Macronutrients in Health and Disease: Fats

Dietary fats are the least required macronutrient, with only a few grams per day needed for the absorption of fat–soluble vitamins A, D, E, and K, among other functions. Foods contain combinations of saturated and unsaturated fats. Substantial quantities of saturated fat are found in dairy products, eggs, meats, and tropical oils, while unsaturated fats predominate in liquid fats (eg, vegetable oils). The latter are subdivided into monounsaturated fats (predominant in olive and canola oils) and polyunsaturated fats (found in nuts, seeds, seed oils, and, to a lesser extent, in meats).

Only the polyunsaturated fatty acids (PUFA) are essential to human nutrition, since the body does not synthesize these. PUFA’s have roles as structural components of cell membranes and as signaling molecules (eg, eicosanoids).⁴¹ The essential fatty acids include linoleic acid (an omega–6 fatty acid) and alpha–linolenic acid (an omega–3 fatty acid).

The Institute of Medicine recommends a ratio of dietary linoleic (omega–6) to alpha–linolenic (omega–3) acid intake of 10:1; however, this figure is controversial (some suggest a somewhat higher intake of omega–3). The suggested ratio of intake for different age groups is listed in Table 1, EFA Requirements (g/day).³⁰

Table 1:
Recommended Ratio of Linoleic Acid to Alpha–Linolenic Fatty Acid,
by Age Group, EFA Requirement (g/day)

The National Academy of Sciences Food and Nutrition Board currently recommends a range of fat intake of 20% to 35% of total energy intake.³⁰ This too is controversial, given that good health outcomes have been achieved with considerably lower levels of fat intake Most individuals can meet their EFA needs by consuming very small amounts of fat per day (~14 g, or 0.5 oz), although many people are eating far more than this (~85 g or 3 oz/day, on average).⁴²

This level of excess consumption of fats is problematic because EFA derivatives are raw materials for eicosanoids (i.e., prostaglandins, leukotrienes, and thromboxanes), hormone–like chemicals with short lived but powerful effects. Eicosanoids play significant roles in immune function, inflammation, thrombosis, proliferation, reproduction, gastroprotection and hemostasis, in addition to other functions.⁴³ The omega–6 polyunsaturated fatty acid linoleic acid and the omega–3 polyunsaturated fatty acid alpha–linolenic acid are metabolized to long–chain fatty acids (arachidonic and eicosapentanoic acids, respectively) that are the precursors for the eicosanoids. These long–chain derivatives are also found in some food products, with arachidonic acid being present in meat, eggs, and dairy products, and eicosapentanoic acid found in fish. These food sources are not required, however, as eicosanoids are produced in the body.

The type and amount of PUFA consumed (omega–6 vs. omega–3) are important considerations. Certain kinds of eicosanoids will predominate when omega–6 fats are in plentiful supply, as is the case with Western diets.⁴⁴ These include prothrombotic thromboxanes (eg, thromboxane A2); immunosuppressive prostaglandins (eg, prostaglandin E2); and proinflammatory leukotrienes (eg, leukotriene B4). Reducing the intake of omega–6 fatty acids (particularly arachidonic acid from animal products) while proportionately increasing the intake of omega–3 fatty acids results in the production of eicosanoids with reduced potential to do harm (eg, thromboxane A3, prostaglandin E3, and leukotriene B5).⁴⁴

The Need to Limit Fat. Of all the macronutrients, fatty acids have the greatest potential to cause disease when consumed in excess. Saturated fats in particular tend to raise cholesterol and triglyceride concentrations (see Obesity and Hyperlipidemia for further information). Reducing saturated fat is important for controlling blood lipids; beyond that, reducing total fat (not just replacing saturated with unsaturated fatty acids) is helpful for reducing the excess weight that contributes significantly to cardiovascular disease and other chronic health problems. Fat provides 9 calories per gram, more than twice that of protein or carbohydrate. Individuals who are successfully maintaining significant weight loss typically consume less than 25% of their total daily calories from fat.⁴⁵

Intake of polyunsaturated and monounsaturated fatty acids should also be limited. PUFA are particularly sensitive to lipid peroxidation, resulting in generation of reactive oxygen species (superoxide and hydroxyl radicals, hydrogen peroxide, singlet oxygen, hypochlorous acid). Diets that are both low in antioxidants (eg, Western diets) and high in PUFA may result in a condition termed “oxidative stress,⁴⁶ that damages DNA, proteins, and carbohydrates, thereby contributing to a spectrum of common chronic diseases.⁴⁷ Peroxidation of PUFA in low–density lipoprotein (LDL) increases the amount of oxidized LDL, a key stimulus for atherosclerosis progression.

Generally speaking, fatty foods provide very few nutrients.  Other than essential fatty acids, vegetable oils provide only vitamins E and K, which can be obtained from other sources. Ideally, fats should not be added to meals; rather, they should be consumed in modest amounts from foods that are a vehicle for other essential nutrients. For example, nuts provide essential fatty acids, magnesium, copper, folic acid, potassium, fiber, and vitamin E.⁴⁸