Dietary Fats: The Basics
Dietary fats are
found in both plant and animal foods. They supply calories and help with the
absorption of the fat-soluble vitamins A, D, E, and K. Some also are good
sources of two essential fatty acids—linoleic acid and α-linolenic acid.
All dietary fats are composed of a mix
of polyunsaturated, monounsaturated, and saturated fatty acids, in varied
proportions. For example, most of the fatty acids in butter are saturated, but
it also contains some monounsaturated and polyunsaturated fatty acids. Oils are
mostly unsaturated fatty acids, though they have small amounts of saturated
fatty acids.
Polyunsaturated fatty acids
(polyunsaturated fats are found in greatest amounts in sunflower, corn,
soybean, and cottonseed oils; walnuts; pine nuts; and sesame, sunflower,
pumpkin, and flax seeds. Only small amounts of polyunsaturated fats are found
in most animal fats. Omega-3 (n-3) fatty acids are a type of
polyunsaturated fats found in seafood, such as salmon, trout, herring, tuna,
and mackerel, and in flax seeds and walnuts. EPA and DHA are long chain n-3
fatty acids found in seafood.
Monounsaturated fatty acids
(monounsaturated fats) are found in greatest amounts in olive, canola, peanut,
and safflower oils, and in avocados, peanut butter, and most nuts.
Monounsaturated fats also are part of most animal fats such as fats from
chicken, pork, beef, and wild game.
Saturated fatty acids (saturated fats)
are found in the greatest amounts in coconut and palm kernel oils, in butter
and beef fats, and in palm oil. They also are found in other animal fats, such
as pork and chicken fats and in other plant fats, such as nuts.
Trans fatty acids (trans fats) are
unsaturated fats found primarily in partially hydrogenated vegetable oils and
foods containing these oils and in ruminant (animal) fats. They are structurally
different from the unsaturated fatty acids that occur naturally in plant foods
and differ in their health effects.
The proportions of fatty acids in a
particular fat determine the physical form of the fat:
•
Fats with a higher amount of
polyunsaturated and monounsaturated fatty acids are usually liquid at room
temperature and are referred to as “oils.”
Fats with a higher amount of saturated fatty acids are
usually solid at room temperature and are referred to as “solid fats.” Fats
containing trans fatty acids are also classified as solid fats, although
they may or may not be solid at room temperature.
A relevant detail in the complexity
of making food-based recommendations that consider nutrients is the difference
between the terms “saturated fats” and “solid fats.” Although they are closely
related terms, saturated fats and solid fats are not synonymous. The term
“saturated fats” refers to saturated fatty acids, a nutrient found in foods,
while the term “solid fats” describes the physical manifestation of the fats in
a food. Some solid fats, such as the strip of fat around a piece of meat, can
easily be seen. Other solid fats are not so visible. For example, the solid
fats in whole milk are suspended in the fluid milk by the process of
homogenization.
Margarines and margarine-like
vegetable oil spreads are food products composed of one or more oils or solid
fats designed to replace butter, which is high in saturated fats. These
products may be sold in sticks, tubs, bottles, or sprays. Margarine and vegetable
oil spreads generally contain less saturated fats than butter. However, they
vary in their total fat and calorie content and in the fat and oil blends used
to make them and, thus, in the proportions of saturated, unsaturated, and trans
fats they contain. It is important to read the Nutrition Facts label to
identify the calorie and saturated and trans fats content of the spread
and choose foods with no trans fats and lower amounts of saturated fats.
The Dietary Guidelines provides recommendations on
saturated fats as well as on solid fats because its aim is to improve the
health of the U.S. population through food-based guidance. It includes
recommendations on saturated fats because of the strong relationship of this
nutrient to a health outcome (CVD risk). It includes recommendations on solid
fats because they are abundant in the diets of the U.S. population, and
reducing solid fats when making food choices is an important way to reduce
saturated fats and excess calories.
Saturated Fats, Trans Fats, and Cholesterol
Saturated Fats
Healthy intake: Intake of saturated
fats should be limited to less than 10 percent of calories per day by replacing
them with unsaturated fats and while keeping total dietary fats within the
age-appropriate AMDR. The human body uses some saturated fats for physiological
and structural functions, but it makes more than enough to meet those needs.
Individuals 2 years and older therefore have no dietary requirement for
saturated fats.
Strong and consistent evidence shows
that replacing saturated fats with unsaturated fats, especially polyunsaturated
fats, is associated with reduced blood levels of total cholesterol and of
low-density lipoprotein-cholesterol (LDL-cholesterol). Additionally, strong and
consistent evidence shows that replacing saturated fats with polyunsaturated
fats is associated with a reduced risk of CVD events (heart attacks) and
CVD-related deaths.
Some evidence has shown that replacing saturated fats with
plant sources of monounsaturated fats, such as olive oil and nuts, may be
associated with a reduced risk of CVD. However, the evidence base for
monounsaturated fats is not as strong as the evidence base for replacement with
polyunsaturated fats. Evidence has also shown that replacing saturated fats
with carbohydrates reduces blood levels of total and LDL-cholesterol, but
increases blood levels of triglycerides and reduces high-density
lipoprotein-cholesterol (HDL-cholesterol). Replacing total fat or saturated
fats with carbohydrates is not associated with reduced risk of CVD. Additional
research is needed to determine whether this relationship is consistent across
categories of carbohydrates (e.g., whole versus refined grains; intrinsic
versus added sugars), as they may have different associations with various
health outcomes. Therefore, saturated fats in the diet should be replaced with
polyunsaturated and monounsaturated fats.
Considerations: As discussed in Chapter 2, the main sources of saturated
fats in the U.S. diet include mixed dishes containing cheese, meat, or both,
such as burgers, sandwiches, and tacos; pizza; rice, pasta, and grain dishes;
and meat, poultry, and seafood dishes. Although some saturated fats are
inherent in foods, others are added. Healthy eating patterns can accommodate
nutrient-dense foods with small amounts of saturated fats, as long as calories
from saturated fats do not exceed 10 percent per day, intake of total fats
remains within the AMDR, and total calorie intake remains within limits. When
possible, foods high in saturated fats should be replaced with foods high in
unsaturated fats, and other choices to reduce solid fats should be made (see Chapter 2).
Trans Fats
Individuals should limit intake of trans
fats to as low as possible by limiting foods that contain synthetic sources of trans
fats, such as partially hydrogenated oils in margarines, and by limiting other
solid fats. A number of studies have observed an association between increased
intake of trans fats and increased risk of CVD. This increased risk is
due, in part, to its LDL-cholesterol-raising effect.
Trans fats occur naturally in some foods and also are produced in a
process called hydrogenation. Hydrogenation is used by food manufacturers to
make products containing unsaturated fatty acids solid at room temperature
(i.e., more saturated) and therefore more resistant to becoming spoiled or
rancid. Partial hydrogenation means that some, but not all, unsaturated fatty
acids are converted to saturated fatty acids; some of the unsaturated fatty
acids are changed from a cis to trans configuration. Trans
fatty acids produced this way are referred to as “artificial” or “industrially
produced” trans fatty acids. Artificial trans fatty acids are
found in the partially hydrogenated oils[23] used in some margarines, snack
foods, and prepared desserts as a replacement for saturated fatty acids.
Although food manufacturers and restaurants have reduced the amounts of
artificial trans fats in many foods in recent years, these fats can
still be found in some processed foods, such as some desserts, microwave
popcorn, frozen pizza, margarines, and coffee creamers.
Naturally occurring trans
fats, known as “natural” or “ruminant” trans fats, are produced by
ruminant animals. Natural trans fats are present in small quantities in
dairy products and meats, and consuming fat-free or low-fat dairy products and
lean meats and poultry will reduce the intake of natural trans fats from
these foods. Because natural trans fats are present in dairy products
and meats in only small quantities and these foods can be important sources of
nutrients, these foods do not need to be eliminated from the diet.
Dietary Cholesterol
The body uses cholesterol for
physiological and structural functions but makes more than enough for these
purposes. Therefore, people do not need to obtain cholesterol through foods.
The Key Recommendation from the 2010
Dietary Guidelines to limit consumption of dietary cholesterol to 300 mg
per day is not included in the 2015 edition, but this change does not suggest
that dietary cholesterol is no longer important to consider when building
healthy eating patterns. As recommended by the IOM,[24] individuals should eat as
little dietary cholesterol as possible while consuming a healthy eating
pattern. In general, foods that are higher in dietary cholesterol, such as
fatty meats and high-fat dairy products, are also higher in saturated fats. The
USDA Food Patterns are limited in saturated fats, and because of the
commonality of food sources of saturated fats and dietary cholesterol, the
Patterns are also low in dietary cholesterol. For example, the Healthy
U.S.-Style Eating Pattern contains approximately 100 to 300 mg of cholesterol
across the 12 calorie levels. Current average intake of dietary cholesterol
among those 1 year and older in the United States is approximately 270 mg per
day.
Strong evidence from mostly prospective
cohort studies but also randomized controlled trials has shown that eating
patterns that include lower intake of dietary cholesterol are associated
with reduced risk of CVD, and moderate evidence indicates that these eating
patterns are associated with reduced risk of obesity. As described earlier, eating
patterns consist of multiple, interacting food components and the
relationships to health exist for the overall eating pattern, not necessarily
to an isolated aspect of the diet. More research is needed regarding the
dose-response relationship between dietary cholesterol and blood cholesterol
levels. Adequate evidence is not available for a quantitative limit for dietary
cholesterol specific to the Dietary Guidelines.
Dietary cholesterol is found only in
animal foods such as egg yolk, dairy products, shellfish, meats, and poultry. A
few foods, notably egg yolks and some shellfish, are higher in dietary
cholesterol but not saturated fats. Eggs and shellfish can be consumed along
with a variety of other choices within and across the subgroup recommendations
of the protein foods group.
