The place that fats should occupy in the diet is a matter of contention. Low fat diets such as the one advocated by Dean Ornish, M.D. suggest a fat intake of 10% of calories, while other medical experts such as Joseph Mercola consume as much as 50% or more of their calories from fat. Who's right?
Let's examine the science and develop our own conclusion.
Fat as one of the three macronutrients (protein and carbohydrates are the other two) comprises various proportions in food depending on the food. Oils, for example, are 100% fat, as is butter. Nuts and high fat animal products derive the majority of their calories from fat. Fruits, vegetables, and legumes have modest to minimal fat content.
The body uses fat for energy. Fatty acids are long chains of carbon molecules which are broken down/oxidized to provide fuel. This oxidation creates harmful metabolites (oxidants) which your body must neutralize with the appropriately named antioxidants, particularly the vitamins A, C, and E and certain minerals.
The problem is, fat generally contains few if any of these oh so important antioxidants, so your source of these nutrients must come elsewhere. From where exactly? Vitamin C is derived exclusively from plant products. Foods highest in this vitamin include peppers, citrus, papaya, broccoli, kale, strawberries, cauliflower, Brussels sprouts, sweet potatoes, and cantaloupe. While some animal products contain vitamin A (liver, herring, salmon, milk, and eggs), the vitamin is much more prevalent in foods of plant origin, including sweet potato, spinach, carrots, pumpkin, cantaloupe, and peppers. The same goes for vitamin E, which is present in greatest quantities in plant foods (nuts, seeds, spinach, chard, broccoli, mango, tomato).
Plant foods are not only highest in the antioxidant vitamins that neutralize free radicals and other products of cellular metabolism and protect your body from aging and disease. They are also the highest in virtually all other vitamins and minerals, making plant foods the most nutritious foods available. The foods heading the list of nutrient density are leafy green vegetables, followed by other vegetables, fruits, legumes, and seeds. These foods derive most of their calories from carbohydrate and to a lesser extent from protein; they are generally lower in fat. Indeed fat content seems to go up as the overall nutritive value of a food decreases. Consider that the least nutritious foods are oils, nut butters, refined carbohydrates (french fries, potato chips), and high fat animal products (cheese, butter, eggs). These foods all derive a substantial portion of calories from fat.
Because the most nutritious foods are lower in fat, emphasizing these foods results in a diet whose total caloric breakdown mirrors what is present in these foods, which ends up being around 70 percent carbohydrate and 10 to 15 percent protein and fat.
Of course it is beneficial to include some fat in the diet. Other than being used as an energy source, some types of fat (the polyunsaturated fatty acids) serve essential functions as precursors to cellular messengers and components of the cell membrane, with a large concentration in nervous and eye tissue. Because your body cannot produce essential fatty acids, you must consume them in your diet, and because fat is present in all foods, you won't have to look far. Leafy greens, seeds, and algae are all good sources.
But not all fatty acids are necessary/beneficial, and the more fat you consume, the more unnecessary and harmful fatty acids you are at risk of taking in. Saturated fats (found in animal products and to a lesser extent in chocolate, coconut, and palm kernel oils), trans fats (margarines, fried foods, pastries) are of no benefit and can actually harm the body in amounts exceeding, well, zero. Translation: don't eat 'em.
Finally, there is the issue of caloric density. If you are calorie-conscious or trying to lose weight or maintain your ideal weight, it is best to opt for nutrient-dense foods over those with a high amount of calories relative to weight, and with few exceptions nutrient-dense foods and calorically-dense foods are mutually exclusive. Fat is twice as calorically-dense as protein or carbohydrate, so foods that derive a large portion of their calories from fat are more caloric, and also tend to have less water and fiber (and vitamins and minerals) than fruits and vegetables. A hundred grams of almonds has nearly 600 calories. The same weight in apples has 52. You do the math.
Congratulations. We are now also experts on fat. Our conclusion: Eat plant products freely and make judicious use of high fat products (avocados, coconuts, olives, seeds) to ensure your diet is maximally nutritious, which we have shown is by definition low in fat.