Most people hate fat-some even die getting rid of it (for instance, by garage liposuction). The death rate in liposuction surgery is twenty to sixty times the death rate of all hospital operations, and 2 percent of gastric-bypass patients die within a few months of surgery. As sobering as these statistics are, they are dwarfed by the death toll from fat's indirect consequences: clogged arteries, high blood pressure, and heart disease-the most common cause of death in the United States. So it may come as a surprise that fat is one of the most useful molecules in your body.
Without fat you wouldn't have enough energy to sleep through the night! With a whopping nine calories per gram, fats (also called lipids) are the most efficient storage molecule in your body, containing large amounts of energy in a small space. When babies are born with a disease that prevents them from digesting fats, they must be awakened at regular intervals to be fed nonfatty foods, or kept on IV glucose, or else they will run out of energy between meals. Failure to metabolize fat is one of the causes of Sudden Infant Death Syndrome, where apparently healthy babies die inexplicably during the night.
You need fats as a day-to-day energy carrier, too. Just consider what it would take to live without them: In order to carry the amount of energy using protein or carbohydrates, a 70-kilogram (154-pound) person would have to weigh more than 100 kilograms (220 pounds). And that extra weight would not be nearly as cushioning and insulating as fat.
Structure Leads to Function: Making Margarine
Fat's high energy comes from its molecular structure. Like wax and gasoline, lipids consist of long chains of carbon and hydrogen that can be slowly "burned" by your body to yield energy, water, and carbon dioxide. Lipids come in different lengths and can be saturated or unsaturated, which affects the shape of the chain and what it does in your body.
Each carbon atom in a lipid chain can hold up to two hydrogens, and a saturated fat is full of them. In unsaturated fat, double bonds between carbons replace some of the hydrogens. If there is one double bond, the fat is monounsaturated. Polyunsaturated fats have multiple double bonds and fewer hydrogens.
Saturated animal fats, like butter and lard, are solid at room temperature because the flat fat molecules pack together tightly and stick to each other. Vegetable fats are generally unsaturated and full of double bonds, giving them a kinked molecular shape; they don't pack together well and remain liquid, so you have to keep them in a bottle.
If you want your vegetable oil to be solid, like margarine, you can either chill it or get rid of the kinks by adding hydrogens. Voila! Partially hydrogenated vegetable oils have more single bonds and are solid at room temperature.
Saturation also plays a role in your arteries, where different shapes of fat flow differently through the blood. Kinked, unsaturated fats don't stick to blood vessels, because blood pushes against their exposed surface and moves them along to where they are needed. Saturated fats, on the other hand, lie flat and occasionally stick to the sides of blood vessels, forming arteriosclerotic plaques. Autopsies of healthy soldiers show that a man in his mid-twenties has usually lost an astonishing 30 percent of the diameter of his large arteries to fatty plaques!
Our cell membranes are made of lipids, too, and once again saturation is key. A combination of saturated and unsaturated lipids allows membranes to be semifluid at room temperature, but they are prone to solidify at cold temperatures, like bacon grease stored in the refrigerator. Frostbite damage begins when oxygen can no longer diffuse through solidified cell membranes. After prolonged exposure to cold, the coldest, outermost cells may die.
So why don't reindeer get frostbite? After all, they live in the arctic, and reindeer get seriously cold feet! The chemical structure of cell membranes in a reindeer leg shows an amazing adaptation: As you progress down the leg toward the hoof, a higher percentage of the lipids in the cell membranes are unsaturated, so they can stay fluid at very low temperatures.
Transport: Oil and Water Don't Mix
Anyone who's made salad dressing knows that fats don't dissolve in water, and they don't dissolve well in your blood, either. Organisms solve this problem with fat chaperones, which are large, water-soluble proteins called lipoproteins. Fats are stored in your body as triglycerides, which are made up of three lipids. When a lipoprotein reaches a target cell, triglycerides must be broken apart into their component lipid chains, pulled across the cell membrane, and reassembled inside.
Fats also travel in the reverse direction-out of storage in fatty tissue and into circulation-to provide energy for other cells. If fat gets mobilized from storage and isn't used, it simply circulates until it is reincorporated into a fat cell. But during the time it spends in circulation, it has a small chance of adhering to an artery wall, potentially causing a heart attack or stroke. Doctors would like to know what makes this artery-clogger mobilize.
Your body's fat cells respond to cues that indicate the body is low on energy, or that the body is about to need more energy than usual. One of these cues is adrenaline, released as part of a stress response. While this stress response is not necessarily useful in an office job, in nature it prepares you to run or fight, so adrenaline mobilizes fat for action. Caffeine has many of the same effects as adrenaline. For this reason, marathon runners often consume caffeine an hour before they begin a race; this puts fat into the bloodstream so athletes can begin burning it for fuel as soon as they begin running. This helps their carbohydrate supply last longer. With only fat to rely on for energy, a runner will feel sluggish and low on energy. Without a caffeine boost, they "hit the wall" earlier and get stuck burning only fat, which is slower to metabolize than carbohydrates.
Most people who drink caffeine are not preparing for a marathon, so the circulating fat doesn't do much good. Consider, for example, a pilot's workday. During takeoff, one of the more stressful parts of a flight, his adrenaline levels are high, resulting in elevated fat circulation. If he drinks coffee first, his circulating fat levels will be even higher. Unlike the marathon runner, though, he'll sit for hours on end, causing fat to circulate without a target before ultimately returning to his fat cells. This losing combination means it's a good thing pilots are required to get an electrocardiogram every year after age forty.
If energy is the currency your body uses to perform its functions, then fat is a savings account. It's bad to have large amounts in circulation in your blood, but fat cells are a great way to store energy that will ultimately be converted into other forms, spent, or hoarded for a rainy day. Given how essential fats are, it's a shame they have such a bad reputation. Still, a gallon of ice cream will probably do you more harm than good.
Lipid reactivity to stress: Stoney, Catherine M.; Niaura, Raymond;
Bausserman, Linda; Matacin, Mala. 1999. "Comparison of chronic and acute stress responses in middle-aged airline pilots." Health Psychology. Vol. 18(3) 241-250.
Hazel, J.R.; Williams, E.E. 1990. Prog Lipid Res 29(3):167-227. "The role of alterations in membrane lipid composition in enabling physiological adaptation of organisms to their physical environment."
Senault, C.; Yazbeck, J.; Goubern, M.; Portet, R.; Vincent, M.; Gallay, J., 1990. "Relation between membrane phospholipid composition, fluidity and function in mitochondria of rat brown adipose tissue. Effect of thermal adaptation and essential fatty acid deficiency." Biochim Biophys Acta 1023(2):283-289.
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