Original Article

European Journal of Clinical Nutrition (2010) 64, 1323–1331; doi:10.1038/ejcn.2010.163; published online 15 September 2010

Effect of a relatively high-protein, high-fiber diet on body composition and metabolic risk factors in overweight women

L Te Morenga1,3, S Williams2, R Brown1 and J Mann1,4

  1. 1Department of Human Nutrition, University of Otago, Dunedin, New Zealand
  2. 2Department of Preventive and Social Medicine, University of Otago, Dunedin, New Zealand
  3. 3Edgar National Centre for Diabetes Research, University of Otago, Dunedin, New Zealand
  4. 4Riddet Institute, Palmerston North, New Zealand

Correspondence: L Te Morenga, Department of Human Nutrition, University of Otago, PO Box 56, Dunedin 9054, New Zealand. E-mail: lisa.temorenga@otago.ac.nz

Received 10 May 2010; Revised 5 July 2010; Accepted 8 July 2010; Published online 15 September 2010.

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Abstract

Background:

 

Obesity and its comorbidities are worldwide problems. Approaches to reducing obesity and its associated metabolic derangements typically emphasize fat and energy restriction, but for many achieving and maintaining weight loss is difficult. Diets that focus on substantially altering macronutrient distribution rather than energy restriction are promising alternatives, but have generally included large amounts of protein, fiber or fat.

Objective:

 

To compare the effects of dietary advice including moderate increases in protein and fiber without specifying energy intake with standard low-fat, high-carbohydrate dietary recommendations on body composition and metabolic risk factors.

Methods:

 

89 overweight or obese women aged 18–65 years were randomized to either a standard diet that was intended to be low in fat and relatively high in carbohydrate (n=42) or to a relatively high-protein (up to 30% of energy), relatively high-fiber (>35g per day; HPHF) diet (n=47) for 10 weeks. Advice regarding strict adherence to energy intake goals was not given.

Results:

 

Participants on the HPHF diet lost more body weight (1.3kg; 95% CI, 0.7, 1.9; P<0.0001), total fat (1.0kg; 95% CI, 0.2, 1.8; P<0.0001) and truncal fat (0.7kg; 95% CI, 0.1, 1.3; P=0.034) than participants on the standard diet. Total cholesterol and low-density lipoprotein (LDL) cholesterol were also significantly lower after the HPHF diet.

Conclusion:

 

An ad libitum diet relatively high in both protein and fiber improved body composition and metabolic risk factors compared with standard dietary advice.

Keywords:

obesity; dietary protein; dietary fiber; body composition; weight loss; women

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Introduction

Overweight and obesity are widely considered to have reached epidemic proportions worldwide, with the comorbidities of excess adiposity, notably diabetes, cardiovascular disease and cancer, stretching healthcare resources in both developed and developing countries to the limit (Yach et al., 2006). Public health and therapeutic approaches have generally emphasized appreciable reductions in energy intake and increases in energy expenditure (World Health Organisation, 2009), but although some countries have registered a halt in the steadily rising rates of overweight and obesity there is little evidence of a decline (Low et al., 2009). Furthermore, in the context of treating individuals who are already overweight or obese it appears to be exceptionally difficult for people to sustain their efforts to reduce energy intake and maintain weight loss when it has been achieved in the short-term (Ayyad and Andersen, 2000). For this reason there is considerable interest in the potential of modifying macronutrient intakes, without specific advice relating to energy intakes, as a means of achieving weight reduction (Abete et al., 2006).

Distribution of macronutrients may influence a range of cardiovascular disease risk factors (Reaven, 2005). For example, high fiber intakes, particularly from wholegrains, have been shown to be beneficial with regard to improving insulin sensitivity (IS) glucose and lipid levels, in addition to facilitating weight loss (Anderson et al., 2009), while high protein intakes appear to facilitate weight loss and fat loss alongside improving lean mass retention, and may have a beneficial effect on plasma lipids in comparison with high-carbohydrate diets (Krieger et al., 2006; Hession et al., 2009). However, experimental diets exploring these effects have generally included large amounts of protein or fiber, which may not be realistic for many individuals. Therefore, we were interested in whether modest increases in both protein and fiber might provide an alternative synergistic approach to achieving metabolic and weight reduction benefits. Thus, we have compared, in overweight women, two diets differing in macronutrient composition; one relatively low in fat and high in fiber-rich carbohydrate as widely recommended by public health agencies, which served as our standard diet and the other relatively higher in protein and dietary fiber.

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Methods

Subjects

Overweight women or women with a family history of type 2 diabetes, aged between 18 and 65 years living in Dunedin, New Zealand, were recruited. The inclusion criteria were either a body mass index (BMI) of greater than or equal to25, or a BMI of greater than or equal to23 if there was a family history of diabetes or for those of Asian ethnicity. Potential participants were excluded if there was evidence of heart disease, cancer, kidney disease or diabetes, if they had participated in a weight loss program or had lost >1kg bodyweight in the previous 2 months. Eligibility for entry into the study was established following completion of personal demographic and health related questionnaire and a 120-min oral glucose tolerance test (75g glucose). Each subject gave informed, written consent and all experimental procedures were approved by the University of Otago Human Ethics Committee.

A total of 94 participants were invited to attend a screening visit, if they appeared to meet inclusion criteria. Three women did not meet the screening criteria and two withdrew before being randomized to treatment. In all, 89 women were randomly assigned to treatment, 6 withdrew before receiving the allocated treatment, 8 withdrew during the treatment period and 75 women completed the entire study (Figure 1).

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Consort figure showing flow of participants through the trial.

Full figure and legend (52K)

Experimental protocol

Participants were randomly assigned to either the intervention diet or a standard diet using sequentially numbered, sealed envelopes containing a computer-generated allocation using random length blocks and stratified by age and BMI. During the initial 4-week phase, intensive advice was given regarding food choices necessary to achieve the required macronutrient composition. During the following 6 weeks they were encouraged to continue the recommended dietary pattern. Participants and researchers involved in the delivery of the treatment could not be blinded, however, laboratory staff and those conducting dual X-ray absorptiometry scans were not aware of group allocation.

Diets

Dietary advice given to the standard group was based on the New Zealand Food and Nutrition Guidelines for Healthy Adults (Ministry of Health, 2003) and developed to construct a diet, in which ~20% of total energy was derived from protein, 50% from carbohydrate and 30% from total fat. Saturated fat was intended to be low (<10%) and dietary fiber intake to be 25g per day or more. The standard group was provided with information available from the Ministry of Health in New Zealand designed to facilitate adherence to these guidelines. These resources are regularly used by dietitians when advising patients requiring standard dietary advice. The intervention diet was intended to be relatively high protein and high fiber (HPHF) and was designed to achieve 30% total energy from protein, 50% from carbohydrate, 20% from fat, and a dietary fiber intake of 35g per day. Individuals in this group were asked to increase their usual protein intake with lean meats, fish or low-fat dairy foods and to choose carbohydrates that were particularly high in soluble fiber, such as oats, certain legumes and nuts, dried fruit and stone fruits, as well as wholegrain breads and cereals. Restriction of fat intake was necessary in order to achieve the dietary fiber and protein goals without increasing energy intakes or requiring a dietary fiber supplement. The HPHF group was given material especially prepared for this study, including recipes and sample diet plans, as relevant material was not readily available. Because the HPHF diet would not have been familiar to the typical New Zealander participants were optionally provided with a variety of pre-prepared frozen main course meals, especially formulated to be high in protein and fiber, as well as 30g per day high-protein whey concentrate powder (NZMP Whey Protein Concentrate 392, Fonterra Co-operative Group Limited, Auckland, New Zealand), wholegrain breakfast cereal and bread, canned beans and canned fish. The meals each provided 2000–2500kJ per serve and participants were advised to supplement these with vegetables and side dishes in order to achieve satiety.

All participants were invited to make appointments with the lead researcher, on an as-required basis, to discuss their progress with following the diet. Participants were asked to maintain their usual levels of exercise for the duration of the study. Participants completed a simple food group checklist everyday and completed a weighed 3-day diet record at baseline, week 4 and week 10.

Measurements

Measurements were made at baseline, week 4 and week 10 after a 10-h overnight fast. Each participant's height, weight, waist circumference and resting blood pressure were recorded. A fasting blood sample was then taken for the measurement of lipids, glucose, insulin and C-reactive protein. Total body fat mass, lean mass, body fat percentage and truncal fat mass were assessed by dual X-ray absorptiometry (DPX-L scanner, Lunar Corp, Cincinnati, OH, USA) using software version 1.35 (Lunar, Cincinnati, OH, USA) at the Dunedin Public Hospital dual X-ray absorptiometry Scanning Unit only at baseline and week 10.

Laboratory analyses

Serum insulin was measured using a specific electrochemiluminescence immunoassay for the Elecsys analyzer (Roche Diagnostics, Mannheim, Germany). Serum total cholesterol (TC) and triglyceride and plasma glucose concentrations were measured enzymatically with kits and calibrators supplied by Roche Diagnostics on a Cobas Mira analyzer (Roche Diagnostics). High-density lipoprotein was measured in the supernatant after precipitation of apolipoprotein B containing lipoproteins with phosphotungstate/magnesium chloride solution (Assmann et al., 1983). Low-density lipoprotein (LDL) was calculated using the Friedewald equation (Friedewald et al., 1972).

Analysis and statistics

Insulin resistance was estimated by the homeostatic model assessment (HOMA)2, using the HOMA2 calculator (Levy et al., 1998) and by the McAuley index (McAuley et al., 2001).

In all, 72 participants were required to detect a 30% difference in HOMA insulin resistance with 80% power at level of significance of 0.05. Statistical analysis was performed using the STATA statistical software package 9.0 (Stata, College Station, TX, UAS). Data were analyzed on a modified intention-to-treat basis (that is, without imputation of missing values). A mixed model using ‘participant’ as a random effect was used to analyze the effect of the treatment over the two intervention phases (weeks 0–4 and weeks 5–10) using baseline values as a covariate (Vickers and Altman, 2001). As there were no significant interactions between the intervention phases and diet group the overall differences between the diet groups are presented. For dual X-ray absorptiometry data the effect of treatment was analyzed by analysis of covariance using baseline values as a covariate.

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Results

More participants were initially randomized to the HPHF group (44 vs 39), they were slightly older and had a higher estimated prevalence of insulin resistance (Table 1). There were 39 women in the HPHF group and 37 women in the standard group for whom clinical and anthropometric data are reported.


The HPHF group consumed significantly more protein and dietary fiber and less total fat and saturated fat than the standard group during the study, but there was no difference in carbohydrate intake, which did not change in either group. Reported energy intakes declined over the 10 weeks in both the standard and HPHF groups, but there was no significant difference between the two groups (Table 2). On average HPHF participants consumed 10.1 (13.5)g per day of whey protein powder providing 7.6g per day protein. Legumes, lean meat and chicken, and fish provided the additional protein consumed by the HPHF group.


At the end of the study the HPHF group had lower total body weight, total body fat and truncal body fat than the standard diet group, but there was no difference in lean body mass. Percent body fat and waist circumference was also lower in the HPHF group relative to the standard group, but the differences were not statistically significant (Table 3). In parallel with these differences, the HPHF group decreased TC and LDL. High-density lipoprotein and fasting plasma glucose also decreased, but the differences between the HPHF and standard groups did not achieve statistical significance (Table 4).



When terms for weight loss were included in the models the differences were −0.32mmol/l (95% CI, −0.53, −0.1mmol/l) for TC and −0.17mmol/l (95% CI, −0.34, 0.002mmol/l) for LDL suggesting that macronutrient composition accounted for the differences rather than weight loss.

Insulin resistance (HOMA) was reduced on the HPHF diet, but the difference was not significant. When assessing IS by the McAuley index (which was developed for assessment in populations comprising mainly normoglycemic individuals) there was a 6% improvement (95% CI, −1, 13%) that almost reached conventional levels of statistical significance (P=0.077). A post hoc subgroup test was performed to determine whether there was a more marked effect of the HPHF diet on IS in obese participants by including an interaction term in the model. This was significant (P=0.038). When considering only those with a BMI of greater than or equal to30kg/m2 at baseline, IS was 11% higher (95% CI, 3, 20%; P=0.009) in participants on the HPHF diet compared with the standard diet group. In those with a BMI <30kg/m2 IS was 4% lower (95% CI, −7, 14%; P=0.505) on HPHF. The interaction effects between BMI greater than or equal to30kg/m2 and diet for other variables were not significant.

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Discussion

This study showed that a modest increase in consumption of both dietary protein and fiber reduced measures of adiposity and several risk factors associated with the metabolic syndrome in overweight women during a 10-week dietary intervention study. This dietary pattern facilitated modest reductions in body mass (1.2kg), total body fat (1.0kg) and central body fat (0.7kg) with no loss of lean mass. Of particular relevance is the fact that weight loss occurred without specific advice to reduce energy intake and lose weight. In parallel with these changes, there was a 5% improvement in total serum and LDL cholesterol concentrations. Although improvement in IS did not reach conventional levels of statistical significance in the group as a whole, in obese women there was a statistically significant improvement of 11%. It is noteworthy that the dietary changes, weight loss and changes in metabolic measures all occurred within the first 4 weeks and were maintained throughout the study period. Furthermore, although reductions in body weight and fat were small they were sufficient to result in metabolic changes that might be expected to translate into clinical benefits. Insulin resistance is considered to be the underlying abnormality in most cases of type 2 diabetes (Chiasson and Rabasa-Lhoret, 2004) and the reduction in TC and LDL cholesterol that we observed would be expected to have a meaningful beneficial effect on coronary heart disease risk (Baigent et al., 2005). In contrast the standard diet, which was based on national dietary guidelines, had no effect other than to maintain weight and most risk factors at baseline levels.

The fact that the intervention and standard diet groups were not treated in an identical manner might be perceived to be a weakness of the study design. However, the aim of the study was to compare the effects of dietary advice intended to achieve a modest alteration in macronutrient distribution and increase in dietary fiber with standard dietary advice to reduce fat and increase carbohydrate. Some meals and additional protein were provided because of unfamiliarity with preparation and food purchasing, required to increase protein and fiber. Sufficient food was provided to enable sharing with the family and as such no advice was given to reduce food intake. Thus, there is no reason to believe that provision of sample foods might have promoted weight loss. Indeed, we could argue that availability of free food could have encouraged increased rather than reduced intakes. As this was an efficacy study the approach seems to be reasonable.

Recruiting overweight women and those with a family history of diabetes enabled us to identify a group at particular risk of diabetes and cardiovascular disease. Prevention and reduction of excess adiposity is the cornerstone of management of such people (Mann, 2006). Most of the studies which have examined nutritional approaches aimed to achieve weight loss and reversal of the clinical abnormalities associated with excess adiposity have included reductions of energy intakes as a core component of dietary advice (Abete et al., 2006). Given the difficulties in achieving maintenance of reduced energy intakes and weight loss, we were interested in exploring advice to alter macronutrient composition in the hope of achieving both weight loss and improved metabolic profile. The New Zealand Guidelines, and many comparable recommendations worldwide, suggest an increase in intakes of vegetables, fruits, wholegrain cereals and low-fat dairy products as a means of achieving a relatively high-carbohydrate, high-fiber, low-fat diet rich in essential nutrients to facilitate weight loss (Ministry of Health, 2003). Such guidelines tend to be based on food groups and utilize food models, such as the food pyramid and plate model. Analysis of the diet records suggests that those given this advice made few changes to their current dietary practices and not surprisingly there was no effect on body fat or metabolic characteristics. Thus, our study suggests that there is little merit in such an approach—at least in the absence of an intensive dietary intervention.

Studies that have shown benefits of a high-carbohydrate high-fiber diet have typically emphasized the benefits of legumes and minimally processed carbohydrates (not simply conventional wholemeal and wholegrain bread) or have used fiber supplements to achieve high fiber intakes. A meta-analysis of ad libitum high-fiber diets suggested the potential of the approach to achieve an weight loss with an increased intake of fiber of ~14g per day (Howarth et al., 2001). More recently, Hays and colleagues reported greater weight and fat loss in overweight older adults with impaired glucose tolerance following an ad libitum high-fiber (57g per day), high-carbohydrate diet compared with those on a control diet (Hays et al., 2004). These effects may be explained largely by reductions in dietary energy density on low-fat, high-fiber diets (Rolls, 2009). Very high-fiber (>50g fiber per day), low-glycemic index diets rich in wholegrains and legumes have also been shown to improve lipid profiles and IS independently of weight loss (Fukagawa et al., 1990; Anderson et al., 1992; Chandalia et al., 2000). Given that the benefits of ad libitum high-fiber diets are more apparent when intakes are appreciably greater than those usually achieved in Western countries, it is not surprising that the standard diet group did not reduce body weight and body fat or show an improvement in indicators of lipid or carbohydrate metabolism. They were given the advice and educational materials typically given to individuals in primary health care settings. If these participants, who were sufficiently motivated to volunteer for a research study, were unable to achieve meaningful changes with such advice it seems unlikely that this approach will be of much value in the population at large without further instruction regarding calorie intake and specific foods.

High-protein diets have been shown to reduce energy intakes, thus facilitating weight loss as well as preserving lean muscle mass during weight loss. A number of studies examining the effect of high-protein diets on weight loss have reported greater weight loss on high-protein diets compared with high-carbohydrate diets, though not all studies have shown a difference (Halton and Hu, 2004). Studies comparing ad libitum high-fat, moderately high-protein diets with energy restricted low-fat, high-carbohydrate diets have shown the most consistent benefits of high-protein diets. One such study, lasting 6 months, reported greater reduction in weight (3.7kg) and fat (3.4kg) in overweight and obese participants following an ad libitum high-protein diet compared with an ad libitum high-carbohydrate, moderate fiber diet. This was attributed to lower energy intakes in the protein group (Skov et al., 1999). This study, however, involved intensive monitoring from study nutritionists. Another study, involving less intensive monitoring also found greater weight loss (2.7kg) and fat loss (1kg) on an ad libitum high-protein diet compared with a low-fat, high-carbohydrate energy restricted diet after 6 months (McAuley et al., 2005). However, by 12 months the difference between the two diets was no longer significant. Studies comparing high-protein with high-carbohydrate diets that have not found a significant difference in weight loss and body composition, have typically compared isocaloric, energy restricted diets (Brinkworth et al., 2004; Johnston et al., 2004; Noakes et al., 2005; Leidy et al., 2007) or were underpowered (Layman et al., 2003; Lasker et al., 2008).

There is evidence that reduced carbohydrate and higher protein diets facilitate greater fat loss for women with features of the metabolic syndrome, including insulin resistance and raised plasma triglyceride concentrations, than do conventional low-fat, high-carbohydrate diets (Cornier et al., 2005; Noakes et al., 2005). A sub-group analysis of our study showing a statistically significant improvement in estimated IS only in obese women on the HPHF diet supports these observations, but further follow-up is warranted.

Recent studies suggest that overweight and obese individuals find it difficult to sustain energy reduction and dietary fiber intakes at levels that vary substantially from usual dietary patterns over the long-term (Brinkworth et al., 2004; McAuley et al., 2006), so it is encouraging to observe that benefits can be achieved with fairly moderate changes in macronutrient intakes and with ad libitum energy intakes. Enhanced satiety and reduction of energy density may explain the potential benefits of a diet relatively high in protein and fiber and low in fat.

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Conflict of interest

The authors declare no conflict of interest.

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Acknowledgements

This study was supported by Fonterra Co-operative Group Ltd. and the New Zealand Government funded this study as part of a New Zealand Foundation of Research Sciences and Technology Project DRIX0401. The Riddet Institute (Palmerston North, New Zealand) provided further funding.

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