Obesity and Weight Control
Excess adiposity has been linked to diabetes mellitus, cardiometabolic syndrome, CVD, and other chronic diseases and has been targeted as a top public health priority.17,18 The trend of rising obesity rates parallels the trend toward increased energy intake among Americans over the last 30 years.19 National efforts to promote weight loss have been only nominally successful, and efforts to maintain weight loss have been an even bigger challenge.
Low Carbohydrate Versus Low Fat in the General Overweight or Obese Population
In 1998, a National Institutes of Health evidenced-based report on obesity evaluation and treatment recommended an energy-restricted diet that focused on a “low-fat” macronutrient approach for weight loss.20 A low-fat dietary approach was used successfully in the landmark Diabetes Prevention Program (DPP) trial to achieve weight loss, and nearly 60 studies on diabetes prevention21 as well as other evidence supported this approach.22 However, data to support claims that a low-fat diet was superior to other approaches were limited.23 An alternate hypothesis, that a low-carbohydrate diet might be an effective weight loss strategy, has emerged; but until 2003, the data to support this approach were limited. The primary overall conclusion of more than a dozen federally funded weight loss trials pitting low fat head-to-head against low carbohydrate was that low-carbohydrate diets were at least as effective for weight loss as low-fat diets. Some differences favor one or the other diet because of metabolic variables.24-27 Another conclusion from these studies was that weight loss peaked at 6 months, followed by variable amounts of recidivism. Average weight loss among study participants who were initially 15 to 100 lb overweight was 5 to 10 lb at 12 to 24 months. Despite modest to nominal weight loss in these randomly assigned groups, there was substantial individual variability in weight change among study participants within all diet groups.28-30
Factors Affecting Variable Individual Weight Loss Results
Recently, at least two explanations have been hypothesized for variable weight loss among individuals assigned to the same weight loss diet. One involves the observation that adults shown to be relatively insulin resistant have been particularly unsuccessful with weight loss when assigned to a low-fat diet (which is, by definition, also high carbohydrate) and more successful when assigned to a low-carbohydrate diet (which is, by definition, also high fat).31-34 Another, more tenuous explanation is the possibility of some level of genetic predisposition to differential success on various diets.35 Before such findings of genetic predisposition can be considered of practical clinical relevance, they need to be further developed and replicated.
Low Carbohydrate for Weight Loss Among Those Who Are More Insulin Resistant
For the insulin resistance theory to be of practical use to health professionals, two important questions should be addressed:
- What practical clinical assessments are available for diagnosing insulin resistance?
- How low is low carbohydrate?
Insulin resistance is a relative term. There are no clinically validated cut points that distinguish people who are insulin resistant from those who are insulin sensitive. The simplest methods of assessing relative insulin resistance make use of fasting insulin level or a fasting triglyceride/high-density lipoprotein cholesterol (HDL-C) ratio; these correlate fairly well with the gold standard methods.36 While there is no specific cut-point for fasting insulin levels to suggest higher risk, McLaughlin and colleagues36 have proposed a cutoff of 3.5 for the triglyceride/HDL-C ratio; above 3.5 is strongly suggestive of relative insulin resistance. Finally, given the likelihood that insulin resistance is an important (or the most important) underlying factor in metabolic syndrome,37,38 the established criteria for metabolic syndrome could be used.39
The second and related question that remains unresolved is how to best define and encourage patients to adhere to a low-carbohydrate diet. How “low” is “low carbohydrate”? Is it 40% or 30% or 20% of energy from carbohydrates? Although there is no formal consensus, an informal poll of experts in this field suggests that it is lower than 40% and higher than 20% (simply because less than that is difficult to maintain long-term). For most individuals, this boils down to a lower-carbohydrate diet, without imposing a specific percentage of energy value.
Conclusions for Obesity and Weight Control
The most important conclusion in the past decade in this area has been that a low-fat approach to weight control can no longer claim to be the single best choice. There is no single diet that is most successful for everyone, or even most people. If anything, the low-fat public health mantra that has dominated nutrition recommendations for the past few decades has likely been a less effective approach to weight loss than a lower-carbohydrate approach for the growing proportion of the population that is insulin resistant. It is more realistic to acknowledge that there will be a wide range of success among individuals with either low-carbohydrate or low-fat diets for weight loss. With either approach, it is not enough to simply recommend “low carbohydrate” or “low fat”; this advice needs to be accompanied by emphasis on high nutrient density and low energy density even when limiting carbohydrate-rich and fat-rich foods. Clearly, this will be difficult for patients, the general public, and health professionals to put into practice if the food environment offers primarily low nutrient density and high energy density foods. Because dietary adherence to these restrictive meal plans may be difficult, it is essential to refer patients to a registered dietitian to help individualize the plan whenever possible.