Fats have one property that distinguishes them from other macronutrients, i.e., carbohydrates and proteins: they are not water soluble but rather fat or lipid soluble. This property means that the body must have specific mechanisms to handle the fat in digestion, transport across membranes, and transport in blood serum. Many different types of molecules in cells and body fluids must contain fatty acids in order to function. A few definitions are necessary first to elucidate the importance of the dietary fats.
Most fats are triglycerides, which consist of three fatty acids linked or bonded to one glycerol molecule. Glycerol is actually a small carbohydrate, but when it is linked to fatty acids the glyceride product is classified as a fat. So, monoglycerides, diglycerides, and triglycerides are all considered fat molecules. Lipid is another broader term used to include the fats or triglycerides, cholesterol, phospholipids, and many other fat-soluble molecules in foods or synthesized in our bodies.
A special class of the lipids is known as eicosanoids or prostaglandins. These molecules take on importance when consideration is given to unsaturated fatty acids, including both the omega-3 and omega-6 fatty acids, which serve as precursors of the eicosanoids, as discussed below.
The 35% or so of energy provided by dietary fat refers only to the triglycerides found in our foods. High-performing athletes typically consume 20 to 25% fat, but they make certain that they get enough unsaturated fats because these fats, especially the omega-3 fatty acids, are generally thought to support immune function and keep athletes from catching common colds and nagging infections.
Of the 35% of energy consumed by most in the U.S. population, the recommended percentage contributions of the specific types of fatty acids, i.e., saturated (SFAs), monounsaturated (MFAs), and polyunsaturated (PFAs), are roughly 10% each. Actual intakes of SFAs in the U.S. tend to be the highest at 13% or more, whereas
MFAs approximate 10% and PFAs are typically less than 10%. The mix of PFAs has also been estimated to have a ratio of 5-10 to 1 of omega-6 FAs to omega-3 FAs. Consumers of mostly animal foods may have an even greater ratio, and vegetarians may have ratios less than 5 to 1, depending on their food selection. High fish eaters, especially those who consume deep-sea fish, have ratios near 5 to 1. More will be covered below on the ratio of these PFAs and their impact on eicosanoid synthesis in reference to athletes.
MFAs have important roles in health, such as in protecting, at least in part, against elevations of serum total cholesterol, but they have limited utility compared to PFAs.
Under the umbrella of the DRIs, the omega-3 and omega-6 PFAs actually have recommendations that are categorized as Adequate Intakes (AIs) because of an insufficient knowledge base, i.e., mean requirements, to assign RDAs.2
Examples of dietary omega-6 FAs are linoleic acid (C182) and arachidonic acid (C204). The major dietary sources of linoleic acid are vegetable oils, and linoleic acid may be converted to arachidonic acid in diverse cells prior to its conversion to prostaglandins of the 2 series.
Examples of dietary omega-3 FAs are a-linolenic acid (ALA; C183), eicosapen-taenoic acid (EPA; C20 5), and docosahexaenoic acid (DHA; C22 6). a-linolenic acid is found primarily in soybean oil and flaxseed oil, but we are more likely to obtain EPA and DHA from fish oils. The latter molecules are converted to prostaglandins (PG3) of the 3 series and leukotrienes (LT5) of the 5 series.
The two series of eicosanoids are illustrated in Table 1.2. The products, depending on their local serum concentrations or amounts secreted, have effects on neighboring cells and tissues. It is the actions of these eicosanoids of one type or another that may have important effects on athletic performance, almost exclusively in the highly trained competitive athletes (see below).
The Two Series of Eicosanoids
Essential fatty acid precursors
Roles in athletes
Linoleic acid (C18:2)a Arachidonic acid (C20:4)a Prostaglandin 2 (PG2)
Heart, arterial constriction, lungs
Omega-3 Series a-Linolenic acid (C18:3)a Eicosapentaenoic acid (C20:5)a Prostaglandin 3 (PG3) Leukotriene (LT5) Blood clotting, immune defense a The number of carbon atoms and number of double bonds (after the colon) are given in parentheses.
The balance between the two types of essential PFAs is best assessed by determining the dietary ratio of these PFAs because the ratio drives the syntheses of the different eicosanoids in the tissues. Low intakes of omega-3 FAs result in an increase in the formation of omega-6 FAs that feed into eicosanoids of this series.4 This balance has important consequences for athletes because greater production of one type of eicosanoid may impact immune defense, tissue healing, heart function, and other organ functions that affect athletic performance — primarily of top-level competitors rather than of recreational athletes.
The amounts of the types of eicosanoids generated, i.e., PG2 vs. PG3 and LT5, depend on the dietary precursors or, for some individuals, the supplements of these two types of PFAs. Except for total vegetarians, i.e., vegans, most individuals cannot consume enough omega-3 fatty acids from foods; they can only get enough if they take a supplement of fish oil or another source rich in omega-3 fatty acids. Even the consumption of deep-sea fish a couple of times a week does not provide enough omega-3 fatty acids to balance the usual intake.
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