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After all, dietary fat is necessary to absorb key vitamins such as A, D, E, and K. And athletes involved in heavy training often need moderate amounts of fat in their diets just to satisfy their monumental daily caloric needs. Fat also assists carbohydrate in providing the fuel needed for endurance performances

However, not all fats are the same. Athletes can choose from saturated, monounsaturated, and polyunsaturated fats, and there are various kinds of polyunsaturates. There's also the possibility of choosing between medium-chain fats, such as those found in many dairy products, and long-chain lipids, like those found in meats and plants. Are certain forms of fat best for maximizing exercise capacity? And which type of fat is superior for your overall health?
Those two key questions have been debated vigorously during the past decade. Lately, the popular press has trumpeted the merits of monounsaturated fat for improved cardiovascular health, and athletic publications have put forth the proposition that medium-chain fats might increase performance under certain circumstances (basically, during ultra-endurance competitions). Health-conscious athletes have been trying to reduce the amount of saturated fat and cholesterol in their diets, often substituting polyunsaturated fats such as corn oil or mono-unsaturated fats like olive oil for animal fat, and many ultra-athletes have initiated the practice of consuming medium-chain fats during their events

Does low-cholesterol drive you mad?
However, the general move in the athletic world toward lower-cholesterol diets isn't without potential problems, because some research has linked low-cholesterol diets with increased rates of depression and suicide. Even more suprisingly, carefully documented research has determined that cholesterol levels are often below normal in habitually violent and impulsive homicidal criminals. Among adolescents, individuals who have an 'aggressive conduct disorder with an attention deficit problem' frequently have below-normal cholesterol concentrations

Why would cholesterol-poor diets and/or low blood cholesterol levels put people into a funk? The American Meat Council claims that the taste of animal flesh is a basic human need which - if denied - leads to aggressive behavior and poor mental health, but their explanation, while short and sweet, is flavored with a sour hint of conflict of interest. A decent biochemical explanation for the connection between low cholesterol and depression is that cholesterol, in spite of its reputation as the 'bad boy' of human nutrition, actually plays some key roles inside the body. One of its important functions is to maintain the integrity of brain-cell membranes

Preserving the integrity of brain cells is a good idea, since keeping the membranes intact keeps the cells working - and the individual possessing those cells alive. Often forgotten, however, is the fact that brain-cell membranes do more than keep the internal contents of brain cells from leaking out; they also contain 'receptors' for key chemical messengers in the brain. The receptors are simply attachment points for these messengers, which permit cell-to-cell communication, and cholesterol helps to keep those attachment points functioning properly and the cells communicating normally with each other

One of the key messengers is a chemical called serotonin, which exerts a calming, anti-depressant effect in the human brain. Serotonin levels are low in many individuals suffering from depression, and extremely violent military men and impulsive arsonists have been shown to have impaired serotonin output. Prozac, a widely-prescribed anti-depressant drug, acts to increase brain serotonin concentrations and improve mood and self-confidence. Overall, augmented levels of serotonin seem to be linked with better mental health, while low levels may be correlated with depression, violence, and the impulse to burn down your neighbour's house

In theory, if your diet were too low in cholesterol, you would have poorly structured brain-cell membranes, reduced numbers of receptors, and therefore brain cells which have a lower capacity to react well with serotonin. In short, you'd get depressed. This link between cholesterol, serotonin, and overall brain function explains why many researchers believe that low-cholesterol diets - such as the ones followed by many athletes - can increase the risk of the blues

But the Finns say no
Sounds good so far, and several studies have linked low cholesterol with depression, but a recent study completed in Finland reached the opposite conclusion. Finnish people who began to consume lower-cholesterol diets actually had reduced rates of depression. In addition, a key problem with the low-cholesterol, high-depression hypothesis is that it means that individuals who are depressed should have lower rates of heart disease (their low cholesterol would downgrade the risk of heart maladies). In reality, depressed people often have higher frequencies of heart troubles

So what's the real relationship between low-cholesterol diets and depression? Why were Finns with less cholesterol delighted instead of dispirited? The answer might be found by looking at the actions of a fat called DHA (docosahexaenoic acid), which is a polyunsaturated, 'omega-3' fat. You may recall that 'omega-3' fats shone brilliantly on the nutritional stage several years ago, when research suggested that they might help prevent heart attacks and strokes. Don't be put off by the term 'omega-3'. To understand what the term 'omega-3' actually means, remember that molecules of fat contain fatty acids, which are long strings of carbon atoms to which hydrogen atoms are attached. Usually, each carbon atom has two hydrogens attached, but sometimes a hydrogen is missing and the carbon is 'double-bonded' to an adjacent carbon. The term 'omega-3' simply means that the first double bond between carbon atoms is three carbons away from one end of a fatty acid

Certain sources of fat - such as fish oils - are fairly high in omega-3 fats, whereas more commonly used fats like vegetable oils have a preponderance of 'omega-6' fats, with the first double bond six carbons away from the end. For now, the only thing you need to remember is that the omega-3 and omega-6 fats are different chemically and play different roles in your body

As mentioned, DHA is an omega-3 fat. It's critical for our story because - like cholesterol - DHA plays a significant role in the construction of brain-cell membranes. As researchers Joseph R. Hibbeln and Norman Salem of the National Institute of Alcohol Abuse and Alcoholism in Rockville, Maryland, point out, people who attempt to bring down their cholesterol levels often do so by reducing the total amount of fat in their diets. This lowers the amount of DHA they're taking in - and therefore the amount of DHA which reaches their brains to build brain-cell membranes. In theory, these people are more likely to get depressed, since their brains are low in DHA. This chain of events would make it look as though low cholesterol were causing depression, even though the real culprit was inadequate DHA

Should you fry in fish oil?
In the Finnish study, in which diminished cholesterol intake led to lower - not higher - rates of depression, the study participants added a twist to the usual story: they didn't lower their cholesterol and saturated-fat intake the usual way - by slipping corn and soybean oil into their pots instead of butter - but primarily by eating increased amounts of fish and less beef. Fish is lower in fat than beef and also turns out to be a rich source of DHA, which may explain why the Finns didn't get depressed as their cholesterol levels dropped. In contrast, corn oil, which many people turn to as an alternative to saturated fat, is low in DHA. It could be that the corn-oil types are getting depressed in droves because of too-little DHA. Does this mean you should fry in fish oil rather than corn or soybean oil?
Maybe so, because Hibbeln and Salem firmly propose that it's the reduction in DHA and other omega-3 fats - not the decrease in cholesterol intake - which is the source of the depression problem. They suggest that the direct link between coronary artery disease and depression is simple to explain: the high saturated-fat diets of many people lead to clogged arteries, and the lack of omega-3 unsaturated fat in the saturated-fat regimen raises the risk of depression. Shifting these saturated-fat eaters over to corn, soybean, or safflower oil will keep the arteries cleaner but won't help the mental side of things, in Hibbeln and Salem's view, because those vegetable oils are low in omega-3s. Hibbeln and Salem even venture into theories of criminality, proposing that violent, impulsive behavior is associated with low levels of omega-3 fats and high quantities of the more popular omega-6 fats and saturated lipids

While the latter claim may seem extreme, it's backed up by some pretty decent research. For example, several years ago researchers at the Helsinki University Central Hospital checked out 34 habitually violent, impulsive male criminals. Eleven of these individuals had committed more than two violent crimes and four were impulsive arsonists. When blood samples from the 34 were compared with those from 16 healthy men from the University staff, it was found that the criminals had significantly higher levels of omega-6 fatty acids and appreciably lower quantities of one of the key omega-3 fats, DHA. In addition, men who had attempted suicide had roughly 20 per cent high omega-6 concen-trations, compared to men who had never tried to take their own lives. Of course, we can't say conclusively that low DHA drove the men to crime or suicide: correlations between variables don't mean that one is the driving force behind the other

Is the future in the past?
However, another interesting observation is that the prevalence of depression in the industrialized world has increased fairly dramatically in the past 100 years or so. In fact, since 1900 each group of people born within a 10-year period has had a higher risk of depression, compared to those born during the previous decade. If you were born between 1950 and 1960, for example, your depression risk is significantly greater, compared to someone born between 1940 and 1950, and appreciably higher than the risk incurred by someone born before 1940. True, the stresses of modern life may contribute to this effect, but it's also true that this century has seen a fairly dramatic increase in human consumption of omega-6 fatty acids, along with a fall in the intake of omega-3 lipids

There are a couple of reasons for this critical dietary swing. First, the nature of agriculture has shifted, so that just a few plant species (primarily corn and soybeans) are utilized as sources of fatty acids. These plants are relatively poor in omega-3 fats. In contrast, during evolutionary history, humans - especially in hunter-gatherer cultures - tended to eat wide varieties of vegetables and therefore took in products with higher amount of omega-3s. Second, commercial livestock are high in overall fat content but pretty deficient in omega-3 fats. For example, a side of beef coming from the cattle pen to your plate usually has a body-fat content of around 30 per cent, similar to a sedentary human, and virtually no omega-3 fat at all. When you eat the thing, you're swamping your body with saturated and omega-6 fat and neglecting omega-3 fats totally

In contrast, the free-range and wild animals (including deer, bison, horses, mammoths, and various grazing herbivores) which made up a larger portion of the human diet over the past million years or so were much richer in omega-3s and lower in overall fat. For example, a free-living African herbivore has a body-fat level of just 4 per cent, like the best human endurance athlete, with a good deal of this fat as omega-3

The result of the change in agricultural practices and human eating habits is that the ratio of omega-6 fat to omega-3 fat in the human diet has changed drastically. In fact, the average ratio of omega-6/omega 3 in the modern diet is now estimated to be somewhere between 10/1 and 25/1, a huge change from the ratio which prevailed during two million years of human evolution, which was probably about one to one! The bottom line is that humans are now eating much less omega-3 fat than they did during their long evolutionary history - and perhaps paying the price from a health standpoint

It's tempting to think that this change in fat intake may be related not only to the modern epidemic of depression but also to the current rampage of coronary artery disease. Critics of the notion that cardiovascular disease is a new thing contend that coronary artery maladies weren't a big health problem for paleolithic humans because they simply didn't live long enough to get into trouble, but it's interesting to note that very young Britons and Americans (age 20 or less) often already show signs of atherosclerosis, whereas currently existing hunter-gatherer tribes in Africa and other parts of the world, with their increased intakes of omega-3 fats, do not. This is in spite of the fact that hunter-gatherers may eat fair amounts of cholesterol, 500-600mg per day by some estimates, about double the amount recommended by the U.S. Senate Select Committee on Nutrition. Individuals from such cultures who reach the age of 60 or more often exhibit little evidence of coronary disease, despite their ample cholesterol intakes

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