TREATMENT STUDIES: THE ROLE OF DIETARY CARBOHYDRATES IN WEIGHT LOSS AND PREVENTION OF WEIGHT REGAIN
9.5 TREATMENT STUDIES: THE ROLE OF DIETARY CARBOHYDRATES IN WEIGHT LOSS AND PREVENTION OF WEIGHT REGAIN
Weight loss and subsequent weight maintenance may be, at least in part, meta- bolically different from prevention of weight gain. Consequently, weight loss and prevention of weight regain may be more difficult to achieve than prevention of weight gain in the first place, and the influence of diet on these three metabolic states may differ. For this reason, the influence of dietary carbohydrates on weight loss will be reviewed separately from the influence of dietary carbohydrates on the prevention of weight gain. We will begin with a discussion of what amount (high or low) and what type of carbohydrates are optimal for weight loss and maintenance, followed by a brief overview of health outcomes associated with
Carbohydrates and Obesity
low-calorie, high- and low-carbohydrate diets. Modifying carbohydrate intake to prevent weight gain and the development of obesity will then be addressed.
Interpreting the research in this area is complicated by the fact that when carbohydrate intake changes, a concurrent change in fat intake typically occurs. As discussed previously, a low-carbohydrate diet is usually associated with a higher fat
intake and vice versa, 8 and when subjects reduce their carbohydrate intake, they typically increase their intake of fat. 96 Protein intake tends to remain relatively constant both within an individual and between individuals, whereas intakes of the other two macronutrients may change. Although the debate about the optimal ratio of macronutrients has focused on carbohydrates vs. fats, there is evidence to suggest that a high-protein diet may be beneficial for weight loss and maintenance, 97,98
possibly by promoting increased postprandial thermogenesis 99 and satiety. 7,12 Further complicating interpretation of clinical studies is the fact that carbohydrate consump- tion varies considerably, most certainly in a laboratory setting, but also among and between individuals in a free-living population. Further, carbohydrates are hetero- geneous as a group, and not all types may be equally advantageous (or disadvanta- geous) for weight loss and prevention of weight gain.
At any given time it is estimated that approximately one third of adults in the U.S. are attempting to lose weight, the majority by altering diet composition. 100 Paradoxically, both low- and high-carbohydrate diets have been recommended and adopted for weight loss. The conventional approach, advocated by the majority of medical groups and health professionals, involves consuming a hypocaloric diet with
a high carbohydrate-to-fat ratio. 101,102 However, a relatively low-carbohydrate, high- fat, and high-protein hypocaloric diet has also been advocated in the popular press. The Atkins diet in particular, which initially gained popularity in the 1970s, has enjoyed resurgence in interest in recent years. 103
9.5.1 L OW - VS .H IGH -C ARBOHYDRATE D IETS FOR W EIGHT L OSS
Total energy, rather than carbohydrate content, has been shown to be the most important determinant of weight loss. Kennedy et al. 64 concluded that weight loss is relatively independent of macronutrient composition, but is dependent instead on
a reduction in total energy intake. This conclusion followed a systematic review of over 200 weight-loss studies of various designs published between 1956 and 2000. In this review, weight loss following consumption of a relatively low carbohydrate intake (ranging from 0 to 85 g/day) was assessed in 20 studies. In the majority of these 20 studies, weight changes were assessed over a relatively short term of approximately 6 weeks in duration. Within this period, weight loss ranging between
2.8 and 12.0 kg occurred in all studies. A similar number (22) of studies of diets relatively high in carbohydrates (6 of which were very low fat, <10% kcal, and 16 moderate in fat, 20 to 30% kcal) caused weight loss to the same extent as reported for the studies of low-carbohydrate diets.
Others have similarly concluded that weight loss in obese subjects is not related to dietary carbohydrate content. 104 Bravata et al. 104 recently reviewed 107 English- language studies published between 1966 and 2003 that manipulated dietary carbo- hydrate in a total of 3268 adult subjects in an outpatient setting. When studies of
Functional Food Carbohydrates
heterogeneous design (e.g., randomized and controlled as well as nonrandomized and noncontrolled trials) were considered altogether (as well as when segregated by energy content of the diet, diet duration, and baseline weight of subjects), weight loss was greater with lower-carbohydrate (60 g/day carbohydrate, mean weight loss of 16.9 kg weight) than with higher-carbohydrate diets (>60 g/day carbohydrate, mean weight loss of 1.9 kg). However, when studies of similar design were com- pared, no difference in weight loss was observed. For example, considering only randomized controlled and randomized crossover trials, mean weight loss did not significantly differ between the lower-carbohydrate diets (3.6 kg (95% confidence interval (CI), 1.2 to 6.0 kg)) and the higher-carbohydrate diets (2.1 kg (95% CI, 1.6 to 2.7 kg)). Rather, the degree of weight loss varied as a function of energy intake and diet duration: the lowest energy intakes for the longest periods of time resulted in the greatest weight loss. Although studies of duration less than 4 days were excluded in this analysis, the majority of studies had a relatively short duration, averaging 50 and 73 days for the lower- and higher-carbohydrate diets, respectively. It must also be noted that a minority of studies involved the consumption of few carbohydrates (<20 g/day), typical of many of the popular diet regimens, such as Atkins. Similarly, very high carbohydrate/very low fat diets were not included.
More recently, results have been published from several randomized controlled clinical trials of subjects on ad libitum diets involving a relatively longer duration of intervention (6 to 12 months). These results lend support to the hypothesis that
a low-carbohydrate diet is superior to a high-carbohydrate diet for weight loss. 105–107 For example, in one 6-month study, subjects on the ad libitum low-carbohydrate Atkins diet (<20 g/day) lost significantly more weight and body fat (8.5 and 4.8 kg, respectively) than subjects following a conventional high-carbohydrate (approxi- mately 54% of calories) and low-fat (approximately 28% of calories) hypocaloric diet (3.9 and 2.0 kg, respectively), a difference that remained significant even when an intention-to-treat analysis including all randomized subjects (i.e., dropouts) was performed. 105 In another trial, unique in that an attempt was made to mimic the approach used by most dieters (i.e., a copy of Dr. Atkins’ book was provided to subjects who otherwise received minimal diet counseling or support), obese adults on the low-carbohydrate diet lost significantly more weight at 6 months (7.0 vs. 3.2% change) than adults on the high-carbohydrate, low-calorie diet. However, by
12 months this gap in weight loss had narrowed (4.4 vs. 2.5% change) and was no longer statistically significant. 106
A variety of advantages have been proposed, but not proven, for a low- carbohydrate diet for weight loss. Interestingly, subjects on the low-carbohydrate diets in these three studies were not given instructions on calorie restriction, but reduced their calorie intake nonetheless to approximately the same degree as subjects instructed on the hypocaloric, high-carbohydrate diets. Proponents of popular low-carbohydrate diets frequently argue that low-carbohydrate (and high- fat and -protein) diets are more palatable, more satiating, and more acceptable to consumers than the conventional counterpart. The purported, but yet to be proven, mechanistic advantage of this low-carbohydrate (<10% of daily calorie intake at inception) diet for weight loss is its ability to promote lipid oxidation, satiety, and
Carbohydrates and Obesity
energy expenditure, attributed at least in part to the induction of ketosis. However, data from clinical trials suggest that ketosis is not a likely explanation for weight loss when consuming a low-carbohydrate diet. For example, ketosis does not appear to persist after the first few months on the diet, and presence of urinary ketones has not been correlated with degree of weight loss. 105,106 It has also been hypothesized that the greater limitation in food choices, simplicity, or novelty of
a very low carbohydrate diet may be responsible for a greater reduction in calorie intake than in the more conventional higher-carbohydrate diet, which allows for intake of a wider variety of foods. 106,107 Similar limitations in food choices may explain the effectiveness of other less conventional diets, including ones that are very low in fat and very high in carbohydrates. 108 Finally, there was a trend in all studies toward lower attrition among subjects consuming a low-carbohydrate diet in comparison to a higher-carbohydrate one. For example, in one study the attrition rate over the course of the 6-month study was 26% in the high-carbohydrate group and 15% in the low-carbohydrate group. 105
In summary, it is premature to endorse a low-carbohydrate diet over a conven- tional low-fat, high-carbohydrate diet for weight loss. Low-calorie intake and neg- ative energy balance can be achieved on a variety of carbohydrate distributions. Regardless of diet composition, diets that are intended to reduce body weight generally achieve only small weight loss, and high attrition is the rule rather than the exception. Further, there are established benefits of dietary fat restriction for reduction of serum cholesterol and risk of cardiovascular disease. Finally, the short- term nature of the majority of studies precludes assessment of risk vs. benefit over extended periods.
9.5.2 L OW - VS .H IGH -C ARBOHYDRATE D IETS FOR P REVENTION OF W EIGHT R EGAIN
For successful weight loss to be maintained, it is critical to determine whether dietary composition is a critical element and, if so, which dietary composition is most likely to prevent excess energy intake over a prolonged period in a free- living population in an ad libitum setting. Unfortunately, most weight-loss studies have been of short duration and have not included long-term follow-up. Data from the National Weight Registry indicate that 90% of long-term weight maintainers consume a low-fat, high-carbohydrate, low-calorie diet and regularly participate in physical activity. 109 This registry is the largest U.S. database available of adults who have lost weight (averaging 30 kg) and maintained a minimum of 13.6 kg weight loss over a period of 5 years. Whether observations of individuals in this registry represent the experience of the general American population of weight maintainers is not known. In no controlled studies has the influence of low- vs. high-carbohydrate diets on maintenance of weight loss been evaluated. Before a low- or high-carbohydrate diet can be advocated, additional studies are warranted in which isocaloric diets of varying carbohydrate contents are provided for extended periods, and during which hunger ratings, diet acceptability, and health- related outcomes in addition to adiposity are monitored.
Functional Food Carbohydrates
9.5.3 T YPE OF C ARBOHYDRATE FOR W EIGHT L OSS
Based on a review of weight-loss studies, Howarth et al. 17 reported that adults on relatively higher fiber diets (fixed or ad libitum) consistently lost more weight than
those consuming lower-fiber diets. This effect was observed even when energy intake was held constant. The amount of weight loss in general was modest (averaging 1.9
kg over 4 months in ad libitum studies and 1.3 kg over 3 months in studies in which fiber intake was fixed), but similar in magnitude to that observed when conventional
low-fat diets are consumed ad libitum. Levine and Billington 110 came to a similar conclusion after finding that 26 of the 38 studies identified reported a decrease in
body weight following a high-fiber regimen. There is also evidence that fiber may play a role in the prevention of weight regain following weight loss, 111,112 although
this has been investigated to a limited extent. No differences were observed between types of fibers (soluble vs. insoluble vs. mixed or fiber supplements vs. high-fiber
foods) in the meta-analysis by Howarth et al. 17 Similarly, Saltzman and Roberts 16 have argued that results are mixed with respect to the impact of soluble fiber on
weight loss, with the numbers of studies reporting no effect equal to the numbers of studies reporting a positive impact on weight loss.
There is evidence that reducing added sugar intake may also be a beneficial weight-loss strategy. Vermunt et al. 113 recently performed a systematic review of
weight reduction trials that focused on replacing added sugar with either low-energy artificial sweeteners (e.g., aspartame) or complex carbohydrates. The cumulative
results suggested that removing added sugar from the diet was beneficial in reducing energy intake and body weight. 113 The authors have cautioned, however, that the
studies addressing added sugars and weight loss were limited in number (<10) and relatively short term (<1 year) and that the results could not be extended to weight maintenance or the prevention of weight regain.
The glycemic response induced by carbohydrate-rich foods may also play a role in weight loss, 114 though the few clinical trials that have been performed to date have yielded conflicting results. 115,116 In one randomized controlled 6-month study, overweight or obese subjects consumed low-fat ad libitum diets that were high in either complex carbohydrates (one fourth of dietary fat replaced by complex carbo- hydrates) or simple sugars (one fourth of dietary fat replaced by simple sugars). Both groups lost significantly greater amounts of weight (1.8 and 0.9 kg, respec- tively) than the higher-fat (30 to 40% of energy), lower-carbohydrate control group, even though none of the subjects were encouraged to reduce their energy intake. Dietary fiber intake did not differ significantly between groups, a fact that was not entirely unexpected given that prepackaged foods (rather than fruits and vegetables) were provided to study participants. 117 Interestingly, in a subset of 46 subjects with symptoms of the metabolic syndrome at baseline, significantly greater weight loss was achieved on the diet high in complex carbohydrates (–4.25 kg) than on both the diet high in simple sugars (–0.29 kg) and the control diet (+1.03 kg). 118 Complex and simple carbohydrates may have differential effects depending on the individual’s metabolic predisposition.
Carbohydrates and Obesity
9.5.4 O THER H EALTH C ONSIDERATIONS OF A H IGH - C ARBOHYDRATE D IET
In addition to weight outcomes, other health and nutrition indicators associated with dietary macronutrient composition must be considered. The merits of high- and low- carbohydrate intakes with respect to cardiovascular disease, diabetes, cancer, and gastrointestinal health are discussed in detail in other chapters. What follows is a brief description of the effects of high- vs. low-carbohydrate diets on the general health of persons undertaking weight loss.
Low-fat, high-carbohydrate diets, particularly those low in saturated and trans fats, have been shown to produce biologically meaningful reductions in blood cho- lesterol levels, without adverse effects on blood glucose. 64 For example, on the basis of a meta-analysis of 37 intervention studies, low-fat, high-carbohydrate diets were shown to reduce blood concentrations of total, low-density lipoprotein (LDL), and high-density lipoprotein (HDL) cholesterol as well as triaclyglycerol. 64,119 However, it is not clear the extent to which some of the beneficial effects were due to weight loss, because weight loss, regardless of how achieved, is frequently accompanied by improved glycemic control, blood pressure, and blood lipid concentrations. In controlled feeding studies in which body weight was maintained, low-fat diets have been associated with decreases in HDL cholesterol and increases in triaclyglycerol, both of which are risk factors for cardiovascular disease. 120 Unfortunately, the major- ity of weight-loss studies performed specifically to test the effects of different carbohydrate intakes have focused on weight and have not included other measures
of health. 64 In the recent systematic review of carbohydrate intervention studies that examined health measures, the majority of which included only overweight or obese adult subjects, alterations in carbohydrate intake were not significantly associated with any adverse changes in serum lipid or glucose concentrations, or systolic blood pressure. 104 However, the authors caution that because of limited sample size, larger and longer-term studies are warranted.
9.5.5 O THER H EALTH C ONSIDERATIONS OF A L OW -C ARBOHYDRATE D IET
A concern voiced by health professionals is that low-carbohydrate diets can result in excess consumption of total fat, saturated fat, and cholesterol with adverse effects on blood lipids. According to several systematic reviews, however, the most consis- tent metabolic effect of a short-term low-carbohydrate diet that produces weight loss is a reduction or no change in serum triglycerides. 64,104 Effects on blood glucose and insulin concentrations and blood pressure were less consistent, while the effects on lipoproteins were the most mixed. 64,104 However, it has been noted that studies of low-carbohydrate weight-loss diets have been relatively short term (i.e., the majority shorter than 90 days) and have involved adults who are relatively young (i.e., younger than 60 years old) and, in most instances, healthy (i.e., free from hyperlipidemia, hypertension, or diabetes). The effects of low-carbohydrate diets on the health of children and individuals with hyperlipidemia, diabetes, or other metabolic disorders merit additional study. Assessments of additional health outcomes, such as renal
Functional Food Carbohydrates
function, bone health, cardiovascular function, and quality of life, are needed. Fur- ther, the lowest-carbohydrate diets (e.g., 20 g/day) have been investigated in very few studies, few of which included measures of blood pressure or blood metabolites. It has also been suggested that low-carbohydrate diets may result in ketosis, high urinary nitrogen loads, impaired liver and renal function, and, in children, myocardial dysfunction. 121 Again, these potential consequences have not been adequately stud- ied. It remains possible that, in the short term at least, the effect of weight loss on blood lipids may override effects due to the macronutrient composition of the diet. 105 If so, very low carbohydrate diets may only have a positive impact on blood lipids during the weight-loss phase. Decades of research suggest that prolonged high-fat intake, especially of saturated fat, increases the risk of heart disease. 122,123
Finally low-carbohydrate diets have been criticized for their lack of adequate dietary fiber and antioxidant vitamins. 105 In an analysis of U.S. national survey data
(1994 to 1996 CSFII), diet quality, as assessed by the USDA Healthy Eating Index (a composite rating based on the U.S. Food Guide Pyramid and Dietary Guidelines),
was higher for high- than for low-carbohydrate diets. 64 The high-carbohydrate group ate more low-fat foods, grain products, fruits, and fruit juices and had higher intakes per 1000 kcal of vitamin A, vitamin C, folate, calcium, magnesium, and iron, and
lower intakes of sodium and alcoholic beverages. 96 On the other hand, adults con- suming lower-carbohydrate intakes had higher micronutrient densities of vitamin
B 12 and zinc and lower contents of sweets and sugars in their diets. 96