The World of Nutrition and Exercise Science: Popular Diets, Part 1

Welcome to the first installment of this new article series. This series will discuss primary research, research reviews, and other science related (specifically exercise and nutrition research) information. I will present key points from the articles discussed. You don’t have to be knowledgeable in statistics or research methodology in order to benefit from reading this series. These articles are for anyone interested in exercise and nutrition.

If interested in furthering your knowledge on the basic ideas of science and how scientific research is conducted, here are some good places to start:

• Scientific and nonscientific approaches to knowledge
• Understanding research methodology

With so much conflicting advice on nutrition, it’s often hard for consumers to decide what is good advice and what is nonsense. In a review paper titled Popular Diets: A Scientific Review (2001), Marjorie Freedman and colleagues did a great job reviewing the research on popular diets. Here are some excerpts:

“Excess weight is associated with increased mortality and morbidity. It is associated with cardiovascular disease, type 2 diabetes, hypertension, stroke, gallbladder disease, osteoarthritis, sleep apnea and respiratory problems, and some types of cancer...Obesity-related conditions are significantly improved with [a] modest weight loss of 5% to 10% even when many patients remain considerably overweight...Calorie balance is the major determinant of weight loss...Diets that reduce caloric intake result in weight loss...Free living, overweight individuals who self-select high fat, low CHO diets consume fewer calories and lose weight...Overweight individuals consuming low fat and very low fat diets lose weight because they consume fewer calories...All low calorie diets result in a loss of body weight and body fat. Macronutrients do not seem to play a major role.

In the short term, however, high fat, low CHO ketogenic diets cause a greater loss of body water than body fat. When these diets end, water weight is regained. Eventually, all reduced calorie diets result in a loss of body fat if sustained long term...Energy restriction, independent of diet composition, improves glycemic control. As body weight decreases, so does blood insulin and plasma leptin levels. Blood pressure decreases with weight loss, independent of diet composition. Moderate fat, balanced, nutrient reduction diets reduce LDL cholesterol, normalize the ratio of HDL/TC, and normalize plasma TGs...Many factors influence hunger, appetite, and subsequent food intake. Macronutrient content of the diet is one, and it may not be most important. Neurochemical factors (e.g. serotonin, endorphins, dopamine, and hypothalamic neuropeptide transmitters), gastric signals (e.g. peptides and stomach distention), hedonistic qualities of food (e.g. taste, texture, smell), genetic, environmental (e.g. food availability, cost, and cultural norms), and emotional factors (e.g. eating when bored, depressed, stressed, or happy) must be considered.

Dietary compliance is likely a function of psychological issues (e.g. frequency of dietary counseling, coping with emotional eating, and group support) rather than macronutrient composition, per se...Proponents of high fat, low CHO diets dismiss the notion that caloric intake is important for either weight gain or weight loss. They claim that ‘most overweight individuals do not overeat’ even as they suggest that high CHO meals leave individuals less satisfied than meals that contain adequate fat, resulting in increased hunger and increased food intake.”

Tune in soon for more discussion on Popular Diets: A Scientific Review.

References Freedman, MR, King J, Kennedy E (2001) Popular diets: A scientific review. Obesity Research9(S1):1–40.