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A large number of popular diets are based on the restriction of certain types of nutrients such as fats or carbohydrates. This article analyzes benefits and disadvantages of these approaches.

Weight loss and dieting is a big business these days. Good books on dieting are making millions of dollars in sales, and weight loss coaching is rapidly developing as a profitable business. Many advertised approaches to the weight loss, however, have not been subject to scientific scrutiny. In fact, 75% of popular diets are not scientifically tested. This article outlines some established scientific facts about two mainstream approaches to weight loss based on restricting the intake of fats and carbohydrates.

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Low-Fat Diets

The World Health Organization Study Group currently recommends that 15% of the daily calorie intake be derived from fat. Therefore, in practical terms, a low-fat diet would be one diet in which ≤15% of daily calories are derived from fat, preferably with an intake of similar amounts of saturated, monounsaturated  and polyunsaturated fatty acids.

The rationale underlying low-fat diets is largely based on the belief that dietary fat is positively associated with body fat, which has fueled a number of popular fat-restricted diets.

The reported increase in the consumption of fats across the world has been positively associated with a number of undesirable events, such as increased incidence of cardiovascular disorders, increased incidence of certain cancers and an overall rise in mortality resulting from cardiovascular complications.

Low-Fat Diets Substantially Reduce Cholesterol Levels

Conversely, interventional studies conducted with the aim of identifying the outcomes of reducing fat consumption (e.g.: the Heidelberg Trial, the Lifestyle Heart Trial, the Pritikin Longevity program) have produced impressive data. For instance, subjects participating in the Heidelberg Trial, after 6 years had their LDL cholesterol (so-called “bad” cholesterol) levels reduced by 6% in the intervention group and elevated by 1% in the control group. HDL cholesterol (“good” cholesterol) levels were elevated by 14% in the intervention group and 12% in the control group.

In the Lifestyle Heart Trial, limiting fat intake to 7% of the total calorie intake allowed for coronary lesions to regress in 82% of the individuals in the intervention arm.

As for the specific effects on body weight, data appear promising as well. The Women’s Health Trial reported reductions in dietary fat intake from 37% to 22% of energy, a change resulting in a 3-kg weight loss over a 2-year period. Researchers reported that intensive instruction to maintain a low-fat diet resulted in reduction of fat intake from 39% to 22% of calories and was accompanied by a 3.2-kg weight loss in 6 months, although this was reduced to 1.9 kg at the end of 2 years study period (the control group lost 0.1 kg after 2 years). However, long-term data suggesting very low fat diets alone will sustain long-term weight loss is lacking.

Low-Fat Diets: Some Potential Risk Factors

So while the results of these studies appear impressive, many questions remain about long-term efficacy and safety. One of the concerns with adopting a low-fat diet is the potential risk for nutrient deficiency. It is important to note that the nutrient adequacy of very low fat diets is highly dependent on individual food choices. Very low fat diets can include nutrient-dense foods such as fruits, vegetables, and whole grains, or more the modern fat-free and low-fat alternatives to traditionally high-fat foods, such as snacks and desserts, which are not so nutrient-dense.

Recommending a lower limit to the fat intake is controversial because of the difficulty in balancing the efficacy of a very low fat diet in decreasing plasma cholesterol levels against the risks of nutrient inadequacy or other adverse effects. Certain populations, such as growing children, pregnant women, lactating women, and the elderly, have special needs for essential fatty acids and caloric density and, as such, should not abide by these diets.

Very-low-fat diets have a very high carbohydrate and fiber content. The American Heart Association cautions against the use of such diets because their high carbohydrate content can increase triglyceride levels. Additionally, the diets can contain over twice (40–70 g/d) the recommended amount of fibers. High fiber intakes can decrease the absorption of zinc, calcium and iron. Complaints of abdominal fullness have also been reported.

Not All Fats Are The Same

Moreover, many lines of evidence indicate that the type of fat is very important to long-term health. For instance, replacing saturated and trans with natural vegetable oils can greatly reduce the risk of heart disease and diabetes. In the Nurses’ Health Study II it was seen that women who consume high amounts of red meat and high-fat dairy foods during their early adult years are at increased risk of developing breast cancer. Making good dietary choices does really matter, but it might be the type of fat, not the amount, that is most important. 

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