Tuesday, May 3, 2011

The Nutrition Debate #20: Know Your Dietary Fats: Saturated and Unsaturated

I am about to embark on a mini-series on fats, and this may be a little dense for many of my readers (assuming I have many readers). I can actually recall a time when “good cholesterol = HDL” and “bad cholesterol = LDL” were not yet fixed associations in my mind, so I am sympathetic if you still find yourself there now. Nevertheless, if you are going to take an interest in your health, as I sincerely hope you will, you are going to have to acquire an understanding of some of the basic science of human nutrition. So, let’s begin with the classification of fats into Saturated and Unsaturated.

Simply put, saturated fats are in solid form at room temperature and unsaturated fats are in liquid form. In the new “Dietary Guidelines for Americans, 2010,” the HHS/USDA has begun to use “saturated fat” and “solid fat” interchangeably. In the same document and in their “food pyramid”, the HHS/USDA describes unsaturated fats as “oils.”

Saturated fats are for the most part animal fats, i.e. fats found in intramuscular tissue as well as under the skin and around the organs. Both in the animals we eat, and in our own physiology, fat serves as both energy source and energy reserve. It also cushions the organs and skeleton from damaging blows and insulates the body from cold. More recently – and this is a development of monumental importance in our understanding of fat metabolism -- fat stores have come to be recognized as an endocrine organ as well. Leptin, a hormone discovered in adipose (fat) tissue in 1994, is an appetite suppressant, sending a strong signal to the hypothalamus at the center of the brain. Our fat is in a constant state of flux as its primary form, triglyceride molecules, are being synthesized (created), or catabolized (broken down).

Two other saturated fats (both solid at room temperature) that are commonly found in foods are vegetable in origin: Both are tropical oils – coconut oil and palm oil. Coconut oil, a medium-chain saturated fat, is appearing with increasing frequency in the Western Diet, but both have been used for millennia in the cuisines of the tropical regions.
Saturated fats are solid because they are structured in a dense way, with every carbon atom along the carbon chain having two hydrogen atoms. In addition, as it has no “double bond” between carbon atoms, they are straight, packed closely together and thus solid. Since there are no “free” atoms, they are also very stable and are not liable to become rancid when exposed to oxygen. It also means that, in their normal state or in cooking they will not create harmful “free radicals” (unpaired electrons) or produce pro-inflammatory AGE’s (Advanced Glycation End-Products) when heated.

Unsaturated fats break down into two sub-classes: monounsaturated and polyunsaturated. The monounsaturated fats are primarily found in olive oil, and to a lesser extent in Canola oil and peanut oil. Additional sources are avocado and many popular nuts (macadamias, pecans, hazelnuts). Because they have only one “double bond” in the carbon chain, they are called monounsaturated. They are fairly tightly packed and fairly stable when heated or exposed to oxygen (as in air). Have you noticed how olive oil becomes solid when refrigerated? It is, therefore, safe to use. Note, however, that medium chain, monounsaturated fats like olive oil, and longer-chain fatty acids, such as those found in polyunsaturated vegetable oils that need bile salts from the gall bladder to be digested, are more likely to be stored as adipose tissue, i.e. body fat. Saturated fats (e.g. butter), in contrast, are more likely to be directly used (oxidized, i.e. burned) for energy.

Polyunsaturated fats are fats that have more than one double bond in the carbon chain (hence poly), and are therefore loosely constructed and always found in the liquid state as “oils.” Because of their loose construction, they are relatively unstable, “reactive,” become rancid rather easily, and are also easily damaged by heating and especially reheating. These are the vegetable oils that have become ubiquitous in our food supply due to the ability to manufacture them cheaply. But the refining and processing has deleterious effects that are becoming increasingly known and understood. Trans fat is only one example. Trans fats (the artificial kind created in the processing phase to make liquid oils solid and more stable for use in margarine and in manufactured baked goods and cooking) are partially hydrogenated vegetable oils. There’s a broad consensus that everyone should strictly avoid artificial trans fats.
It is important to recognize that all fats are combinations of the three types: saturated, mono and polyunsaturated. Butter, for example is 66% saturated, 30% mono and 4% poly; Tub margarine is 23% saturated, 55% mono and 22% polyunsaturated; olive oil is 15% saturated, 70% mono and 10% poly. Soybean oil, used in 70% of processed foods, including restaurant meals in the US, is 15% saturated, 22% mono and 62% poly. Canola oil, a genetically engineered variant of rapeseed oil, is 7% saturated, 65% mono and 28% polyunsaturated. What are we getting ourselves into now!

The next column will be “The Dangers of Polyunsaturated Fats.” I will not be espousing the government’s position here.

© Dan Brown 5/1/11