“If you can't explain it to a six year old, you don't understand it yourself.”
For many years, the idea that low-fat diet may help us prevent coronary heart disease has been deeply ingrained in our minds. Since pretty much all high calories come from fats, protein or carbohydrates, reducing consumption of one, means an increased consumption of another. Clearing our plates of meat and eggs, the major source of fat and protein, has encouraged us to find a replacement and eat more grains, pasta and starchy vegetables, all of which are carbohydrates.
Over the past 40 years, we cut fat intake by 25 % and increased carbohydrates consumption by more than 30%, according to a new study. The consequences are tangible. It’s not difficult to find research showing that a high-carb foods rich in refined grains and sugar increase the risk of obesity, diabetes and coronary heart disease. In fact, the risk is much higher than a diet high in fat and cholesterol.
It’s interesting to see that the obesity epidemic started about the same time as the low-fat guidelines first came out. Looking back in 1977, the low-fat diet was recommended both in Europe and in the States. Although this doesn’t prove that low-fat diet was the culprit, we know that since early 80s people have started replacing traditional foods, like meat or butter with processed low-fat foods high in sugar.
Fig.1. Overweight and obesity in US population by age between 1971-2005. National Center for Health Statistics (US). Health, United States, 2008: With Special Feature on the Health of Young Adults. Hyattsville (MD): National Center for Health Statistics (US); 2009 Mar. Chartbook.
Let's review the research. A randomised, controlled trial published a few years ago was looking at what diet, low-carb or low-fat, may have a bigger effect on weight loss and cardiovascular events in women. Scientists concluded that the low-fat diet was associated with a lower weight loss and lower risk of major risk factors for cardiovascular disease and diabetes than the low-carb diet. Since then, many studies have confirmed these findings.
A Cochrane Review of all randomised controlled trials for reduced dietary fat interventions has shown that replacing fat with carbohydrates is not protective against cardiovascular disease and it doesn’t reduce mortality. There is still evidence supporting the idea that the type of fat we consume may have an impact on our health and reducing the amount of saturated fat could reduce the number of cardiovascular events.
Researchers concluded that:
“dietary change to reduce saturated fat and partly replace it with unsaturated fats appears to reduce the incidence of cardiovascular events, but replacing the saturated fats with carbohydrate (creating a low-fat diet) was not clearly protective of cardiovascular events. The protective effect was seen almost exclusively in those who continue to modify their diet over at least two years, and in studies of men (not those of women).”
Fig.2. Mean body weight of women randomised to very low carbohydrate and low fat diets. Source: Brehm BJ, et al. A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women. The Journal of Clinical Endocrinology & Metabolism.
Dietary interventions focused on diet alone are difficult to design as other factors, like physical activity, smoking or stress level influence the outcome no matter what dietary changes are being implemented. A few attempts have been made however and in 2007 a randomised clinical trial enrolled 311 people looking at what diet: high-fat, high-protein or low-carb may have the biggest impact on cardiovascular disease, blood pressure and HDL cholesterol level. The study has shown that the high-fat, high-protein, low-carb diet was associated with more than 30% lower risk of cardiovascular events, lower blood pressure and lower HDL cholesterol level than the low-fat diet.
A recently published multicentre PREDIMET trial has also supported the idea that fat is a friend, not an enemy. People who ate a high-fat Mediterranean-style diet for nearly five years were less likely to experience serious cardiovascular events compared with those who avoided fat completely.
Fat is not all bad and it shouldn’t be eliminated. It's the type of fat we choose to eat, not the amount that really matters. Since the very first nutritional guideline to limit dietary fat and cholesterol was released in 1961, we have been experimenting with our foods relying on small epidemiological data.
The consequences are not very encouraging. Before large clinical trials are available, it would be wise to keep it simple and eat what previous generations recommended: fewer grains, less sugar, more raw vegetables and fruit, fish, meat and eggs.