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We've already told you that not all types of dietary fat are the same in relation to body-fat increases. For example, omega-3 fatty acids, commonly
found in fish oil, appear to actually inhibit fat gains in lab rats and mice. This relates to the incorporation of omega-3 fatty acids into cellular
membranes. When instilled in cell membranes, omega-3 fats favor a greater degree of insulin sensitivity due to increases in fluid cell receptors for
the hormone. In doing so, insulin activity becomes more effective, which translates into less bodyfat accretion.
Saturated fats, such as those found in meats and cheeses, work in an opposite fashion to omega-3 fats. Instead of increasing cell-membrane fluidity, saturated fats promote denser cellular membrane. This, in turn, interferes with insulin cell-receptor actions, inducing insulin resistance and a consequent greater risk of bodyfat gains.
But while omega-3 fats are polyunsaturated in structure, there is a third basic type of fat to consider: omega-9 fats, represented by monounsaturated fats, so termed because they contain one open bond along their fatty-acid structure. These differ from polyunsaturated fats, which have several open bonds. Saturated fats, as the name implies, have no open bonds.
Most nutrition researchers consider monounsaturated fats "neutral" in biological activity. This refers to their actions in the synthesis of fat-derived chemicals called eicosanoids, a general term encompassing several hormonelike chemicals, such as prostaglandins, leukotrienes and thromboxanes, which are involved in a myriad of vital body functions. Monounsaturated fats don't influence the synthesis of these important eicosanoids to the same extent as do either polyunsaturated or saturated fats, hence the neutral label.
Unlike saturated fats, which are associated with higher liver-cholesterol synthesis, or polyunsaturated fats, which tend to lower blood-cholesterol levels, monounsaturated fats don't affect cholesterol much one way or the other. One advantage monos have over polys is that the latter type of fat also lowers protective high-density lipoprotein cholesterol, while monos do not.
The beneficial effects of a gener-ous intake of monounsaturated over saturated fats is reflected in the so-called Mediterranean diet, which features the mono variety. People in countries such as Italy, where this style of eating is popular, usually show lower rates of cardiovascular disease and cancer. The lowered cancer risk results from the fact that monounsaturated fats (such as olive oil) are less prone to oxidation than polyunsaturated fats. Oxidation of fats leads to the formation of noxious byproducts called free radicals that attack the fatty elements of cell membranes, breaking the cohesion of this cell barrier and leaving the cell open to the mutations that characterize cancer.
All of these favorable factors associated with monounsaturated fats have resulted in some suggestions that such fats be the mainstay of dietary fat intake. A salient exam-ple of this advice is offered in the megaselling diet book written by biochemist Barry Sears, PhD, called The Zone. Sears strongly advises the consumption of monounsaturated fats - about 30% of daily caloric intake - with some polyunsaturated fats thrown in. His reasoning is based on the beneficial effect such fats have on insulin (i.e., little or none) and the way they promote the synthesis of what he calls "good" eicosanoids that favor fat oxidation through increased insulin control.
Sears has been consistently at-tacked by scientists skeptical that consumption of a specific type of fat can truly affect either bodyfat or insulin control. However, emerging evidence appears to confirm Sears' ideas that all fats are not created equal. For instance, several animal studies show that consuming omega-3 fatty acids and monounsaturated fats positively influences eicosanoid synthesis. Since there's no ( argument that eicosanoids are in fact made from dietary-fat building blocks, it's difficult to understand why Sears would be attacked for this view.
A recently published study involving lab mice (Journal of Nutrition, 127:2006-10, 1997) shows that a combination of exercise and specific dietary-fat intake does indeed inhibit bodyfat gains in mice. The rodents in this case were given one of three diets: 1) a diet low in fat, containing 11.5% of calories from fat; 2) a diet high in saturated fat, containing 40.8% of calories derived from beef fat; and 3) a diet high in monounsaturated fat, containing 40.8% of calories from canola oil.
Some of the mice exercised using a wheel device in their cages. Others simply lay in their cages eating high-fat diets. The latter inactivity is typical of many human adults and children. As expected, these sedentary, high-fat-eating mice gained more bodyfat than the mice fed either the low-fat or mono diets. Mice that were allowed to exercise showed lower levels of bodyfat, regardless of which type of fat dominated their diets.
The interesting part of the research, however, was that the mice consuming the monounsaturated-fat diet gained the least bodyfat of all of the groups. This was true despite the fact that both the mono mice and the saturated-fat mice had unrestricted access to food. This confirms earlier findings that saturated fat makes you fatter than other fats.
As to why saturated fat is so fattening, scientists look to several mechanisms. One has to do with how fast fats are oxidized or burned in the body. Monounsaturated fats are relatively fast burning, as are the omega-3 fats represented by fish oils or the alpha-linoleic fatty acid found in flax oil that is converted into omega-3 fats in the body. Because saturated fats are slower burning, they tend to accumulate, leading to greater bodyfat gains.
Another factor is the insulin resistance promoted by saturated fats discussed earlier. Insulin resistance means that more insulin is secreted by the body. Insulin is a storage hormone that favors the uptake and retention of dietary protein, fat and carbohydrate. In practical terms, when insulin is secreted, bodyfat is either maintained or synthesized.
On this basis, it's easy to understand why saturated fat favors bodyfat gains. The high caloric content of saturated fat, coupled with its slow-burning characteristics and its effect on inhibiting optimal insulin activity, favor bodyfat gains. Other fats, such as the omega-3 fats and mono fats, are both fast-burning and insulin-promoting, thereby making such fats beneficial to those seeking to lose or control bodyfat levels.
Another notable aspect of this investigation was that all of the exercising rats, regardless of the type of fat they consumed, exhibited lower bodyfat levels. The specific type of exercise they did was the human equivalent of moderate-intensity, longer-duration aerobics. Studies indicate that this type of aerobics favors greater usage of fat as fuel.