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The Best Weight Loss Diet – The Verdict Is in

By Jeffrey S. Dunham, M.D., Ph.D., M.P.H.

A two-year trial comparing the safety and effectiveness of weight loss diets has concluded that the low-carbohydrate diet is better than the low-fat diet or the Mediterranean diet for losing weight, improving cholesterol and triglyceride levels, and lowering the risk of heart disease.

The study, published in the July 17, 2008 issue of the prestigious The New England Journal of Medicine, randomly assigned 322 moderately obese subjects to one of three diets:

• A low-fat, calorie-restricted diet based on the American Heart Association guidelines;
• A moderate-fat, calorie-restricted Mediterranean diet; or
• A low-carbohydrate, non-calorie-restricted diet.

Subjects in the low-fat and moderate-fat Mediterranean diet groups were restricted to 1800 calories per day for men and 1500 calories per day for women. Those in the low-carbohydrate diet group had no restrictions on intake of total calories, fat or protein, but were restricted to 120 grams of carbohydrates per day.

While other previously published weight-loss studies have been limited by small size, short follow-up times, and high dropout rates, this study followed subjects for two years and had a dropout rate of only 4.6% at one year and 15.4% at two years. The trial was conducted at a workplace in Israel with an on-site medical clinic, and is the longest and most significant study published to date comparing the three diets.

What is particularly striking about the results is that while the subjects in the low-fat and Mediterranean group were limited in total intake of calories, those in the low-carbohydrate group could consume as many calories as they wanted, but still ended up losing the most weight. Even more significantly, while subjects in the low-carbohydrate group had the highest intake of total fat, saturated fat, and total cholesterol, they had the lowest blood levels of total cholesterol, triglycerides, and C-reactive protein (an important risk factor for heart disease) at the end of two years.

Over the last five years there have been a number of other studies that have shown that it is the amount of carbohydrates, and in particular the refined carbohydrates found in processed and starchy foods, that make us fat and increase our risk of heart disease.

In 2003, two articles addressing this question were published in the New England Journal of Medicine. The first, by Samaha, et al, compared a low-carbohydrate diet to a low-fat diet in 132 severely obese subjects. After 6 months, the low-carbohydrate group lost more weight, had lower triglyceride levels, and had a greater improvement in insulin sensitivity (insulin resistance is the hallmark of adult-onset diabetes) than the low-calorie, low-fat group.

The second study, by Foster, et al, randomly assigned 63 obese men and women to either a low-calorie, low-fat, high-carbohydrate diet or a low-carbohydrate, high-protein, high-fat (Atkins) diet. Again, after 6 months, the subjects in the low-carbohydrate group lost more weight, had lower triglyceride levels and higher HDL (good cholesterol) levels than those on the conventional low-fat diet.

In 2006, another study (again published in The New England Journal of Medicine) evaluated the bi-annual dietary questionnaires completed by 82,802 women followed for 20 years in the Nurses' Health Study. Since Atkins first proposed his low-carbohydrate diet over 30 years ago, there had always been the concern among physicians that those individuals on low-carbohydrate diets would consume more fat and have an increased risk of heart disease.

After analyzing 20 years worth of dietary questionnaires and comparing them with the incidence of heart disease in these women, the authors concluded that the women who had diets with the highest carbohydrate intake, and especially the highest glycemic load (carbohydrates that increase blood sugar levels the fastest), had the highest risk for heart disease regardless of the fat intake. (Carbohydrate-containing foods with the highest glycemic load include bread, pasta, potatoes, rice, cereals, and sugary drinks like fruit juices and soft drinks; those with the lowest glycemic load include fresh whole fruits and vegetables. The glycemic load of foods containing only fat and protein, like meat, cheese and eggs, is zero).

So how does one explain all of these results? How could people eat bacon and eggs for breakfast and not only lose more weight than people on a low fat, calorie-restricted diet, but also have lower triglceride levels and a better cholesterol profile?

It turns out that fat storage is determined more by hormone levels than by total calories consumed or even calories burned with exercise. There is only one hormone that is primarily responsible for fat storage, and that hormone is insulin. There is only one stimulus for insulin secretion by the pancreas, and that is increasing blood sugar levels. And there is only one type of food that increases blood sugar levels, and that food is carbohydrate. The more easily the carbohydrate is digested and absorbed, the higher the blood sugar level, and, in turn, the greater the insulin secretion and fat storage. Therefore, it is the type of calories we consume, not the total amount of calories we consume or expend by exercise, which determines how much body fat we store.

If you try to make people fat by forcing them to eat excess calories in the form of fat and protein (e.g., cheese and meat), they just can't do it. The experiment has been tried, and the people just refuse to eat the excess calories in that form. If you then add carbohydrates in the form of bread, pasta, rice and potatoes, they can easily consume 8000 to 10,000 calories a day, since they are able to put all of those excess calories into fat because of higher insulin levels. Look around the gym. How many people do you see day in and day out doing nothing but exercising on a treadmill or taking spinning classes and still never seem to able to get lean?

High insulin levels force the calories we consume into fat cells and make them unavailable to use as energy. Low insulin levels are a powerful stimulus to release fat from fat cells and make them available to use as fuel for our bodies. High insulin levels not only allow sugar to be transported into fat cells and stored as fat, but also stimulate the liver to produce triglycerides from the excess carbohydrates and release them into the blood to be transported to fat cells for storage. The bottom line: the fewer carbohydrates you eat (and especially those with the highest glycemic load), the leaner you will be, the lower your triglyceride level will be, and the higher your HDL (good cholesterol) will be.

Unfortunately, these studies have not translated into clinical practice because most doctors still believe, erroneously, that it is the fat in our diet (and especially the saturated fat) that is the cause of obesity and heart disease, and continue to recommend a low-fat diet for their patients. Thirty years ago this hypothesis seemed to make sense, but if you look back at the scientific studies done in the past thirty years, you realize that there is very little or no evidence to support this notion. In fact there was substantial evidence even then to support the idea that it is excess carbohydrates in our diet, and not fat, that causes obesity, diabetes and heart disease. This evidence was ignored at the time because the low-fat hypothesis was accepted as dogma by the medical establishment (the American Medical Association, the American Heart Association, and other professional medical associations).

Gary Taubes has written an excellent book entitled "Good Calories, Bad Calories" which documents why this low-fat hypothesis came to be accepted and why it now should be discarded. As a nation, we are eating less fat now than we ever have, and yet are fatter than we ever were. This current study in The New England Journal of Medicine provides the strongest evidence yet that decreasing our intake of carbohydrates (and especially those rapidly absorbed carbohydrates in starchy and processed food) is the best way to reverse the current epidemic of obesity and lower our risk for all the diseases of aging associated with it.

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