The lipids are the third class of macronutrient.
They are as ubiquitous in the diet as proteins and carbohydrates,
where they occur predominantly as storage lipids called
triglycerides(formed from fatty acids), and cholesterol, a lipid with
roles in both cell structural and communication.
Fatty acids are the building blocks of most lipids. These
energy-rich molecules come in a variety of configurations,
each with different chemical and physical properties to serve
a diversity of functions. The long, straight, tightly stacked
molecules of saturated fatty acids predominate in the rigid
solid fat deposits in animals; the loose associations of the
molecularly kinked unsaturated fatty acids allows them to stay
liquid and form the oils of vegetables, seeds, and fish. The kinks
in unsaturated fatty acids are a result of the type of chemical
bonds between atoms within the fatty acid (double bonds for
the chemically-inclined). Fatty acids can vary in length, and
amount of saturation (monounsaturated fatty acids have one
double bond, polyunsaturated can have several). Unsaturated
fatty acids can also be in cis- or trans- configurations, which
refers to the nature of the double bonds within the acid. Trans-fatty
acids produced by the chemical hydrogenation of oils have
gained particular notoriety for their potential to increase the risk
of heart disease, while naturally-occurring trans fats (such as
conjugated linoleic acid from milk products) can be beneficial.
Although fatty acids can be taken up from the diet, the
body is well equipped to synthesize its own for energy storage
or structural purposes. The exception is the two essential fatty
acids which cannot be synthesized by mammals and must be
obtained through the diet: alpha-linolenic acid, an omega-3 fatty
acid; and linoleic acid, an omega-6 fatty acid. Given adequate
amounts of these two compounds, and the body’s enzymatic
machinery, the entire repertoire of fatty acids can be constructed.
Much of the fats and oils in the diet are in the form of
triglycerides, which are storage lipids formed of a molecular
complex of glycerol and three fatty acids (unsaturated,
saturated, or mixtures of the two). Other sources of dietary
fatty acids include phospholipids (which form cell membranes),
mono- and di-glycerides. Whatever the source, dietary fats
and oils are broken down by the action of digestive enzymes
secreted from the pancreas and intestines (called lipases) to
release individual fatty acids, which are absorbed from the small
intestine, packaged into new triglycerides or phospholipids, and
distributed throughout the body to serve their various roles.
Cholesterol is a type of lipid distinct from the fatty acids,
triglycerides, or phospholipids. It is a waxy, steroid compound
that has critical roles in metabolism. The human body makes
the majority of its own cholesterol, although some is obtained
through the diet: dietary sources of cholesterol are exclusively
animal products (plant sources of cholesterol are extremely rare
and appear to be limited to certain types of algae).1 To partition
cholesterol into “good” and “bad” varieties is a bit misleading;
there is only a single type of cholesterol in nature. Elevated
levels of blood cholesterol (or more accurately, elevated levels of
cholesterol that are contained in specific types of lipid particles
called low density lipoproteins) have been implicated in the
progression of atherosclerosis, the progressive accumulation of
fatty deposits in arteries that can lead to serious cardiovascular
consequences. However, cholesterol’s association with heart
disease risk obscures its many critical roles in metabolism, from
serving as a building block to many hormones, to ensuring the
proper firing of nerve impulses.
Roles of Fats and Cholesterol in Normal Metabolism
Dietary lipids have several potential fates in human metabolism:
Fuel Source and Fuel Storage. Triglycerides, and the fatty acids
they contain, are a rich source of cellular energy. While glucose
is the preferred energy source for most cells, glucose is a bulky
molecule that contains little energy for the amount of space it
occupies (glucose is stored in the liver and muscles as glycogen,
which is used to power the brain and muscles between meals).
Not only are fatty acids better sources of energy on a per-weight
basis (one gram of carbohydrate contains 4 kilo-calories of
energy, compared to 9 kilo-calories/gram for triglyceride), but they are denser, giving them the ability
to store massive amounts of energy in
fat deposits. The average human, for
example, can only store enough glucose
as glycogen in the liver for about 12 hours
worth of energy, but can store enough fat
to power the body for significantly longer
(up to several days during starvation).
Because fatty acids are superior for
storing excess energy, excess dietary
carbohydrates are converted into fatty
acids and packaged into triglycerides
for long-term storage. However, this
conversion is one-way; fatty acids
cannot be converted back to glucose.
This presents a problem for some cells
(like neurons in the brain), which do
not metabolize fatty acids, and do not
have access to the vast energy stores of
adipose tissue. In times of carbohydrate
deficit (such as during starvation or
low-carbohydrate dieting), fatty acids
are converted into ketone bodies, an
alternate source of fuel for brain cells.
Building blocks of cell membranes.
Lipids form the bulk of the membranes
which surround each cell in the body;
these lipids are predominantly formed
of phospholipids and cholesterol. It is
here that the importance of cholesterol
becomes apparent; its “waxy” nature
helps to keep the membranes around
cells fluid, so that cells are viable at
a wider range of temperatures. Some
cells have a particularly heavy reliance
on cholesterol for proper function:
The membranes surrounding healthy
liver cells, for example, are almost one-third
cholesterol,2 and cholesterol is a
major component of the myelin sheath
that surrounds neurons and allows
for these cells to transmit electrical
impulses rapidly over long distances.3
Mitochondria, the centers of energy
generation in most cells, also rely
heavily on cholesterol to insulate
against the loss of electrolytes during
their generation of ATP.4
Precursors to hormones. Cholesterol
is used as the starting material in the
synthesis of all of the bodies steroid
hormones, which include the sex
hormones (testosterone, progesterone,
and the estrogens), mineralcorticoids
(which control the balance of water and
minerals in the kidney) glucocorticoids
(which control protein and carbohydrate
metabolism, immune suppression,
and inflammation), and vitamin D
(which controls calcium and phosphate
balance). Omega-3 and omega-6 fatty
acids from the diet are used to synthesize
eicosanoids, a set of hormone-like
molecules that are important for a diverse
set of metabolic functions, including
inflammation/anti-inflammation, blood
clotting, immune function, response to
allergens, protection from stomach acid,
and parturition (labor).
Digestion and Absorption of Fats.
Cholesterol is the starting material for
bile acids, a group of detergent-like
molecules that are synthesized in the
liver, and used to emulsify/solubilized
many dietary fats and cholesterol. This
facilitates the absorption of dietary lipids
in the intestines.
SPECIFIC HEALTH BENEFITS OF FATS AND CHOLESTEROL
Many of the health benefits realized by modifying lipid intake
involve altering patterns of consumption: reducing intake of
saturated, trans-fats, and cholesterol, and increasing intake of
mono- and polyunsaturated fats. Omega-3 intake, in particular,
has been the subject of hundreds of studies in humans and
animals, in part for its ability to successfully reduce the risk of
several diseases by different mechanisms:
Reducing Chronic Low-level Inflammation. Diets high in
saturated fats have been associated with an increase in proinflammatory
markers in some studies, particularly in diabetic
or overweight individuals.5, 6 The intake of synthetic trans-fats has
also been associated with increases in markers of inflammation
in some studies,7, 8 although this data is conflicting and may be
more pronounced in individuals that are also overweight.9, 10
Omega-3 fatty acids have been studied for their prevention of
cardiovascular disease and mortality in tens of thousands of
patients; the anti-inflammatory effects of omega-3’s are thought
to contribute to this activity.11 Several small studies of omega-3
fatty acid consumption have demonstrated their beneficial effects
against other inflammatory diseases, particularly asthma, IBD,
and rheumatoid arthritis.12, 13 These clinical trials are supported
by several large observational trials encompassing thousands
of patients, which have revealed inverse relationships between
fish oil/omega-3 consumption and markers of systemic
inflammation within diverse populations.14-17
Promoting Healthy Blood Pressure. Omega-3 fatty acids
consumption has led to significant reductions in blood pressure
across several clinical trials. In a survey of 36 clinical trials on the
effects of omega-3 supplementation in over 2000 individuals, a
median intake of 3.7 g/day of fish oil demonstrated an average
reduction of blood pressure of 2.1 mm Hg (systolic) and 1.6
mm Hg (diastolic).18 In hypertensive individuals, the average
reductions in blood pressure were much greater, amounting
to -4 mm Hg (systolic) and -2.73 mm Hg (diastolic). When
compared to low-fat diets, Mediterranean diets (moderate
fat diets characterized by high intakes of monounsaturated
fats from nuts and olives, and polyunsaturated fats from fish)
demonstrated average reductions of 1.7 mm Hg (systolic) and
1.5 mm Hg (diastolic) over six studies including more than 2600
individuals.19 While these reductions may seem very small, it is
important to remember that even modest reductions in blood
pressure can have significant effects on cardiovascular health;
for example, lowering diastolic blood pressure by 5 mm Hg has
been estimated to lower the risk of stroke death by 40 percent
and the risk of death by heart disease or other vascular causes
by 30 percent.20
Promoting Healthy Levels of Blood Lipids. The effects of
fish oil fatty acids on the reduction of serum triglycerides (a
cardiovascular risk factor) are well established. Forty-seven
studies with over 15,000 patients, have demonstrated an
average triglyceride reduction of 30 mg/dL, at an average intake
of 3.35 g fish oil over 24 weeks.21 The effects of fish oil fatty
acids on LDL and HDL cholesterol are equivocal.22 (Prescription
fish oil uses a highly concentrated esterified fatty acids that
provides a dosage of 3.36 g of omega-3 in 4 capsules; its
degree of triglyceride reduction—up to 45 percent—is similar
to non-prescription fish oil at a similar dose, but requires less
capsules.23) High monounsaturated and polyunsaturated fatty
acid intake typical of Mediterranean diets have demonstrated
reductions in serum triglycerides and total cholesterol, when
compared to low-fat diets.19
HOW MUCH FATS AND CHOLESTEROL SHOULD I BE GETTING?
As with the case of carbohydrates, there are opposing opinions
on the optimal amount and composition of lipid for a healthy
diet. There is general agreement that synthetic trans-fats
should be avoided. The majority of expert opinions suggest
that cholesterol and saturated fat intake (particularly from
animal sources) be minimized; the USDA RDIs suggest these
values at < 300 mg/day and < 20 grams/day (~10 percent of
total calories in a 2000 calorie diet) respectively. The National
Cholesterol Education Program’s Therapeutic Lifestyle Changes
(TLC) diet suggests more conservative daily intakes of < 200 mg/
day cholesterol and < 7 percent of total calories from saturated
fats.24 Beyond these recommendations, however, is a substantial
schism between proponents of low-fat/higher carb and low-carb/
higher fat diets. There is evidence that both diets can reduce
disease risk and maintain a healthy body mass index.25
As for the benefits of omega-3 fatty acids in particular,
the majority of clinical studies on the effects of fish oil for
cardiovascular health and inflammation looked at intakes of 3–4
grams of omega-3 fatty acids per day, which is a considerable
amount to consume from fish alone (200 –400 grams of fatty
fish per week provides between 500 and 800 mg of omega-3’s
per day).26 Significant health benefits have been observed at
these lower levels of fish oil consumption in some observational
studies.16, 26
To read the series on Macronutrients:
References download: http://totalhealthmagazine.com/references/june2012.pdf
