Can someone tell me the difference (if there is one) between the endogenous glycation (through the Maillard reaction) that takes place when sugar is ingested compared to searing a steak (as in the browning reaction) which is also supposed to be detrimental to health? If health compromising glycation occurs when sugar and proteins cross link then how is browning of fat or meat where there is no sugar comparable and equally destructive?
My take on this (as a chemist, not a biochemist or a nutritionist) is that endogenous glycation is bad - it's irreversibly binding YOUR proteins to a sugar and makes them not good at their job (whatever that job was when it was a free protein). I don't think exogenous glycation (browning of meat, e.g.) could be bad, as the reaction is already done, i.e., you've already IRREVERSIBLY bound the meat protein into another compound. It's not going to be re-reacting in your body, as glycation is irreversible. A chemist would say that the glycated protein is much more stable and unreactive. So I can't see a problem with eating glycated proteins. However, when browning meat you could get other bad thingslike PAHs and whatnot, but I'm not including them in this discussion.
I was actually wondering about this myself recently, looking for an excuse to say BBQ is ok, but after considering it, here's what I conclude:
We've had fire for about a million years, or possibly more. Presumably, we used fire to cook meats with, and may have developed immunity to exogenous AGEs because of the time span.
While the gycation process may produce free radicals, as that process occurs externally, its free radicals would react with whatever they can from the environment (air, CO2, Carbon Monoxide from the fire, melting fats from the meat, etc.), before we eat them. Glycation does damage protein, but the danger we care about is the damage they cause to our own proteins, and not the damage done to the surface of the meat we're about to digest anyway.
As miked notes above, any highly reactive components are likely to first react with the proteins, sugars, and the fats from the meat itself, or the environment before we take our first bite, and any remaining, with our saliva as we chew them (and of course, possibly skin cells in our mouth, esophagus), and the acid + digestive enzymes in our stomachs before being absorbed in our intestines.
And of course we can't eat meat hot off the spit, or grill, because it would burn us, and so we have an incentive to let it cool, plus it tastes better if we let it rest for 5 minutes to let the juices redistribute, instead of running out. And some of us may add a pat of butter, or A1 sauce, etc. So there's plenty of opportunity for those free radicals, and AGEs to react with something else, and thus become stabilized, and no longer dangerous.
So I doubt that there's much danger in BBQ. So I'd be more worried about consuming carbs, which get broken down into glucose, and this is reactive to our proteins (endogenous AGEs), than I would about steak or BBQ.
Another clue is that steak tastes better to us, than boiled, steamed, or raw meat. It's as if our senses are telling us that it's a better source of nutrients, because it's easier to digest since the connective tissues have already been broken down.
I think that as long as we don't fool our sense of smell/taste with neolithic agents of disease, we can trust them.
The problem is that our bodies do have receptors (RAGE) for AGEs, which are equally activated by endogenous and exogenous AGEs. RAGE triggers inflammatory pathways that will injure the kidneys and pancreas (this is why some say protein is bad for the kidney, especially protein from animal sources). It is not necessarily protein that is bad for kidneys but the AGEs that are formed from cooking meat with high and dry cooking methods are the specific culprit.
Unfortunately our genes don't have much use for our bodies after our offspring have reached sexual maturity, so I don't think you can fall back on well if it taste better it must be preferred by the body philosophy.
Exogenous AGEs is likely why saturated fat is implicated in heart disease. Saturated fat is usually paired with a cooked animal protein. However the health message was to just reduce saturated fat, when really cooking methods should have been altered. Most fast food is cooked twice (high in exogenous AGEs). Baby formula is very high in AGEs. All school food is baked or fried processed proteins. Pizza (broiled cheese) is very high. Fried Bacon is very high (however microwaved is not). Hamburgers and fried chicken are top of the list too.
One of the main amino acids that undergoes glycation is lysine and it happens more readily in an alkaline environment. To reduce the amount of AGEs formed when cooking meats, marinate in an acidic liquid like vinegar or lemon juice.