Against the backdrop of the obesity epidemic and the inability of most individuals to sustain weight loss induced by calorie-restricted diets [1
], alternative dietary approaches to achieve short- and long-term weight loss have become of increasing scientific interest [2
]. Up until recently (2015), the Dietary Guidelines for Americans recommended that macronutrient intake consist of 45–65% of daily energy intake from carbohydrates, 20–35% from fats, and 10–35% from protein [3
]. In line with these recommendations, the results of the U.S. National Health and Nutrition Examination Survey (NHANES) showed that carbohydrate consumption increased from 39% of total energy intake in 1971 to 51% in 2011. During this same time period, however, the percentage of overweight Americans also increased dramatically (from 42% to 66%) [4
]. Based in part on such trends in weight gain, the creators of many popular diets (e.g., Atkins, Zone) have suggested that diets in which carbohydrate intake is significantly higher than other macronutrients are not an optimal approach for weight loss and may even contribute to weight gain. Most of these diets are published and promoted by one or more health and wellness “experts” who attest to the health and weight loss benefits observed when following their recommended diet.
Despite their popularity among the general public, the efficacy of many popular diets for weight loss has been called into question by researchers, nutrition experts, and health care professionals [5
]. A meta-analysis by Johnston et al. (2014) previously attempted to answer the question of whether any popular diets were effective in producing weight loss over the short term (six months or less) and/or long term (12 months) [2
]. The primary findings of this meta-analysis were that reductions in calorie intake were the primary driver of weight loss and that differences between diets differing in macronutrient composition were relatively small.
Although the findings of the Johnson et al. (2014) meta-analysis are of high importance, a potential factor confounding the interpretation of these findings was that this review included studies in which participants were specifically instructed to reduce their caloric intake and/or increase physical activity levels, beyond the recommendation of the popular diet [2
]. To our knowledge, the effectiveness of specific popular diets on weight loss outcomes in dietary interventions that did not include specific calorie targets and/or structured (i.e., supervised) physical activity recommendations has not been examined. Therefore, the purpose of our review was to examine the effects of the most widely recognized popular diets of 2016, in their proposed format, on both short- and long-term weight loss outcomes in overweight and obese individuals, based on findings from clinical trials that did not include specific calorie targets, meal replacements, supplementation with commercial products, and/or structured exercise programs.
The purpose of this review was to examine the clinical evidence supporting the effectiveness of current popular diets that did not include specific calorie targets, meal replacements, supplementation with commercial products, and/or structured exercise programs on both short-term (≤six months) and long-term (≥one year) weight loss outcomes. There were a number of important findings of this review. First, clinical trials that tested popular diets as recommended (without specific calorie targets) were available for only seven of the 20 eligible popular diets in the 2016 U.S. News & World Report. This indicates that the majority of popular diets have not been rigorously empirically tested in human clinical trials as they are currently recommended. Thus, it is difficult to evaluate the efficacy of the vast majority of popular diets based on evidence from clinical trials at the present time. Second, there was a large disparity in the evidence base for these seven diets, with the Atkins Diet having substantially more support than the other seven empirically tested diets (i.e., the DASH Diet, the Glycemic-Index Diet, the Mediterranean Diet, the Ornish Diet, the Paleolithic Diet, and the Zone Diet). Specifically, findings from nine of 10 clinical trials supported the efficacy of the Atkins Diet in producing clinically meaningful short-term weight loss, with findings from six of eight trials supporting the ability of this diet to produce long-term weight loss.
The findings of this review are not in line with current recommendations of the Dietary Guidelines Advisory Committee, which state that diets with less than 45% of calories as carbohydrates are not more successful than other diets for long-term weight loss (12 months) [35
]. As noted above, we found that the Atkins Diet produced substantial long-term weight losses in a number of clinical trials [20
]. Additionally, the Paleolithic diet, another diet that advocates less than 45% of calories being consumed as carbohydrates, was also found to produce substantial short- and long-term weight loss in a recent clinical trial [13
]. Although we found diets with low carbohydrate content to be effective at producing short- and long-term weight loss, the safety of this dietary approach needs to be critically examined [36
When considering the findings of this review, it is important to remember that successful clinical weight loss was reported according to generally accepted criteria for clinically meaningful weight loss (≥5% body weight) in overweight and obese adults instead of significant weight change from baseline [38
]. Weight losses of this magnitude have been found to produce beneficial changes in blood pressure, blood glucose, lipid profiles, and psychological well-being [39
]. Noteworthy, lifestyle interventions involving caloric restriction typically produce mean weight losses of 5 to 10 kg over the course of four to six months [40
]. Thus, the magnitude of weight loss achieved by the popular diets is in line with that typically achieved for calorie-restricted diets.
Although a recent meta-analysis by Johnston et al. (2014) used similar criteria to define clinically meaningful weight loss, our findings differed from their review which concluded that “These findings support recent recommendations for weight loss in that most calorie-reducing diets result in clinically important weight loss as long as the diet is maintained [2
]”. In contrast, the findings of our review indicated that clinically meaningful short- and long-term weight loss can be achieved without restricting calories per se but rather by following the recommendations of some popular diets. One likely reason for the discrepancy in findings is the difference in eligibility criteria used to select studies. In contrast to the Johnston et al. [2
] meta-analysis, studies in which the dietary interventions incorporated specific calorie and/or exercise recommendations were not included in the present review. We chose not to include these studies because specific caloric targets and/or supervised exercise programs are likely to produce weight loss and thereby confound potential effects of popular diets on weight loss outcomes. Additionally, the majority of popular diets do not include specific caloric recommendations (Table 1
), so individuals following these diets would not typically set caloric intake goals.
A critical question related to which popular diet is the most effective for producing weight loss is, “What are the potential mechanisms through which the popular diets promote weight loss?
” Some diet advocates (e.g., Atkins Diet) assert that limiting carbohydrate consumption is the primary driver of weight loss [40
], while others argue that restriction of specific macronutrients can lead to a reduction in total calorie intake, and that calorie restriction is the primary driver of weight loss. Although it is clear that calorie restriction produces short-term weight loss, a growing body of research supports low-carbohydrate, high fat dietary approaches for healthy weight management [41
]. These findings have led to increasing interest regarding the potential mechanisms through which dietary macronutrient content may promote or discourage weight loss. For example, Ebbeling et al. (2012) demonstrated that following weight loss, low-fat, high carbohydrate diets produced greater reductions in resting and total energy expenditure than other diets, whereas diets with low-carbohydrate and higher fat content produced the smallest reductions in energy expenditure during isocaloric feeding following weight loss [42
]. In line with the findings of Ebbeling et al. (2012), the findings of the present review suggest that high fat, low carbohydrate diets are most advantageous for promoting long-term weight loss.
There are several limitations to the present review. First, there were a limited number of clinical trials available from which to evaluate weight loss outcomes of popular diets that did not have specific calorie targets or structured exercise programs. Due to the limited number of published studies, we were not able to statistically compare weight loss differences between individual diets. The small number of clinical trials examining the efficacy of many popular diets is concerning, as it indicates relatively little empirical evidence exists to support many current popular diets available, which are heavily marketed to the public.
A second limitation is that our analyses were based only on the randomized dietary assignment and did not account for adherence to the actual macronutrient composition of the specific diet. Unfortunately, there is a lack of information on adherence to popular diets as well as weight loss outcomes. In a few of the studies included in this review, attrition levels were high (>40%), which suggests individuals had trouble adhering to the diet. For example, long-term results of the study conducted by Truby and colleagues were based on a 12-month follow-up of only nine out of 57 randomized participants who volitionally chose to adhere to the Atkins diet after completing the initial six-month intervention [26
]. It is noteworthy that only a small percentage (16%) of the individuals in the randomized sample chose to remain on the diet following the intervention.
Another limitation of the review is that we reported all weight changes as weight change from baseline rather than as a difference from a control group. An additional limitation is that weight loss was the only outcome included in this review. Changes in waist circumference, BMI, and body composition might provide more evidence from which to evaluate the efficacy of these diets. Additionally, assessment of the effects of popular diets on cardiovascular, metabolic (e.g., blood pressure and serum lipid concentrations), and functional outcomes could reveal information on the safety of these diets. During aging, there is typically an increase in body fat mass and a corresponding loss of muscle mass and strength [43
]. In line with this, individuals who are “normal weight” or “healthy weight” but have a high body fat percentage have recently been recognized as an at risk group, [45
] as they often show signs of metabolic dysregulation normally associated with obesity [46
] and have increased risk of cardiovascular disease [48
]. Such findings highlight the importance of assessing body composition and not just body mass (weight) in future weight loss intervention trials.
This review also had a number of strengths. First, to our knowledge, the present review is the first to compile the findings from clinical trials that objectively measured weight loss associated with popular diets in the absence of explicit calorie restriction targets and/or structured exercise components. Additionally, the inclusion criteria for the clinical trials in this review were rigorous. The reason for such explicit criteria was to ensure that only methodologically-strong studies were included. However, our study eligibility criteria may have eliminated some studies and clinical evidence that might provide a broader perspective on the differences between the popular diets on weight loss outcomes.