CINCINNATI— It’s still too early to tell if popular diets that focus on eliminating some essential nutrients while loading up on others will lead to long-term health benefits, according to researchers at the University of Cincinnati (UC).
Bonnie Brehm, PhD, professor in the College of Nursing, says diet plans, such as the Atkins, (low-carbohydrate, high-fat) and Ornish (low-fat, high-carbohydrate) diets, have stirred a debate about the health effects of macronutrient distribution in the diet.
In fact, diets that include extreme ranges of macronutrients—carbohydrate, protein or fat—might not be what the doctor orders to manage weight and diabetes, and improve cardiovascular health.
"With the rise of obesity and diabetes, there is concern about whether popular diets are the best prescriptions for optimal health,” says Brehm. "There’s an intense debate about the ideal combination of carbohydrate, protein and fat for effectively managing obesity and its metabolic consequences such as diabetes, and for improving body composition and overall health.
"But it’s difficult to reach an agreement when there’s been a lack of long-term studies to determine the benefits and risks of each diet,” she says.
Brehm, and co-author David D’Alessio, MD, professor of medicine at UC, published a literary review of the benefits of diets with varying fat and carbohydrate content in the March 2008 edition of Nature Clinical Practice.
In studies published over the last 20 years, Brehm and D’Alessio found that while low-carb, fad diets may help shed pounds and lower cardiovascular risks in the short term, longer studies are needed to fully determine if these diets are really effective and safe—and whether they warrant nutritional recommendations from health providers.
"The studies we examined provided additional, but not yet sufficient, evidence for modification of current nutritional guidelines, which suggest how much fat, protein and carbohydrate a person should consume to lose weight or improve their overall health,” Brehm says. "Despite a recent flurry of research, many fundamental questions related to dietary macronutrients and their effects on energy balance and metabolism remain unanswered.”
Macronutrients—carbohydrate, protein and fat—are nutrients the body needs in large quantities to provide energy for normal functions.
Carbohydrate is the body’s preferred energy source, with most diets providing 50 percent or more of total calories in the form of starches and sugars. Protein, found in meat, poultry, fish, eggs, dairy products and beans, supports tissue growth and maintenance of the body. Fats tend to be the most confusing due to the variations of "good” and "bad,” but in general they provide energy, some vitamins and essential fatty acids that the body can't make itself.
Brehm and D’Alessio focused their review on diets either low in carbohydrate or high in monounsaturated fat or the "good fats” generally known to reduce "bad” cholesterol levels in the blood and lower the risks of heart disease and stroke.
Brehm says that while each diet claims to offer a distinct health benefit over another, some may come with associated risks, which might not be worth taking in the long run.
"We found evidence that low-carbohydrate diets lead to weight loss in 12-month studies,” says Brehm, "but these same studies showed that these diets don’t affect everyone’s metabolism the same way and there’s potential for increased LDL-cholesterol or the "bad” cholesterol.”
Brehm points out that the medical community traditionally has supported moderately low-fat, high-carb diets for cardiovascular health, but the success of these diets has only recently been tested in well-designed clinical studies.
"The unceasing demand by patients and health care providers for effective weight-loss methods has led to the use of a wide number of unproven strategies,” says Brehm. "A year might seem long for those conducting or participating in a study, but it comprises only a short time in the life of an overweight person. In essence, the long-term benefits remain unknown.”
Brehm says the same can be said for high-monounsaturated fat diets, such as those typical of the Mediterranean region. While these diets may help manage cardiovascular risk, Brehm says, there’s a steadfast belief among health practitioners that high-fat diets may cause increased energy intake and weight gain. However, recent studies have shown that high-monounsaturated fat diets are associated with at least as much weight loss as lower-fat diets.
Until more long-term research is conducted, Brehm recommends sticking with the diet recommendations of expert bodies such as the Institute of Medicine, American Heart Association and American Diabetes Association. These, she says, are based on clinical expertise and scientific data by groups of leading practitioners.