Low Carbohydrate Versus Low Fat Diets

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Patients often ask which kind of diet they should follow in order to lose weight: low carbohydrate or low fat? Representing the low-carbohydrate approach are books like Atkins for Life, which advocates consumption of animal foods such as meat, bacon, poultry, fish, cheese, butter, and eggs, and severely limits carbohydrates to 20 g per day during the first phase of the diet. Dr Atkins attributed weight gain to consumption of what he called "bad carbohydrates" like rice, potatoes, pasta, or anything made with sugar [1]. His reasoning was that these carbohydrates trigger a hyperinsulin condition that causes excess calories to be stored in fat cells, which in turn makes the person obese.

Until recently, the medical community considered this diet potentially dangerous. The prevailing thought was that a diet high in fats would raise blood cholesterol levels and occlude arteries. This reasoning supported a lawsuit filed in Florida by a man who claimed the diet raised his cholesterol from 146mg/dl to 230mg/dl and that he needed angioplasty as a result. Logic would suggest that a high-fat diet would do what the lawsuit claimed. But is this supported by science? What should physicians tell their patients who want to go on a low-carbohydrate diet?

A low-fat diet is promoted by the American Heart Association [2] and by the federal government's previous Food Guide Pyramid, which places carbohydrates at the base of the pyramid, with 8 to 11 daily servings recommended [3]. The conventional low-calorie, low-fat dietary recommendation is to maximize complex carbohydrates and limit total fat to 30%, of which 10% can be saturated fat. The problem is that America has followed this dietary recommendation for decades and now is in the midst of an obesity epidemic. If the nation is following a low-fat diet, then why are most Americans overweight or obese? Is there scientific evidence that this diet is effective for weight loss? What should a physician tell patients who want to follow a low-fat diet?

As a primary care physician, I need simple, clear, and effective dietary tools that I can share with patients in 10 to 15 minutes. I need to know the scientific evidence behind what I recommend, but I do not have the time to discuss in detail the pros and cons of the various dietary positions for promoting weight loss. This chapter will review the current studies regarding the effectiveness of low-carbohydrate versus low-fat diets, as well as low-glycemic versus high-glycemic diets. The second half of this chapter will present a commonsense dietary approach that will enable the physician to help the patient reduce calories and lose weight, no matter which dietary approach the patient chooses to follow.

When patients ask about the various diets and how one diet compares to another, I draw a line representing a continuum. One end of the line is marked "animal diet" endorsed by Dr Atkins' book. The other end of the line is marked "plant diet" endorsed by Dr Dean Ornish's Eat More, Weigh Less book, which

Protein

Protein

Low fat diet Low carbohydrate diet

Figure 3.1. Pie charts showing examples of a low-fat diet of 60% carbohydrates, 15% protein, and 25% fat and a low-carbohydrate diet that is 45% carbohydrates, 20% protein, and 35% fat.

Carbohydrate

Protein

Protein

Low fat diet Low carbohydrate diet

Figure 3.1. Pie charts showing examples of a low-fat diet of 60% carbohydrates, 15% protein, and 25% fat and a low-carbohydrate diet that is 45% carbohydrates, 20% protein, and 35% fat.

promotes a low-fat diet [4]. Then I tell the patient that every reasonable dietary approach to lose weight fits on the line somewhere between these two diets. An example is the Zone diet, which recommends that total caloric intake should be apportioned as carbohydrates 40%, protein 30%, and fat 30% [5]. Then I ask the patient where on this spectrum would they likely find their dietary preference.

Next I draw a pie chart with three sections (Figure 3.1). Each section represents one of the three macronutrients: carbohydrate, protein, and fat. I show the patient that the uniqueness of each diet is determined by how big a particular section of the pie is. I explain that a low-carbohydrate diet by definition means the carbohydrate section of the pie is smaller, thus making the percentage of the diet derived from the other two macronutrients larger. I demonstrate that the opposite applies to a low-fat diet. If less than 30% of the total calories is from fat, an increased percentage is from carbohydrates.

When patients see the pie charts, they understand that any dietary approach recommended by a diet book does not reflect any new or hidden science. The author has simply applied his or her theory for weight loss to a particular way of dividing the macronutrient pie. If cutting the pie represents each author's personal preference on how to lose weight, then what is a patient to do? The various dietary proposals in popular diet books are contradictory, and the low-fat versus low-carbohydrate debate is confusing. What kind of diet should a physician advise for overweight patients?

Consider the possibility that the solution for long-term weight loss is not how the macronutrient pie is cut, but the size of the pie itself. Most overweight or obese people eat too much food, whether that food is animal based or plant based and whether food choices are healthful or unhealthful. When calories ingested exceed calories expended, then weight gain occurs.

As a physician, I do not condemn or defend any of the popular weight loss programs. I need to stay focused on the patient and not on particular food recom mendations. Before considering how a patient-centered focus is the dietary answer for long-term weight loss, let's take a closer look at the debate between low-carbohydrate and low-fat diets.

A study of the low-carbohydrate diet at Duke University included 51 overweight or obese subjects who lost an average of 20lb. They were restricted to 10g of carbohydrates per day until they lost 40% of their weight loss goal, then the carbohydrate allowance was increased to 50 g per day. At the end of 6 months, most participants had lost 10% of their weight and had decreased their low-density lipoprotein (LDL) cholesterol. Though calorie counting was not required, participants ate an average of 1450 calories per day [6].

The results of the low-carbohydrate diet in this study are not in doubt: people lost weight using this dietary approach. What is unclear is whether the weight loss resulted from carbohydrate restriction leading to ketosis, or was the result of fewer calories consumed by restricting certain foods.

A recent study of 120 overweight, hyperlipidemic volunteers compared a low-carbohydrate diet to a low-fat, low-calorie diet. The low-carbohydrate group initially consumed only 20 g per day of carbohydrates and took vitamin supplements. The low-fat, low-calorie group consumed less than 30% of total calories from fat and less than 300 mg of cholesterol per day and ate 500 to 1000 fewer calories per day. Both groups were encouraged to exercise three times per week, keep a food diary, and attend weekly educational meetings. After 24 weeks, the low-carbohydrate group had lost more weight with better improvement of lipids than the low-fat, low-calorie group. Weight loss averaged 12.9% in the low-carbohydrate group compared to 6.7% in the low-fat group. On average, triglycerides were reduced by 72 mg/dl and high-density lipoprotein (HDL) rose by 5.5 mg/dl in the low-carbohydrate subjects compared to a reduction of 27.9 mg/ dl in triglycerides and a drop of 1.6 mg/dl in HDL for the low-fat subjects [7].

Limitations of this study include the fact that it involved a relatively healthy population that was followed for only 24 weeks. The long-term weight loss effects in this group are not known, and the effectiveness of this diet for less healthy patients cannot be assumed.

A study at the Philadelphia Veterans Affairs Hospital evaluated weight loss and lipid changes among severely obese (BMI > 35) patients of whom 39% had diabetes and 43% had metabolic syndrome. The 6-month study started with 132 patients. The low-carbohydrate group (N = 68) consumed less than 30g of carbohydrates per day, while the low-fat, low-calorie group (N = 64) reduced their caloric intake by 500 calories per day with less than 30% of total calories from fat. At 6 months, the results favored a low-carbohydrate diet with a mean weight loss of 5.1kg compared to a mean loss of 1.9kg for the reduced calorie, low-fat group. Triglyceride levels fell by 20% and insulin sensitivity improved by 6% for the low-carbohydrate group compared to only a 4% drop in triglyceride levels and a 3% reduction in insulin sensitivity for the low-fat group. An attrition rate of 47% at 6 months was a significant limitation of the study [8].

A 1-year follow-up study of the same population showed no statistically significant weight loss difference between the two groups (5.1kg for the low-carbohydrate group and 3.1kg for the low-fat group). However, there remained a favorable difference for the low-carbohydrate group in terms of triglycerides, HDL, and hemoglobin A1C [9].

Are these data sufficient to prove a low-carbohydrate diet is more effective than a low-fat diet for weight loss or improving lipids? There is no debate as to whether people lose weight following a low-carbohydrate diet. There is lack of scientific evidence regarding long-term weight loss sustained for more than 1 year by low-carbohydrate dieters. On the other hand, representing the low-calorie, low-fat position are members of the National Weight Control Registry (NWCR), who total over 4200, and report losing an average of 67 lb for an average duration of 6 years. According to Dr James Hill, one of the founders of the NWCR, members report that 56% of their calories come from carbohydrates, 19% from protein, and 25% from fat [10].

The NWCR results do not mean the low-fat diet is more effective for weight loss either. A meta-analysis using the Cochrane Library, MEDLINE, EMBASE, and the Science Citation Index in 2001 and 2002 compared low-fat diets with other calorie-restricted diets. Four studies measured follow-up at 6 months, five studies at 12 months, and three at 18 months. The average weight loss at 6 months for the low-fat diet groups was 5 kg and for the calorie-restricted groups the average weight loss was 6.5kg; at 1 year the average loss was 2.3kg for the low-fat groups versus 3.4kg for the calorie-restricted groups; and at 18 months the low-fat groups' average loss was 2.3 kg versus a weight gain of 0.1 kg in the calorie-restricted groups. At the end of 18 months, the results showed no statistical difference between a low-fat diet and other calorie-restricted diets in terms of weight loss, serum lipids, blood pressure, and fasting plasma glucose [11].

Only one study has compared the four popular diets mentioned: the Atkins low-carbohydrate diet; the Ornish high-carbohydrate, low-fat vegetarian diet; the Weight Watchers low-fat, high-carbohydrate diet [12]; and the Zone 40-30-30 system. This 1-year study included 160 volunteers. At the end of the study, the dropout rate was 50% for both Atkins and Ornish participants and 35% for the Weight Watchers and Zone participants. Weight loss occurred and lipids improved on all the diets. Physical activity was not assessed in the study. Commenting about the study's results, Dr Dansinger said, "The good news about this study is that we demonstrated that all these diets work. That means physicians can work with patients to select the diet that is best suited to the patient" [13]. The fact that weight loss can occur through various dietary approaches supports the hypothesis that each of these diets addresses the same cause of weight gain: the intake of excess calories.

Between 1971 and 2000, the prevalence of obesity in America went from 14.5% to 30.9% [14]. The Centers for Disease Control and Prevention report that during this same time caloric energy intake increased for both men and women. Men consumed an average of 2450 calories per day in 1971 and in 2000 consumed an average of 2618 calories per day, an increase of 168 calories per day. Women consumed an average of 1542 calories per day in 1971 and an average of 1877 calories per day in 2000, an increase of 335 calories per day. For both genders, the increase in calories was from an increase in carbohydrates. For men the percentage of total calories from carbohydrates went from 42.4% to 49% and for women from 45.4% to 51.6% [15]. In 1971, caloric sweeteners accounted for 39% of the carbohydrates consumed, grain products 35%, vegetables 10%, and dairy products and fruits 6% each; grain products and sweeteners together totaled 74% of carbohydrates consumed.

By 1994, grain products and sweeteners together accounted for 78% of all carbohydrates consumed. Protein intake decreased from 16.5% to 15.5% in men and from 16.9% to 15.1% in women. The percentage of total calories from fat decreased from 36.9% to 32.8% for men, and from 36.1% to 32.8% for women, with both genders having a decrease in saturated fat, too [15].

The increase in daily energy per capita is the result of an increase in sweeteners and grain consumption. According to the United States Department of Agriculture (USDA), per capita use of corn sweetener was 19lb in 1970 then rose to 87 lb in 1997. Flour and cereal products went from 136 lb in 1970 to 200lb in 1997. Consumption of wheat flour increased by 35%, corn flour by 79%, and high-carbohydrate snacks by 200% [16].

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