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Low-Carbohydrate Diets vs Low-Fat Diets: A comparison

Weight loss, General, Diets | October 19, 2014 | Author: The Super Pharmacist

diet, weight loss

Low-Carbohydrate Diets vs Low-Fat Diets: A comparison

The theory of dietary fat as a major factor in weight gain and ill health may be overly simplistic. Carbohydrates are complexes of relatively simple molecules whick provide energy in the diet, and may be converted into fat molecules. A new body of research indicates that excess carbohydrate intake may contribute to fat storage.

Glucose, a sugar widely used in commercial food preparation and sweetening, may be regarded as a single 'unit' of carbohydrate. These are broken down in the body to be incorporated into new fat molecules. 

Carbohydrate-rich foods, such as table sugar and very sugary foods, that can be easily broken down into glucose subunits are known as high glycaemic index foods, just as complex carbohydrates are termed low glycaemic index foods.

Therefore, glycaemic index may be a factor in the risks of obesity and disease, as are the different types of fat molecules.

Conventional low-fat diet

Theories state that eating patterns that allow for a large proportion of fats are linked to the increased risk of obesity, cardiovascular disease and general ill health. Saturated fatty acids are a type of dietary fat particularly rich in energy that has been strongly linked to this risk. However, large-scale population studies have found no significant association between saturated fat intake and coronary heart disease.

Mono-unsaturated fats have been associated with an increased cardiovascular disease risk.

Trans fats, a modified form of the unsaturated variety used in large quantities in the production of processed foods, are also strongly associated with the risk of cardiovascular disorders.

Polyunsaturated fatty acids have a less well-defined role in heart health, but may be associated with reduced risk. There are two main types of polyunsaturated fat; omega 3- and omega 6-fatty acids.

  • Omega-3 fatty acids are associated with positive effects on cardiovascular health, whereas omega-6 fatty acids are associated with the opposite.

In addition, there are other forms of dietary fats, such as short- and medium-chain fatty acids, which are linked to variable (but largely positive) effects on the risks of cardiovascular disease and obesity.

Low-Fat Diets

Low-fat diets should, in theory, induce a reduction in the concentration of dietary fatty acids and cholesterol in the bloodstream, both of which are biological markers of cardiovascular disease.

A review and analysis of 8 studies on low-fat diets in pre- and post-menopausal women concluded that low-fat diets significantly reduced total cholesterol in comparison to control subjects following their usual eating plans. However, this beneficial association was not so clear-cut; there was a significant reduction in HDL-type cholesterol, which is associated with improved cardiovascular outcomes, as well as LDL-type cholesterol, associated with increased risks.

However, another review of 32 trials comparing the effects of high-fat and low-fat diets found that low-fat diets were associated with a significantly greater reduction in LDL cholesterol (and total cholesterol), although high-fat diets were associated with significant increases in HDL cholesterol and decreases in fatty acid in the blood.

Low-fat diets may not be suitable for individuals for whom glucose metabolism is already compromised and are at a higher risk of cardiovascular disease, i.e. diabetes patients. An analysis of 14 trials including over 1,700 participants found that a high-fat diet was associated with significantly reduced blood pressure and blood glucose levels. Diabetic patients on high-fat diets also had higher levels of 'beneficial' HDL cholesterol in comparison with those following low-fat diets. Another review of 15 randomised-controlled trials on low-fat diets including non-diabetic subjects found no significant effect on bodyweight, LDL, HDL or other bio-markers of cardiovascular risk such as C-reactive protein.

Low-Carb Diets

Carbohydrate intake may influence the storage of fat in the body, and may be associated with increased cholesterol and fatty acids in the bloodstream, as the energy they provide negates the necessity of breaking the existing fat molecules down for fuel instead. Low-carb diets may drive the physiological mechanisms by which this is done.

A review of four studies on reduced carbohydrate diets indicated a significant decrease of total LDL cholesterol in response to this intervention, but only in the short term (i.e. 5 to 12 weeks). No effects on the concentrations of HDL or fatty acids in the blood were reported.

A meta-analysis of 20 trials including approximately 3,000 subjects found that low-carb diets had a significant, positive effect on glucose control and HDL levels in diabetic patients, and also resulted in significant reductions in bodyweight.

Another analysis of 17 studies with over 200,000 participants found a significant association between low-carbohydrate diets and the risk of all-cause mortality, but not cardiovascular mortality or the incidence of cardiovascular disease. This may be worrying, but is the result of just one analysis, with these associations coming from the minority of its constituent studies. However, this may indicate the need for further study into the effects of reduced carbohydrate intake in the long term.

Low-Carb vs. Low-Fat Diets

A meta-analysis of 48 diets including over 7,200 participants found that low-carb diets were associated with slightly superior weight loss in comparison with low-fat diets; an average of 8.7kg compared to 8kg respectively after 6 months of dieting, but after 12 months these averages were nearly identical (7.27kg and 7.25kg respectively).

Analysis of 13 trials comparing very low carbohydrate diets (those allowing for a maximum of 50g carbohydrate per day) with low fat diets showed that the former achieved a greater decrease in bodyweight and blood pressure in the long term, although both HDL and LDL cholesterol were increased.

Some other studies appear to contradict the hypotheses that carbohydrate intake increases cardiovascular risk.

A study including approximately 22,000 male smokers aged between 50 and 69 years (and therefore at a high risk of cardiovascular disease) substituting carbohydrates for fats found that this was associated with reduced risks. Differences in glycaemic index appeared not to influence cardiovascular risk. In addition, the replacement of saturated and trans fats with carbohydrates was also associated with a decreased risk, but the replacement of monounsaturated fats was associated with an increased risk. This may indicate that the effect of carbohydrate intake on the probability of cardiovascular disorders depends on the types of fat also included in the diet.

The Mediterranean diet, in which a large proportion of daily food intake is made up of high-glycaemic index and omega 3-rich foods, is associated with decreased risks of several life-threatening conditions, including several cancers. In addition, a meta-analysis of 17 studies showed that consistent adherence to this eating pattern was associated with a 23% decrease in the risk of type II diabetes onset.

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