From a nutritional standpoint, the body utilizes six different dietary
components: protein, carbohydrates, fats, vitamins, minerals, and water.
Protein, fats, and minerals serve to make up major structural components,
while protein, carbohydrates, and fat can serve as fuels by providing
calories for the execution of physiologic functions. This article will focus
on fat, specifically its function within the body as well as the impact on
health consequences.
Fat consists of a heterogenous collection of chemically related substances.
The fundamental unit of fat is the fatty acid. In a simplified view, a fatty
acid is made up of a long chain of carbon atoms on which hydrogen atoms are
attached. At one end of the chain is an acid group. While this structure
appears relatively simple, the potential variation in structure results in a
large set of distinct entities that behave quite differently in the body, but
still fall under the term fatty acid.
The major distinction among fatty acids is the degree of saturation.
Saturation refers to the amount of hydrogen atoms that are attached to the
carbon chain. If all the available space is occupied, the fatty acid is
called a saturated fatty acid. If less than the maximum amount is found, then
the fatty acid is unsaturated. Unsaturated fatty acids result from a chemical
structure called a double bond. One double bond in the fatty acid is called a
monounsaturated fatty acids, while greater than one is referred to as a
polyunsaturated fatty acid.
Rather than merely a classification scheme by chemists, the grouping of fatty
acids this way has demonstrable effects. The degree of saturation influences
the melting point of the fatty acid. Saturated fats have a high melting point
and consequently are solid at room temperature. The fat that is visible in
beef is due to the high content of saturated fat. Monounsaturated fats are
typically liquid, but will become cloudy when placed in the refrigerator.
Polyunsaturated fats stay liquid even in the cold because their melting point
is lower than monounsaturated fats. Cooking oils (which are natural mixtures
of various fatty acids) are mostly monounsaturated and/or polyunsaturated
fatty acids. Olive oil is a good example of mostly monounsaturated fatty
acids, while canola oil has considerable polyunsaturated fatty acid.
One final aspect of fatty acid classification deals with a variant of
unsaturated fat that has been introduced largely as a consequence of food
processing. Because of the chemical features of double bonds, there are two
ways, referred to as cis and trans, to make them. Most naturally occurring
unsaturated fatty acids exist as cis fatty acids. Trans fatty acids are quite
rare being limiting to a small amount in milk as a consequence of gut
bacteria in dairy cows. As mentioned above, unsaturated fatty acid, tend to
be liquid. In order to "firm up" mixtures of unsaturated fatty acids, food
processors will "partially hydrogenate" them. This processing will produce
trans fatty acids.
Nutrition labels will sometimes break down the fatty acid composition. The
nutritional utility of this distinction will be discussed below. Besides the
degree of saturation, fatty acids are also classified by the length of the
carbon chain. This is also important because different fatty acids within a
class can exhibit different actions physiologically.
Fatty acids are the basic building blocks for fat functions in the body.
Fatty acids perform a variety of specific and necessary functions. First and
foremost, fatty acids are an integral component of the plasma membrane of
every cell in the body. The plasma membrane is the shell around a cell, much
like the skin of a balloon. The membrane of a cell is a very active location
because signals from outside the cell are directed to the cell at its
surface. The composition of the cell membrane in terms of the fatty acids
that make up the membrane are known to affect the quality and degree of
signaling across the membrane. This function is crucial since cellular
responses to hormones, uptake of nutrients, and discharge of waste all
require activity at the membrane.
Another function of fat is as a source of energy. Fatty acids are very
calorically dense with 9 calories per gram of fat. Carbohydrates and protein
on the other hand are only 4 calories per gram. In addition, the body has
limited ability to store carbohydrate (in the form of glycogen) because it
requires a lot of supporting water to maintain the structure. In fact, the
average person carries around about 1 pound of glycogen with an additional 4
pounds of water. This is why calorie restricted diets are diuretic in nature
for the first few days; as the glycogen is used up (and not replaced), the
water is lost along with the it. The body does not store protein and so must
breakdown protein for energy if necessary. Muscle is the most readily
available source. But again, muscle tissue is 72% water by weight, so a
little loss of muscle protein, causes a large loss of total weight.
Unfortunately, when eating levels are returned to normals, these compartments
are restored with a concomitant retention of water.
Alternatively, fat can be efficiently stored (much to the dismay of dieters).
Because of the chemical structure, fat does not require large amounts of
water to sustain it. In adipose tissue (the storage form of fat), about 85%
of the total weight is actual fat. Thus, from the body's energy perspective,
fat is the obvious choice as a storage form of excess energy.
Fat has several other functions that are necessary for optimal health. Fat is
a major component that forms the barrier to water in the skin. Fat is also a
critical component of nerves which are coated with fat. This coating serves
to speed up conduction down the nerve. This is a critical function and
requires much fat and much time. Newborns have not yet completed this
process. In fact, the lack of toilet capabilities in young children is due to
the incomplete nervous system in that their nerve signals travel too slow to
permit development of feedback to control their bowel and bladder.
A final function of fat is to serve as the substrate for a whole set of
hormones known as eicosanoids. Although less well known than other hormones
like insulin or growth hormone, eicosanoids are critical for many diverse
functions that regulate things like blood pressure, inflammation, blood
clotting, and labor. In fact, a pregnant, fat-deficient animal cannot go into
labor.
Thus, fat has a place in our diets and a major role to play in basic
physiology. From a nutritional standpoint, fat may appear a non-issue since
our bodies can actually manufacture fat as needed. The problem here is that
not all the necessary fats can be produced. There is a subset of dietary
fatty acids, specifically linoleic acid (known as omega-6) and linolenic acid
(known as omega-3) that is essential to consume. These two types of
polyunsaturated fatty acids that our bodies cannot manufacture are not
interconvertible with each other and must be derived from the diet.
The focus on fats in the diet has been gradually building momentum since the
1950's when the rate of cardiovascular diseases began to rise. Cholesterol
levels in the blood were identified as a major predictor of risk; however,
attempts to reduce serum cholesterol by reducing dietary cholesterol were
uniformly disappointing. Research to understand the factors controlling
cholesterol levels led to attention on dietary fats. Unfortunately, while a
considerable effort has been devoted to focussing attention on cholesterol
and fat intake, more recent research has shed additional light on this
complex issue.
Traditional nutritional advice advocates a reduction in fat intake as a means
to reduce cholesterol levels; however, this simple prescription is misguided
for several reasons. In the first place, the nutritional advice parroted by
mainstream nutrition pundits is derived from clinical trials that require
large numbers of people and take a long time to perform. The design and
intent of these studies are based on nutrition and dietary principles that
are quite old - at least 10 to 15 years. Since these studies have the most
validity, they are relied on for having the most concrete results. The real
problem, however, is that dietary principles have evolved considerably in the
last decade and so while study results may appear conclusive, the original
design of the study may be flawed. In fact, while large scale epidemiological
studies and cross cultural studies have tended to support the traditional
advice on dietary fat intakes and cardiovascular diseases, smaller - more
focussed - clinical trials that have attempted to intervene by altering dietary
fat intake (in the direction of lower fat intakes to reduce risk for disease)
in persons at high risk for cardiovascular disease have in general been
unsuccessful in terms of positively influencing cardiovascular disease
incidence.
Another factor that is largely ignored by mainstream nutrition pundits is
movement away from a single factor for heart disease. Clearly, the emphasis
now is on the total cholesterol number. In fact, total cholesterol values are
not very informative for heart disease risk prediction. Cholesterol consists
of two major components, LDL (the "bad" cholesterol) and HDL (the "good"
cholesterol). Cardiologists today are tending to use individual LDL and HDL
values for predicting risk, rather than the total value. In this view, rather
than striving for a total cholesterol below 200, cardiologists are looking
for LDL below 160 (and ideally under 130), with HDL above 45 (and ideally
over 60).
Mainstream nutrition pundits call for reductions in fat intake as a means to
lower cholesterol values. Current guidelines suggest limiting fat intake to
no more than 30% of caloric intake, but many advocates of low-fat eating
stress even further reduction (to the range of 20%). Several proponents even
suggest lowering fat intake to 10% or less of caloric intake. This level of
fat intake is not only difficult to achieve without sacrificing food and
palatability choices, but also runs the risk of resulting in a deficiency of
essential fatty acids as well as several fat soluble vitamins. Such however
is the result of a single focus of heart disease and cholesterol. This point
of view only examines the absolute level of fat intake without regard to fat
composition of the diet. The composition of the fat in the diet is surely as
important, if not more so, than absolute amounts. Most mainstream
nutritionists point to Japan as a low fat population with less cardiovascular
disease compared to the United States. However, they ignore the fact that the
Japanese have a dramatically different fatty acid composition of their diet
in addition to the lower level of fat. Noticeably absent from the discussion
of fat intake by mainstream nutritionists, however, is Greece where fat intake
is comparable if not higher than the US, but still enjoys a lower level of
cardiovascular disease. In this case, the fatty acid composition of the
typical Greek diet is also quite different from the US in spite of similar
levels of total fat intake. In the US, a large clinical study has revealed
that in a group with low levels of smoking, the diet component associated
with the greatest reduction in cardiovascular mortality was frequent nut
consumption - a relatively high fat food - but again with a different
(better) fat composition than the typical American diet.
The question for fat intake and cholesterol then becomes how do the various
fatty acids (the composition of the fat in the diet) influence cholesterol
levels? The answer to this question is not as straightforward as it might
seem. In the first, case animal studies while informative, do not provide the
real test, so that studies must be performed in humans. Dietary studies are
difficult to conduct, because palatability is always an issue and compliance
to the eating regimes is critical. In addition, changing dietary parameters
entail either adding a component which replaces something else or simply
adding something. Both options introduce problems with the subsequent
analysis. In the former case, one has a degree of uncertainty whether the
effect seen is due to withdrawing something bad or adding something good;
while in the latter, changing the total caloric intake will affect other
parameters such as weight changes. Over the years, multiple studies of these
types have been performed in many different ways in a an effort to arrive at
justified conclusions with robust results.
In terms of cholesterol, all fats are not created equal. Saturated fats
possess the ability to raise both LDL and HDL levels. Polyunsaturated fats
will lower both, while monounsaturated fats are relatively neutral with a
small tendency to raise HDL and lower LDL. This recognition and appreciation
suggests dietary approaches for influencing cholesterol levels beyond simply
blind fat reduction. As mentioned above, cardiologists are now tending to
view LDL and HDL independently, with less concern for the total cholesterol
value. Elevated cholesterol levels (at least those that merit concern)
typically are elevated because of elevated LDL levels. Elevated LDL levels
can be addressed by a reduction of saturated fat intake with a concomitant
increase in either monounsaturated or polyunsaturated fats.
The evidence that this manipulation works is shown by the Greek incidence of
cardiovascular disease. As mentioned above, Greeks have fat intakes similar
to the US, but in their case, they consume much less saturated fat and
correspondingly more monounsaturated fat (in the form of olive oil). The
story is similar for the Japanese who consume very little saturated fat, but
much more fish which is a rich source of polyunsaturated fat. Thus, the low rates of
cardiovascular disease in other countries has less to do with total fat
intake and more to do with fat composition, specifically low intake of
saturated fat. The Japanese provide even more data since although they
average a lower fat intake than the US, within Japan the incidence of
cardiovascular disease among fishing villages (where high intakes of fatty
fish occur) is lower than farming villages (with lower fat intakes due to
more rice consumption).
Lest the take home message be that saturated fat is the only culprit and the
all efforts should be directed to lowering saturated fat intake, the story is
slightly more complicated. In the first place, not all saturated fat is
linked to elevated cholesterol levels; only a subset of saturated fatty acids
have this effect. Secondly, the effect of saturated fat intake on
cardiovascular disease incidence is only mediated through its effect on
raising LDL levels. If cholesterol levels are controlled for, then saturated
fat has no independent effect. What this means is that if the LDL and HDL
levels are adequate, then adjustment of saturated fat intake will have little
benefit. This is an important concept because earlier work suggested that
saturated fat intake was an independent factor, but more recent work has
shown that much of this effect was due to failure to account for fiber intake
which typically declines as saturated fat intake increases.
Finally, the role of trans fatty acids needs clarification. Trans fatty acid,
although they are technically classified as polyunsaturated fats
because of their chemical structure, in general behave more along the lines of saturated
fats with regard to their effects on cholesterol. In addition to this
property, they also appear to have negative effects on hormonal regulation by
interfering with eicosanoids due to their similarity with polyunsaturated
fats. Since nutrition labels typically do not list trans fatty acid amounts,
look for term "partially hydrogenated" to disclose its presence.
In general, fat intake limited to 30% of total calories is more than adequate
for reducing cardiovascular risk, provided that the fatty composition of the
diet is appropriate and that fiber intake is adequate. Recommendations for fiber
are in
the range of 20 - 25 grams per day. Fiber intake is the major
reason that high fat was earlier associated with cardiovascular disease risk,
since usually as fat intake (and saturated fat intake in particular) goes up,
fiber intake goes down. In fact, attention to fat composition, by increasing
consumption of monounsaturated fats at the expense of saturated fat, while
maintaining adequate polyunsaturated intake (10% of total calories), results
in better lipid profiles reflected in LDL and HDL levels and greater
cardiovascular risk reduction. With more attention towards the fat
composition, higher levels of fat in the diet can be tolerated and result in
overall better health.
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