Monday, November 8, 2010

High-heat cooking will AGE you, if you eat food deep-fried with industrial vegetable oils

As I said before on this blog, I am yet to be convinced that grilled meat is truly unhealthy in the absence of leaky gut problems. I am referring here to high heat cooking-induced Maillard reactions (browning) and the resulting advanced glycation endproducts (AGEs). Whenever you cook a food in high heat, to the point of browning it, you generate a Maillard reaction. Searing and roasting meat usually leads to that.

Elevated levels of serum AGEs presumably accelerate the aging process in humans. This is supported by research with uncontrolled diabetics, who seem to have elevated levels of serum AGEs. In fact, a widely used measure in the treatment of diabetes, the HbA1c (or percentage of glycated hemoglobin), is actually a measure of endogenous AGE formation. (Endogenous = generated by our own bodies.)

Still, evidence that a person with an uncompromised gut can cause serum levels of AGEs to go up significantly by eating AGEs is weak, and evidence that any related serum AGE increases lead the average person to develop health problems is pretty much nonexistent. The human body can handle AGEs, as long as their concentration is not too high. We cannot forget that a healthy HbA1c in humans is about 5 percent; meaning that AGEs are created and dealt with by our bodies. A healthy HbA1c in humans is not 0 percent.

Thanks again to Justin for sending me the full text version of the Birlouez-Aragon et al. (2010) article, which is partially reviewed here. See this post and the comments under it for some background on this discussion. The article is unequivocally titled: “A diet based on high-heat-treated foods promotes risk factors for diabetes mellitus and cardiovascular diseases.”

This article is recent, and has already been cited by news agencies and bloggers as providing “definitive” evidence that high-heat cooking is bad for one’s health. Interestingly, quite a few of those citations are in connection with high-heat cooking of meat, which is not even the focus of the article.

In fact, the Birlouez-Aragon et al. (2010) article provides no evidence that high-heat cooking of meat leads to AGEing in humans. If anything, the article points at the use of industrial vegetable oils for cooking as the main problem. And we know already that industrial vegetable oils are not healthy, whether you cook with them or drink them cold by the tablespoon.

But there are a number of good things about this article. For example, the authors summarize past research on AGEs. They focus on MRPs, which are “Maillard reaction products”. One of the summary statements supports what I have said on this blog before:

"The few human intervention trials […] that reported on health effects of dietary MRPs have all focused on patients with diabetes or renal failure."

That is, there is no evidence from human studies that dietary AGEs cause health problems outside the context of preexisting conditions that themselves seem to be associated with endogenous AGE production. To that I would add that gut permeability may also be a problem, as in celiacs ingesting large amounts of AGEs.

As you can see from the quote below, the authors decided to focus their investigation on a particular type of AGE, namely CML or carboxymethyllysine.

"...we decided to specifically quantify CML, as a well-accepted MRP indicator ..."

As I noted in my comments under this post (the oven roasted pork tenderloin post), one particular type of diet seems to lead to high serum CML levels – a vegetarian diet.

So let us see what the authors studied:

"... we conducted a randomized, crossover, intervention trial to clarify whether a habitual diet containing high-heat-treated foods, such as deep-fried potatoes, cookies, brown crusted bread, or fried meat, could promote risk factors of type 2 diabetes or cardiovascular diseases in healthy people."

Well, “deep-fried potatoes” is a red flag, don’t you think? They don’t say what oil was used for deep-frying, but I bet it was not coconut or olive oil. Cheap industrial vegetable oils (corn, safflower etc.) are the ones normally used (and re-used) for deep-frying. This is in part because these oils are cheap, and in part because they have high “smoke points” (the temperature at which the oil begins to generate smoke).

Let us see what else the authors say about the dietary conditions they compared:

"The STD was prepared by using conventional techniques such as grilling, frying, and roasting and contained industrial food known to be highly cooked, such as extruded corn flakes, coffee, dry cookies, and well-baked bread with brown crust. In contrast, the STMD comprised some raw food and foods that were cooked with steam techniques only. In addition, convenience products were chosen according to the minimal process applied (ie, steamed corn flakes, tea, sponge cakes, and mildly baked bread) ..."

The STD diet was the one with high-heat preparation of foods; in the STMD diet the foods were all steam-cooked at relatively low temperatures. Clearly these diets were mostly of plant-based foods, and of the unhealthy kind!

The following quote, from the results, pretty much tells us that the high omega-6 content of industrial oils used for deep frying was likely to be a major confounder, if not the main culprit:

"... substantial differences in the plasma fatty acid profile with higher plasma concentrations of long-chain omega-3 fatty acids […] and lower concentrations of omega-6 fatty acids […] were analyzed in the STMD group compared with in the STD group."

That is, the high-heat cooking group had higher plasma concentrations of omega-6 fats, which is what you would expect from a group consuming a large amount of industrial vegetable oils. One single tablespoon per day is already a large amount; these folks were probably consuming more than that.

Perhaps a better title for this study would have been: “A diet based on foods deep-fried in industrial vegetable oils promotes risk factors for diabetes mellitus and cardiovascular diseases.”

This study doesn’t even get close to indicting charred meat as a major source of serum AGEs. But it is not an exception among studies that many claim to do so.


H Birlouez-Aragon, I., Saavedra, G., Tessier, F.J., Galinier, A., Ait-Ameur, L., Lacoste, F., Niamba, C.-N., Alt, N., Somoza, V., & Lecerf, J.-M. (2010). A diet based on high-heat-treated foods promotes risk factors for diabetes mellitus and cardiovascular diseases. The American Journal of Clinical Nutrition, 91(5), 1220-1226.


David Isaak said...

I enjoy oven-roasted cauliflower and brussels sprouts--lightly oiled (coconut or olive) and then roasted at 425 F.

But I have to say it never occurred to me to toss them in a deep frier.

Direct roasting of vegetables (usually by burying them under a fire) seems to go back a long way in human history--as does roasting or grilling meat.

I know the raw-food crowd think that fire is the root of all evil.

Me like fire. Fire good.

Anonymous said...

And that Ned, is why you are so good at what you do! Excellent post.

Tom Passin said...

I have always been doubtful that AGEs *in food* would be harmful. That's because it seems that the harmful effects in the body related to AGEs occur when unwanted glycation occurs *in body tissues*. The AGEs in food have already undergone their glycation reactions; they seem unlikely to undergo promote further glycation once ingested.

Kindke said...

I havent done much research into the subject of AGES ( as it relates to meat browning ) mainly because of the below logical fallacy I havent taken it seriously.

The fallacy I refer to is that according to wikipedia, the Maillard reaction occurs between something called a reducing sugar and an amino acid.

As far as im aware meat has no reducing sugars in it, or atleast very small amounts. Further, meat is composed of complex polypeptides, not free amino acids.

Am I missing something here?

Aaron Blaisdell said...

If I was given a forced choice between consuming foods cooked in industrial vegetal seed oils or smoking cigarettes, I'd choose smoking the cigarettes. That's just how bad industrial vegetal oils are turning out to be for human health! Great analysis Ned!

Jenny said...

It's not true that the A1c is a measure of AGEs. It's a measure of the glycosylation of hemoglobin cells but the measurement will rise if red blood cells live longer than normal lives. Conversely, people with anemia will get low values that tell you nothing except that their blood cells are turning over very quickly.

The misinterpretation of A1c test results is very dangerous to people with diabetes. Recent studies find it a very poor index into actual blood sugar levels in individuals.

I'm with you in agreeing that the bad rap should go to the Omega-6 oils used in frying not the food that gets fried.

Ned Kock said...

Tom and Kindke raise very good points that I think need to be addressed by anyone doing research on this exogenous AGEs threat hypothesis.

Another interesting point is that heating generally makes food more digestible. This includes protein-rich foods, through a process called "denaturation".

Ned Kock said...

Thanks Jamie and Aaron.

I guess it is a tough choice between cigarettes and industrial vegetable oils.

Many isolated populations that are free from the diseases of civilization smoke. This includes the longest-living group in the world, the traditional Okinawans.

Having said that, these isolated populations often do not inhale the smoke into their lungs. They seem to keep the smoke in their mouths before expelling it. And incidence of related cancers (e.g., oral cancer) seems to go up due to this.

The Okinawans don't consume industrial vegetable oils. But they consume plenty of pork, which is not so low on omega-6 fats.

Looks like there are quite a few myths that have been creeping in lately.

Ned Kock said...

David, have you heard of the theories by Richard Wrangham? He is a Harvard anthropologist with an evolutionary orientation.

Wrangham believes that cooking dates back to 1.9 million years ago. Well before the emergence of Sapiens.

It is a controversial theory, but there is evidence supporting it.

Ned Kock said...

Hi Jenny.

Even though A1c has its problems, including the ones you mentioned, it may well be the best measure of glycation we have at the moment.

It is a good predictor of various complications associated with the endogenous formation of AGEs. For example, chronically elevated A1c is a good predictor of diabetic retinopathy.

Ned Kock said...

A fascinating area of research is that of glycation (or lack thereof) in birds. I am surprised that there doesn't seem to be much emphasis on finding what birds have that reduces glycation, and how that could be used in the treatment of humans.

For example, the average glucose level for the Anna’s (a hummingbird species) is about 324 mg/dl. Even at that high level, well above the level of a diabetic human, the Anna’s hummingbird species has an HbA1c of less than 5, which is lower than that for most insulin sensitive humans:

And that is not due to the Anna’s "running on sugar" only. Falcons are carnivores and have very high blood glucose levels too. This seems to be a common trait of birds. And birds tend to live longer than mammals of similar size.

David Isaak said...

Thanks for the Wrangham tip. I just ordered his book!

Anonymous said...

I am currently working on a Master's in Exercise Physiology and one topic we recently discussed was intramyocellular triacyglycerides (IMTG). Apparently, endurance atheletes AND obese and diabetic individuals store fat as IMTG in their muscles - obviously one is due to training and the other is due to a metabolic dysfunction. One thing that some of the studies seemed to say was a "high fat" (65% cals) was a factor in IMTG and even a relation with "insulin resistance". It would seem that one is pathological and one is physiological (Peter has a great series at Hyperlipid) and this parallel is seen with AGEs. One is ingested and probably not an issue whereas the development within the body is something to take serious. I like how it relates back to IMTG. I try to get at least 50% of my calories from fat and I am ~12-14% bodyfat. Just because you eat a lot of fat doesn't mean you're going to be diabetic because you have a level of "insulin-resistance" - it's not pathological. And just because you eat AGEs doesn't mean you're wreaking the same havoc as unwanted glycation.

Ned Kock said...

Interesting parallels Jeromie. A good example of thinking outside the box.

Exercise physiology is one of my favorite topics here. I am surprised that EP is not one of the pillars on which MDs usually build their practices.

Medical practice would benefit a lot, in my opinion, from incorporating EP-inspired strategies for the treatment of many degenerative diseases.

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