper (OP)]

Okay, but how much truth is there then in the idea that the body releases more energy in response to eating something sweet in anticipation of being able to absorb sugar from that food later once it's left the stomach? Some, we have established, is absorbed before it reaches the stomach, but most of it will likely be trapped inside the food as it's swallowed and will likely not be available to use as energy for some time if it gets held up in the stomach, but I know, for example, that glugging down a drink with glucose in it gives a powerful boost when you're running out of steam on a long cycle run. What proportion of that boost comes from directly absorbing it before it's reached the stomach and what proportion of that boost might actually come from energy released elsewhere in the body as a response to the drinking of that sugary stuff being detected on the way through the mouth?

(And I'm not expecting anyone to research this for me to find out, but merely asking in the hope that someone already knows the answer.)

Thanks for clearing up the bit about ptyalin and pepsin. (I was not thinking of either of those, but was taking on trust the first sentence of reply #1.)

[chris]

There are certainly "anticipatory" responses to eating and drinking. You feel less thirsty the moment that a drink hits the back of your throat, despite none yet being absorbed systemically. It's the same with food. The taste system integrates its responses to what you put into your mouth with memories of how many calories those sorts of flavours return, on average; this is subtracted from the hunger signals coming from elsewhere in the brain. This puts in train a host of other responses, including hormone release, that gear metabolism to receive the calorie burden. This almost certainly accounts for part of the rush that accompanies eating.

[Tomassci]

Saliva kills bacteria, breaks carbohydrates. That is my answer