Evidence-Based Evaluation of Popular Weight Loss Diets

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Evidence-Based Evaluation of Popular Weight Loss Diets
© 2004 Feinberg School of Medicine, Northwestern University

According to the Centers for Disease Control, one out of two adult Americans exceeds the upper healthy
weight limit (body mass index or BMI > 25). Moreover, 20% of US adults are clinically obese (BMI > 30).
Health problems associated with our country's obesity epidemic are well established: type 2 diabetes,
cardiovascular disease, stroke, gallbladder disease, osteoarthritis, sleep apnea and possibly cancer. The
61% increase in the incidence of US adult obesity from 1991 to 2000 has coincided with a 49% rise in
type 2 diabetes among adult Americans.

The proliferation of top-selling diet books published in the past five years is an ironic twist to the health
concerns among overweight Americans striving to lose weight. These popular diets typically offer dieters
"empty" promises such as quick weight loss, increased energy level and absence of hunger/food
deprivation. But few dieters actually try to improve their eating habits. The Centers for Disease Control
report that only 17.5% of dieters try to consume fewer calories and increase physical activity, two key
factors for weight control. Furthermore, approximately one out of four dieters consumes the
recommended five servings of fruits and vegetables daily. Considering that dieters fail to practice sound
weight loss practices, approximately 70% of these individuals regain at least half of their weight loss
within two years.

Despite the apparent poor success rate of fad diets, these reducing eating plans continue to grow in
popularity among overweight Americans wishing to shed some pounds. In fact, over half of the 50 best-
selling diet books have been published since 1999.

What is the "magic" bullet for successful weight loss? Researchers from the United States Department of
Agriculture (USDA) and other institutions have conducted an extensive review of scientific literature to
evaluate the efficacy of popular diets. Based on their analysis of existing data collected on reducing diets,
calorie restriction in itself is the "key" ingredient for successful weight loss. Unfortunately,
evidence for the long-term health safety and maintenance of weight loss associated with fad diets
remains unclear. Highlights of research findings on popular diets are described below.

1. CALORIES: Eating less calories leads to weight loss.

Total daily caloric intake averaging 1400-1500 calories daily results in weight loss
regardless of the macronutrient composition (i.e. breakdown of protein, carbohydrate and fat) of
the weight-reducing diet. Although physical activity can enhance weight loss, dieters who
adhered to a 1500 calorie/day diet still lost weight because they expended more energy than that

Dieters who adhered to a low-fat, high carbohydrate diet rich in fruits, vegetables and
grains consumed less calories than those on other types of diets (see table below). In
short, weight loss study results indicate an inverse relationship between carbohydrate intake and
body weight; heavier people consistently consume less carbohydrate. These findings dispel the
myth among dieters that carbohydrates are "fattening".

Moderate-fat weight loss diets also result in loss of body weight and body fat even when
food is consumed ad libitum. For example, the mean caloric intake of individuals following a
general diet using the USDA Food Guide Pyramid guidelines (diet composition: 55%
carbohydrate, 25% fat, 20% protein) was approximately 1895 calories daily compared to 2166
calories/day for those consuming a high-fat, low-carbohydrate dietary regimen. Furthermore,
those who regularly adopted a high-carbohydrate, moderate fat diet had a lower body mass
index (BMI) than individuals consuming higher fat diets. These findings suggests that individuals
who consistently opt for foods containing mostly carbohydrate and some protein (1 gram of
protein or carbohydrate = 4 calories
) are consuming less calories over time compared to those
who eat more energy-dense fatty foods (1 gram fat = 9 calories).

Evidence-Based Evaluation of Popular Weight Loss Diets (page 2)
© 2004 Feinberg School of Medicine, Northwestern University

2. Nutrition Adequacy

• High-fat,
diets are deficient in several nutrients. These regimens are
consistently low in vitamins A, B6, D and E, thiamin, folate, calcium, magnesium, iron,
potassium and dietary fiber. Hence, supplementation is highly recommended for those who
adhere to this type of weight-reducing regimen.

• Moderate-fat, low-carbohydrate diets are nutritionally balanced per the USDA Food Guide
Pyramid guidelines. If one or more of the six food groups (i.e. grains, fruits, vegetables,
meats, dairy and fats & sugars) is eliminated or severely restricted, inadequate intake of
nutrients provided from that food category is likely to occur.

• Low-fat,
diets are often deficient in vitamin E, vitamin B12 and zinc. Eating
foods fortified with these nutrients or a supplementation may be necessary for those adhering
to low-and very-low fat regimens.

• Table below summarizes the nutrient differences of popular diets.

Approximate Macronutrient Breakdown of Popular Diet Plans*

Type of Diet
Fat grams

(% calories)
Nutrition Adequ
(% calories)
(% calories)
Typical American Diet
85 (35%)
275 (50%)

High Fat,
1414 96
(60%) 35 (10%)
105 (30%) Low in several nutrients:
Low-Carbohydrate Diet
Fat Level
Vitamins A, B6, D, E, thiamin, f

magnesium, iron, zinc, potassi
• Dr. Atkins Diet
fiber. This type of diet also con
• Zone Diet
amounts of total fat, saturated
• Sugar Busters
cholesterol. Nutritional supplem
• Protein Power
highly recommended.
Moderate Fat Diet
1450 40
(25%) 218 (60%)
54 (15%)
Usually a nutritionally balanced

Fat Level
assuming the dieter eats a var
• USDA Food Guide Pyramid
from all food categories. Howe
• DASH Diet
certain food categories can lea
• American Diabetic Association
in certain nutrients especially c
• Weight Watchers
• Jenny Craig
Low- and Very Low-Fat Diet
1450 20
235-271 (70%) 54-72 Deficient in zinc and vitamin B

Fat Level
infrequent meat consumption.
• Volumetrics
varies: 10-20%
type of diet can be inadequate
• Dean Ornish’s
nutrient found in oils, nuts and
Eat More, Weigh Less
in fat.
• New Pritikin Program
*Source: Adapted from: Freedman, M., King, J. and Kennedy, E. Popular Diets: A Scientific Review. J of Obesity Research. 2001: Suppl 1.

Evidence-Based Evaluation of Popular Weight Loss Diets (page 3)
© 2004 Feinberg School of Medicine, Northwestern University

3. Relationship between Weight-Reducing Diet and Body Composition

• Weight loss in itself results in loss in body weight, body fat and even lean muscle mass.
Total daily calorie restriction (~1500 calories/day) sustained over a period of time promotes a
loss of body fat, despite the macronutrient composition of the diet.

• Physical activity is strongly recommended for weight management because it facilitates loss
of body fat by increasing (1) energy expenditure, (2) lean muscle mass and (3) metabolism.

4. Physiological Changes During Weight Loss

• Weight loss is directly correlated with a decrease in (1) total blood cholesterol, (2) LDL-
cholesterol and (3) plasma triglycerides. Greater reduction in LDL-cholesterol is observed
during weight loss with a low saturated fat intake. The types of dietary fat consumed and
duration of the weight loss program affected HDL-cholesterol levels.
• Caloric restriction regardless of the macronutrient content of the weight loss diet improves
glycemic control by lowering blood sugar and insulin levels.

• Weight loss in itself decreases blood pressure. However, in the absence of weight loss, the
DASH diet (a low-fat diet rich in fruits, vegetables, whole grains and low-fat dairy foods)
promotes a reduction in blood pressure. This effect is magnified when the DASH diet is
combined with a low sodium intake.

5. Hunger, Satiety and Adherence to Weight Loss Diets

• Insulin and leptin regulate long-term weight regulation. Insulin stimulates the uptake of glucose
and proteins into the cells as well as possibly increasing appetite. Leptin, on the other hand, is
a hormone released from fat cells, which helps suppress appetite and increase metabolism.
Insulin and leptin secretion is influenced by the macronutrient composition of the diet although
the actual mechanisms remain unclear. During weight loss, blood insulin and leptin levels drop

• All fat-restricted diets offer satiety value, contrary to claims made by high-protein and high fat
diet gurus. In fact, dieters who follow both a calorie and fat-restricted regimen commonly
complain of having “too much food to eat”. This may be due to the high fiber, high water
content of low-fat foods, which may account for dieters’ enhanced feeling of fullness during and
between meals. Additionally, those who adhere to a low-fat regimen over time appear to
develop an aversion to fatty foods, which may also contribute to long-term success in weight
control. Furthermore, high dietary fiber intake is correlated with lower insulin levels and long-
term maintenance of weight loss.

Neurochemical factors, gastric signals, emotional factors, individual taste preferences are other
contributing factors which can account for individual differences in appetite, food intake and
body weight.

6. Food Guide Pyramid Comparison of Weight Reducing Regimens

Diet quality (total fat, saturated fat, cholesterol, sodium and menu variety) for high-
carbohydrate, low- to moderate-fat diets scored significantly higher compared to that of high-
fat, low-carbohydrate regimens. Moreover, vegetarian high-carbohydrate diets provided the
greatest nutritional value.

Evidence-Based Evaluation of Popular Weight Loss Diets (page 4)
© 2003 Feinberg School of Medicine, Northwestern University

• Percent of energy from carbohydrate indirectly correlates with intake of fat, protein, dietary
cholesterol and calories. The carbohydrate content of selected diets reviewed also revealed
an indirect relationship with body mass index. In short, traditional high carbohydrate diets
(>55% total calories) generally provide less fat and calories, more nutrients and greater menu
variety compared to popular high-fat, low-carbohydrate diets.

• Sample menu illustrating a nutritionally balanced, high-carbohydrate, low-fat reducing diet fits
into the framework of the USDA Food Guide Pyramid is shown below.

USDA Food Guide Pyramid Adapted for Weight Reduction: Nutrition Analysis*

6 oz orange juice (1 fruit)
Turkey Sandwich:
3 ounces grilled white fish (1 meat)
1 cup wheat flakes cereal (1 grain)
2 oz fat-free turkey breast (1 meat)
1 cup steamed brown rice (2 grain)
1 cup nonfat milk
1 reduced-fat cheese (1/2 dairy)
(1 dairy)
1 cup grilled mixed fresh vegetables
2 leafs Romaine lettuce
½ banana (1 fruit)
(2 vegetable) seasoned with ½ tablespoon of
2 slices tomato
olive oil (1-1/2 fats)
1 tablespoon dijon mustard
2 slices whole wheat bread (2 grains)
12 baby carrots
2 cups Romaine lettuce (2 vegetables)
(2 vegetable)
5 cherry tomatoes; 4 slices cucumber
1 ounce baked tortilla chips (1 grain)
(1 vegetable)
1 cup unsweetened ice tea
2 tablespoons reduced fat dressing (2 fats)
Beverage: Herbal Tea
Dessert: 1 cup fat-free frozen yogurt (1/2
Nutrition Analysis:
Nutrition Adequacy
Food Guide Pyramid

Total calories: 1592

Sample Menu: Food Guide
Meets or exceeds the Daily
Protein: 77 g (19% calories)
Reference Intake (DRI) for 16
6 Grains 6-11 Grains
Carbohydrate: 252 g (63% calories) vitamins and minerals based on a
5 Vegetables 3-5
reference premenopausal female
Fat: 37 g
(targeted group that typically “tries” several
(21 % calories)
2 Fruits 2-4 Fruits
weight loss diets).
Saturated Fat: 7 g (4 % calories)
2 Meats/Proteins 2-3 Meats
Cholesterol: 105 mg
2 Dairy 2-3 Dairy
Dietary Fiber: 29 g
3-1/2 Fats Sparingly
Sodium: 3024 mg
*Nutrient Data System 2.93 software used for
Potassium: 3875 mg
analysis of sample menu


Mokdad, Ali, Bowman, B., Ford, E. Vinicor, F., Marks, J. and Koplan, J. The continuing epidemics of obesity and diabetes in the United States.
JAMA. 2001; 286:1195-1200.

Kennedy, E., Bowman, S, Spence, J. Popular diets: Correlation to health, nutrition and obesity. J of Am Dietetic Asso 2001; 101:

Freedman, M., King, J. and Kennedy, E. Popular Diets: A Scientific Review. J of Obesity Research. 2001: Suppl 1.

Ludwig, D. et al. Dietary fiber, weight gain, and cardiovascular disease risk factors in young adults. JAMA 282: 1539-1546.

Denke, Margo. Metabolic effects of high-protein, low-carbohydrate diets (editorial). Am J Card 2001;88: 59-61.