Tuesday, 29 May 2012
Partially hydrogenated vegetable oils have been an increasingly significant part of the human diet for about 100 years (particularly so in the latter half of the 20th century), and some deleterious effects of trans fat consumption are scientifically proven.
The exact biochemical methods by which trans fats produce specific health problems is still being continually researched. For example, the mechanisms through which trans fats contribute to coronary heart disease are fairly well understood, while the mechanism for trans fat's effect on diabetes is still under investigation.
Coronary heart disease
The primary health risk identified for trans fat consumption is an elevated risk of coronary heart disease (CHD). A comprehensive review of studies of trans fats was published in 2006 in the New England Journal of Medicine that concludes that there is a strong and reliable connection between trans fat consumption and CHD.
The major evidence for the effect of trans fat on CHD comes from the Nurses' Health Study (NHS) — a cohort study that has been following 120,000 female nurses since its inception in 1976.
Hu and colleagues analyzed data from 900 coronary events from the NHS population during 14 years of follow up. He determined that a nurse's CHD risk roughly doubled (relative risk of 1.94) for each 2% increase in trans fat calories consumed (instead of carbohydrate calories). By contrast, it takes more than a 15% increase in saturated fat calories (instead of carbohydrate calories) to produce a similar increase in risk. Eating non-trans unsaturated fats instead of carbohydrates reduces the risk of CHD rather than increasing it.
Hu also reports on the benefits of reducing trans fat consumption. Replacing 2% of food energy from trans fat with non-trans unsaturated fats more than halves the risk of CHD (53%). By comparison, replacing a larger 5% of food energy from saturated fat with non-trans unsatured fats reduces the risk of CHD by 43%.
There are two accepted measures of risk for coronary heart disease, both blood tests. The first considers ratios of two types of cholesterol, the other the amount of a cell-signalling cytokine called C-reactive protein. The ratio test is more accepted, while the cytokine test may be more powerful but is still being studied. The effect of trans fat consumption has been documented on each as follows:
Cholesterol ratio: This ratio compares the levels of LDL (so-called "bad" cholesterol) to HDL (so-called "good" cholesterol). Trans fat behaves like saturated fat by raising the level of LDL, but unlike saturated fat it has the additional effect of decreasing levels of HDL. The net increase in LDL/HDL ratio with trans fat is approximately double that due to saturated fat. (Higher ratios are worse.)
C-reactive protein (CRP): A study of over 700 nurses showed that those in the highest quartile of trans fat consumption had blood levels of CRP that were 73% higher than those in the lowest quartile.
Another study considered deaths due to CHD, with consumption of trans fats being linked to an increase in mortality, and consumption of polyunsaturated fats being linked to a decrease in mortality.
There has been suggestion that the negative consequences of trans fat consumption go beyond the cardiovascular risk. In general, there is much less scientific consensus that eating trans fat specifically increases the risk of other chronic health problems:
Cancer: There is no scientific consensus that consumption of trans fats significantly increases cancer risks across the board. The American Cancer Society states that a relationship between trans fats and cancer "has not been determined." However, one recent study has found connections between trans fat and prostate cancer.
Diabetes: There is a growing concern that the risk of type 2 diabetes increases with trans fat consumption. However, consensus has not been reached. For example, one study found that risk is higher for those in the highest quartile of trans fat consumption. Another study found no diabetes risk once other factors such as total fat intake and BMI were accounted for.
Obesity: Research indicates that trans fat may increase weight gain and abdominal fat, despite a similar caloric intake. A 6-year experiment revealed that monkeys fed a trans-fat diet gained 7.2% of their body weight, as compared to 1.8% for monkeys on a mono-unsaturated fat diet. Although obesity is frequently linked to trans fat in the popular media, this is generally in the context of eating too many calories; there is no scientific consensus connecting trans fat and obesity.
Liver Dysfunction: Trans fats are metabolized differently by the liver than other fats and interfere with delta 6 desaturase. Delta 6 desaturase is an enzyme involved in converting essential fatty acids to arachidonic acid and prostaglandins, both of which are important to the functioning of cells.