Probably more widely debated than any other diet in scientific and consumer literature is the high-protein/low carbohydrate diet. These plans have been a part of diet lexicon since the mid-1800s, with William Banting’s Letter on Corpulence (2). Billed as the “world’s first diet book,” Banting’s work recommended eating lots of meat, a few vegetables, and avoiding foods that he previously overconsumed.
Today the term “low-carb diet” is often thought of as synonymous with the Atkins diet, named after cardiologist Dr Robert Atkins. Also known as just “Atkins,” the diet restricts carbohydrate consumption by eliminating most carbohydrates (rice, bread, pasta) and replacing them with meats, poultry, eggs, and dairy products (3). During the late 1990s and early 2000s, low- carbohydrate diets became some of the most popular diets in the U.S., and versions of this diet (the Zone Diet, Protein Power Lifeplan, Go Lower Diet, and South Beach diet, among others) remain popular today. In most formats, the carbohydrate-modified (low-carbohydrate/high protein) diet is a ketogenic diet, which induces a state of ketosis through a severe limitation of dietary carbohydrates. Ketosis occurs in metabolism when the liver converts fat into fatty acids, and ketones (the byproduct of incomplete fat metabolism) reach high levels in the blood (4).
A compendium of research has examined the manipulation of macronutrient content to produce a “metabolic advantage” for weight loss. However, there is no consensus in the literature that low-carbohydrate diets produce significantly greater rates of weight loss or longer-term weight loss maintenance when compared with more conventional low-fat diets (5-7). Additionally, the American Heart Association warns people that the high protein and saturated fat content of the Atkins diet can be harmful to the heart.
If high-protein diets produce a greater amount of weight loss in some studies, what are the possible mechanisms? Scientists suggest that several mechanisms may be responsible for the weight loss seen with low-carbohydrate diets:
- The severe restriction of carbohydrate depletes glycogen (stored carbohydrate) supply, leading to excretion of bound water.
- The ketogenic nature of the diet may suppress appetite, leading to reduced caloric intake.
- The high protein content of low-carbohydrate diets may provide greater hormonally mediated satiety, thereby reducing spontaneous food intake.
- The self-selection from limited food choices may lead to a decrease in caloric intake.
In a review article of 107 published research studies designed to evaluate changes in weight among adults using low-carbohydrate diets in the outpatient setting, Bravata (8) found that weight loss while using low- carbohydrate diets was principally associated with decreased caloric intake and increased diet duration, but not with reduced carbohydrate content. In other words, the calorie reduction and negative energy balance that occurs on these diets induce weight loss. It is worth noting that many studies in this area are limited by a high attrition (drop-out rate) and by a lack of adherence to the diet.
Where does this leave health and fitness professionals with regard to low-carbohydrate diets and questions from clients? The acceptable macronutrient distributions range (AMDR) is 45 to 65% of total calories from carbohydrates. According to science supporting the AMDR, anyone eating an adequate energy provision for weight loss from nutrient-dense foods, with 45% to 65% of total calories from carbohydrates, will fall inside of the recommendation.
That said, the emphasis of nutrition counselling today is to preserve (as much as possible) the way clients like to eat. Weight loss requires habit changes, but behaviourists who specialise in weight loss suggest that if client preferences can be preserved, they should be. In other words, if a client loves sweet potatoes, whole- grain bread, oatmeal, and legumes (all nutrient-dense carbohydrates), they may not be very compliant with a program that provides only 45% of total calories from carbohydrates. They also do not need to be on a low-carbohydrate diet to lose weight, as any reduction in calorie intake below daily expenditure will induce weight loss.
Low-carbohydrate diets may work for some people, and as long as the diets are within the AMDR and provide nutrient-dense foods, they should not present a health risk. But they are not for everyone. Flexibility on behalf of the counsellor and client is crucial to weight loss success (9, 10). In other words, the difficulty for most people lies not in the diet, per se, but in adherence to the diet. The closer the weight loss program is to the way clients like to eat, the more successful they are going to be. Alternatively, clients can choose to simply decrease the frequency of meals, portion sizes of the foods they normally consume, high-fat/energy-dense foods, or make any acceptable dietary changes to reduce caloric intake.