Thursday, March 18, 2010
Saturated Fats/Unsaturated Fats
Since Dr. Ancel Keys and his colleagues formulated and promulgated the diet-heart hypothesis in the 1960's, the idea of eating saturated fat has become anathema in most nutritional circles. When Americans were told that that consumption of saturated fat was positively correlated with the incidence of heart disease, they began to eat more of the "heart-healthy" mono- and polyunsaturated fats and fewer of the saturated ones. In spite of that, the number of hospital discharges with cardiovascular disease as the first listed diagnosis has continued to increase in the U.S. This is especially surprising in light of the fact that the percentage of U.S. adults who smoke has declined from over 40% in 1965 to about 20% in 2007. Is it possible that saturated fats are not as evil as they have been portrayed?
To begin the discussion, it is important to understand that a saturated fat is not saturated with calories or with cholesterol. In this case, "saturated" is a chemical term, and it means that the molecule in question is saturated with hydrogens--that is, it contains the maximum number of hydrogens it can hold. Here are two fatty acids, one saturated and the other unsaturated:
In the fatty acid at the top, the carbon-carbon bond between the two green C's is a single bond. Each green carbon holds two hydrogens, and they are saturated with hydrogen. In the fatty acid at the bottom, there is a double bond between the two green C's. Each of those carbons holds one hydrogen. The carbons do not hold as many hydrogens as they possibly could and they are therefore unsaturated. This particular fatty acid has only one unsaturated carbon-carbon bond, so it is monounsaturated. If it had two or more unsaturated bonds, it would be polyunsaturated.
The important thing about unsaturated fatty acids is that the presence of a double bond weakens the carbon-hydrogen bonds on the carbons next to the double bond. In the picture above, those carbon-hydrogen bonds are marked with green asterisks. That doesn't sound particularly interesting until we understand what happens when those hydrogens are removed by something like oxygen, heat or metal ions. As soon as we remove one of the vulnerable hydrogens, our heart-healthy unsaturated fatty acid becomes a free radical. In other words, it contains an unpaired electron and it becomes extremely chemically reactive.
Once the first free radical is formed, the generation of free radicals from unsaturated fatty acids happens in a self-propagating manner. One free radical can interact with other unsaturated fatty acids to produce more free radicals, which in turn produce even more free radicals, and so on. Besides damaging the fatty acids, these free radicals can also destroy other molecules, including vitamins and proteins. In addition, the free radicals are able to react with oxygen to produce hydroperoxides. These eventually break down into aldehydes, which produce the odors and flavors associated with rancidity.
The reactivity of fatty acids increases with the number of double bonds they contain. Stearic acid is an 18-carbon saturated fatty acid. If we add one double bond, it becomes one hundred times more likely to form a free radical. If we add three double bonds, it becomes 2500 times more likely to form a free radical. The health effects of saturated versus unsaturated fatty acids won't be addressed until the next blogpost, but it is certain that saturated fatty acids are far more stable than their unsaturated counterparts.
There are several ways to decrease the likelihood of free radical formation and rancidification in fatty acids. One is to be sure that heat is not used to extract the fatty acid from its source. In the case of unrendered animal fats, this is not a problem. In the case of vegetable fats, cold pressing ensures (at least it does in the EU) that the oil will not be heated above about 80 degrees Fahrenheit. Unfortunately the U.S. definition of cold pressed is not particularly rigorous, so it may be necessary to check websites or make telephone calls to the manufacturer to determine the temperature a particular brand of oil reaches as it is extracted. When fat is used for cooking, it is important to realize that the higher it is heated and the longer it is heated, the more likely it will be to form free radicals.
Another strategy to avoid free radical formation and rancidification in fats and oils is to be sure that they are kept away from light, particularly UV light. It is also helpful to keep fats and oils away from oxygen. They should not be stored for long periods, and once a container is opened, it should be used up as quickly as possible.
As we have already seen, some polyunsaturated fats are necessary for growth and for optimal health. However it pays to know which fats are which and to be careful with respect to the amounts and types of dietary fats we consume. In closing, here is a table that presents the approximate composition of some common fats, arranged from the lowest to the highest percentage of polyunsaturated fatty acids.