Download Dietary Fat, Endurance Exercise, and Health

Dietary Fat, Endurance Exercise, and Health
When sports nutritionists devise dietary recommendations for endurance athletes, fat is typically
relegated to third-string status behind carbohydrates and protein. In practical terms, you’re advised to
consume enough carbs to replenish glycogen stores and enough protein to meet demands, including the
need for muscle tissue repair and building. It’s only after carbohydrate and protein needs
have been met that fat even enters the discussion. But its third-string status doesn’t change the fact that
fat is an important and plentiful source of muscle fuel, and it’s also a source of essential nutrients that are
vital to good health. This article explains the role of fat as a muscle fuel and how it teams with
carbohydrates to keep you going during exercise. You’ll also learn why you need certain fats to be
healthy, and which dietary fats are best for you and which are to be avoided.
Fat from Food: Absorption to storage
In chemical terms, the fats found in food are primarily triglycerides. A triglyceride is made up of
biochemical building blocks. The backbone of a triglyceride is a molecule called glycerol, and it has room
to attach to three biomolecules known as fatty acids, of which there are various kinds. When you eat
food, any fats (triglycerides) present are digested or broken down into their glycerol and fatty acid
building blocks. These building blocks are then absorbed into cells that line the small intestine. Once
inside the intestinal cells, the fatty acids and glycerol are repackaged into triglycerides and then
transported in the bloodstream by special carriers. A major storage site for triglycerides is your adipose
tissue — the layer of fat found under your skin and between your organs. Triglycerides are also
deposited in lipid droplets found within skeletal muscle tissue. These lipid droplets are referred to in
scientific terms as intramyocellular lipids. Even lean athletes have an ample supply of fat stores. In fact,
from adipose tissue alone, you’ve probably got somewhere around 80,000 calories’ worth of fat stored
away, another 2,500 calories’ worth as lipid droplets in muscle, and 80 or so calories as triglycerides
attached to special carriers circulating in the bloodstream. By comparison, your carbohydrate cupboard is
not so amply stocked. Fully loaded, you only have about 1,500 calories’ worth of carbs in the form of
muscle glycogen, 400 calories socked away as glycogen in the liver, and a measly 20– 40 calories as
glucose circulating in the bloodstream.
Fueling Muscle with Fat versus Carbs
Whether your muscles primarily burn stores of fat or carbs as fuel depends on the intensity of your
physical activity. At rest, the vast majority of energy needed to fuel muscle comes from fatty acids
summoned from your adipose tissue stores by way of the bloodstream. Very little fuel comes from lipid
droplets in muscle or blood glucose, and virtually none comes from muscle glycogen. When you get off
the couch and exercise at an intensity equivalent to a walking pace, here again the vast majority of the
muscle fuel comes from fatty acids mobilized from your adipose tissue stores. Augmenting this are small
amounts of fuel contributed from muscle lipid deposits and blood glucose, while again, virtually none
comes from muscle glycogen. If you then bump up your exercise intensity up to a moderate level that
you could sustain for a number of hours, the use of fatty acids from adipose tissue continues to remain
about the same. However, there’s a big jump in the use of lipid droplets within muscle tissue. And at the
same time, muscle glycogen and glucose make a significant contribution. In fact, at this moderate level of
exercise intensity, carbohydrates and fat are just about matched in terms of their contributions to fueling
your muscles. Finally, when you then ratchet up the exercise intensity even higher, to a level equivalent
to a racing pace for a 90-minute endurance event, the fat from adipose tissue and muscle lipid droplets
simply can’t keep pace. In fact, fat as a fuel doesn’t account for any of the increase in exercise intensity.
Instead, virtually all of the extra fuel needed to shift from moderate-intensity to high-intensity exercise
comes from muscle glycogen stores and circulating blood glucose. However, given your limited
carbohydrate reserves, you can only maintain a high-intensity endurance pace for about 90–120
minutes. Without replenishing those carbohydrate reserves, muscle glycogen stores run dry. At that
point, liver glycogen stores are relied upon to maintain blood glucose levels. But when liver glycogen
stores become depleted, blood glucose level drops, and you are forced to stop or slow down
considerably. In fact, you’re forced to slow to a pace where fat can again be your primary muscle fuel
source. In short, fat clearly is not the primary fuel that powers high-intensity endurance exercise. Instead,
that’s a role for carbohydrate fuel reserves. However, glycogen stores and blood glucose happen to be in
short supply, even under the best of conditions. And that’s the reason nutritionists focus so intently on
getting you to consume enough carbs each day. Without adequate carbohydrates, you don’t replenish
your glycogen stores, therefore you can’t get to and maintain that high intensity gear you need to be first
across that finish line or to train at your best. That being said, fat is an important fuel source at rest and
during low- to moderate-intensity exercise. And while your adipose tissue stores aren’t significantly
depleted by an endurance event or a tough training session, the same isn’t true for intramyocellular
lipids, those triglyceride droplets found in muscle tissue.
Muscle Lipid Reserves need Replenishment too
Researchers have found that 2–3 hours of moderate-intensity exercise results in a 60–70% drop in
muscle lipid stores. It turns out that those tiny lipid droplets in muscle cells are usually found adjacent to
mitochondria within cells. Mitochondria are the metabolic energy generators for cells. The fact that the
lipid droplets are so close in proximity to these energy factories has scientists thinking that this supply of
triglycerides is a key fuel source during moderate-intensity exercise. This may be helping to spare
glycogen stores for later in exercise. Interestingly, researchers have also found that when endurance
athletes consume very high carbohydrate diets with little fat, muscle lipid deposits don’t get replenished
after exercise. It’s still too early to know the performance implications of this finding, but it seems prudent
to make sure that your diet isn’t so low in fat that you fail to replenish depleted muscle lipid stores.
Fat as a Source of essential Nutrients
There are other important reasons to keep fat intake on your radar screen. Fats play important functional
and structural roles in the body. They are a major constituent of brain matter and are important to brain
development. Fats are found in the membranes of cells throughout your body, and they’re critical to the
normal functioning of your lungs, eyes, and nerves. The fat in your diet also serves as a source of, and
helps with the absorption of, fat-soluble nutrients such as vitamin D for strong bones, vitamin K that helps
blood clot, the antioxidant vitamin E, along with vitamin A and plant sources of vitamin A known as
carotenoids. In addition, a couple of the fatty acids that are common building blocks for triglycerides are
essential to your diet. This means these fatty acids are critical to good health, but we can’t make them
ourselves. Instead, we have to consume them from the diet. These fatty acids play important hormonelike roles that influence vital physiological processes like normal blood clotting and inflammation.
Calorie Balance
So far, we’ve made the case that you need enough fat in your diet each day to replenish your muscle
lipid stores depleted during exercise and to be healthy, but you also want to avoid consuming too much
fat. Why? Because fat is especially rich in calories. On a per gram basis, carbohydrates and protein
deliver 4 calories, while fat delivers a whopping 9 calories. So that plate of salad loaded with leafy
lettuce, ripe tomatoes, and a host of tasty chopped up vegetables is pretty low in calories. But add a halfcup of creamy salad dressing, made up largely of fat, and suddenly you’ve transformed your light salad
into a calorie bomb. Consuming calories consistently in excess of what you require daily leads to weight
gain and an increase in body fat. As an endurance athlete, having to cart around more weight than you
need results in greater exertion during exercise and slower times. The bottom line is that consistently
taking in too many calories, whether they’re from fat, protein, carbs, or some combination, will lead to
weight gain. But fat gets extra attention because it is so rich in calories. As such, it can be easy to
inadvertently exceed your calorie needs when eating fatty foods. Balance is key. Having some fat in your
diet is important, but control your intake so that you meet your needs for fat without over consuming
calories.
Get Your Fats Straight
So, what are the good fats and which ones should you avoid? To differentiate the good from the bad, you
need to know the different kinds of fat. Earlier we covered the fact that triglycerides are composed of
three fatty acids attached to a glycerol molecule. Fatty acids come in various shapes and sizes, and they
are categorized as monounsaturated, polyunsaturated, saturated, or trans. The fats found in foods are
actually a mixture of different types of fatty acids, but they get categorized based on the predominate
type of fatty acid they contain. For example, olive oil contains a mixture of monounsaturated,
polyunsaturated, and saturated fatty acids, but olive oil is referred to as a monounsaturated fat because
monounsaturated fatty acids are in the majority. Fat sources that primarily feature monounsaturated and
polyunsaturated fats get a thumbs-up. These fats tend to lower LDL or ―bad‖ cholesterol and/or raise
HDL or ―good‖ cholesterol. When consuming fats, choose from among these: Common monounsaturated
sources include: olive oil, canola oil, peanut oil, avocados, almonds, pecans, hazelnuts, sesame seeds,
and pumpkin seeds. Common polyunsaturated sources include: corn oil, sunflower oil, safflower oil,
soybean oil, walnuts, flaxseeds and flaxseed oil, and fatty fish. Fatty fish such as salmon, tuna, herring,
mackerel, sardines, and anchovies deserve their very own bullet point. The polyunsaturated fatty acids in
these fish are referred to as omega-3 fatty acids and go by the acronyms EPA (standing for
eicosapentaenoic acid) and DHA (standing for docosahexaenoic acid). Diets rich in these fish oils are
associated with a reduced risk of cardiovascular disease. Experts recommend eating two or more
servings of these types of fatty fish each week to obtain the benefits of the omega-3 fatty acids. Fat
sources rich in saturated fats and trans fats get a thumbs-down. These fats raise ―bad‖ LDL cholesterol
levels, while trans fats also lower ―good‖ HDL cholesterol levels. The experts recommend limiting
saturated fat intake as much as possible and steering clear of trans fats altogether. Another type of fat in
the diet is cholesterol. For many individuals, the link between cholesterol in the diet and blood cholesterol
is not that strong. Saturated fats and trans fats have a much stronger negative influence on cholesterol
levels. Thus, health organizations like the American Heart Association recommend limiting cholesterol
intake to less than 300 mg daily, rather than trying to avoid it entirely. However, if you have heart disease
or your LDL cholesterol level is 100 mg/dl or greater, it’s recommended that you limit your cholesterol
intake to less than 200 mg a day. Common saturated fat sources include meat and dairy products. You
can cut back on saturated fat intake by choosing low-fat and nonfat dairy foods and lean
cuts of meat. Coconut oil and palm oil, and products made with them, are also sources of saturated fat.
For the most part, trans fats are man-made through the hydrogenation or partial hydrogenation of
vegetable oils. Trans fats can be found in fried foods; solid fats like stick margarine and partially
hydrogenated vegetable shortening; baked goods like cookies, biscuits, cakes, muffins, and crackers;
snack foods like chips, candy, and microwave popcorn; and in pre-mixed products like cake and pancake
mixes. Common cholesterol sources include eggs, dairy products, meat, and poultry. Egg yolks and
organ meats (liver, kidney, sweetbread, and brain) are particularly high in cholesterol. Fish generally
contains less cholesterol than other meats, but some shellfish are high in cholesterol. Foods from plants,
such as vegetables, fruits, grains, cereals, nuts, and seeds, contain no cholesterol. To navigate fats in
food, make a habit of examining the Nutrition Facts box on the labels of packaged foods. If you do, you’ll
see that Saturated Fat and Trans Fat are all listed. You’ll also see Cholesterol, Calories, and Total Fat
listed. The point isn’t for you to become a human nutrient calculator. Instead, use the information to
become familiar with how much and what types of fat are in the foods you’re buying.
Tips for Choosing Healthy Fats and Controlling Fat Calories
The goal is to make smarter, healthier food choices that will help you achieve your full athletic potential
and help keep you healthy. Rather than avoiding fat in your diet, replace unhealthy fats with the healthy
variety, and control your overall fat intake to keep calories in balance. Use plant-based oils for cooking
and baking. Olive oil, canola oil, and other plant-based oils are rich in heart-healthy monounsaturated
and polyunsaturated fats. Steer clear of trans fats by reading the labels of packaged foods before buying.
Avoid products with partially hydrogenated oils. In restaurants, ask if trans fat–free oils are used for frying
and baking. Choose lean cuts of meat and low-fat or nonfat dairy products. Eat fish rich in omega-3 fatty
acids at least twice a week. Limit your intake of deep-fried foods and ask for sauces and gravies on the
sideso you can better control your intake of calories from fat.
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