Relationship of fruit and vegetable intake with adiposity: a systematic review
T Ledoux, USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, 1100 Bates Street, Houston, TX 77030, USA. E-mail: email@example.com
Fruit and vegetable (FV) intake has been proposed to protect against obesity. The purpose of this paper was to assess the FV consumption to adiposity relationship. Twenty-three publications were included. Inclusion criteria: longitudinal or experimental designs; FV intake tested in relation to adiposity; child, adolescent or adult participants; published in English-language peer-reviewed journals. Exclusion criteria: dietary pattern and cross-sectional designs; participants with health concerns. Experimental studies found increased FV consumption (in conjunction with other behaviours) contributed to reduced adiposity among overweight or obese adults, but no association was shown among children. Longitudinal studies among overweight adults found greater F and/or V consumption was associated with slower weight gain, but only half of child longitudinal studies found a significant inverse association. Limitations in methods prevented a thorough examination of the role of increased FV intake alone or mechanisms of effect. An inverse relationship between FV intake and adiposity among overweight adults appears weak; this relationship among children is unclear. Research needs to clarify the nature of, and mechanisms for, the effects of FV consumption on adiposity. Whether increases in FV intake in isolation from lower caloric intake or increased physical activity will result in declines or slower growth in adiposity remains unclear.
Fruit and vegetables (FV) are rich in water and fibre, and low in energy density; therefore, FV consumption has been proposed as an obesity prevention strategy (1–4). FV may be protective from adiposity due to the displacement of energy-dense foods (5,6); the satiating effect of fibre resulting in fewer calories consumed (7,8); and the modulation of dietary glycemic load, affecting postprandial hormonal shifts (9,10). However, previous reviews (two narrative (11,12), one systematic (13)) came to conflicting conclusions (11–13). One found no support for the protective effect of FV consumption on childhood obesity; however, the quality of research methods of articles in the review was unknown (11). A systematic review of adult intervention studies concluded that interventions targeting FV consumption resulted in reduced adiposity (13); however, most studies involved participants with chronic health conditions, thereby limiting generalizability (13). A narrative review of epidemiologic studies identified too many inconsistencies in the literature to draw definitive conclusions (12), but most of the studies were cross-sectional, a weak design for drawing causal inferences. As a result, the present review assessed whether FV consumption was related to reductions or slower gains in adiposity among adults and children in experimental or longitudinal studies (stronger designs than cross-sectional), and assessed research method quality. Potential mechanisms of effects (e.g. fibre, energy density) were also discussed.
A total of 772 articles were located using PsychInfo and PubMed databases from 1980 to January, 2009. Key words included: fruit, vegetable, obesity and weight. Articles referenced in key articles were also considered. Inclusion criteria were longitudinal or experimental designs (most rigorous and suggestive of causative associations) in which the relationship between FV (whole FVs, not fruit juice alone) intake and an adiposity indicator (e.g. body mass index) was tested; studies using healthy human participants (children, adolescents or adults); and studies published in English-language peer-reviewed journals. Exclusion criteria included dietary pattern (did not examine associations between adiposity and FV consumption in isolation from other foods like whole grains, low fat dairy, etc.) and cross-sectional studies (cannot infer causation); and studies using participants with chronic diseases, pregnant or post-partum women, or children with developmental delays as they involve special physiological or dietary concerns.
Higher FV consumption was expected to be related to lower weight status, or increases in FV consumption were expected to be related to decreases in weight status. Studies were categorized into those that found this expected inverse association between FV and adiposity, those that found mixed results (i.e. relationships in some groups but not others, or for F or V but not both), or those that found an unexpected positive or no association. Across these categories, research findings and their validity were compared by critiquing research methods. Research factors determined to enhance study validity included: rigor of study design, validity of measures, statistical adjustment of potential confounding variables (including dietary reporting bias), and sufficient sample size to detect hypothesized relationships. Rationales for specific indicators of research method quality included the following. The most valid approach to self-reported dietary assessment is the multiple pass 24-h dietary recall (diet records and food frequency questionnaires are less valid) (14). A study's FV classification system (what foods were considered FV) is important to consider for purposes of comparing, interpreting and generalizing results. Because over- or under-reporting of dietary intake significantly alters results (15), using an equation that adjusts for this (e.g. Huang equations) (16), reduces more error than removing outliers. BMI is an accepted indicator of adiposity (17), but underwater weighing and dual energy x-ray absorptiometry (DXA) are more precise (18), and self-reported weight and height are prone to more error than values obtained by trained technicians (19,20). Circumference and skin-fold measures estimate fat distribution (19,20). As the relationship between FV intake and adiposity has been confounded by energy expenditure (EE) (21), energy intake (EI) (22), socioeconomic status (23), ethnicity (23), age (23), gender (23) and social desirability of response (24), research should control for these variables statistically or through experimental methods (sample selection, matching, etc.). EE is most precisely measured by techniques like doubly labelled water, which is impractical in larger studies. Hence, physical activity (PA; one component of EE) measures are often used; and of these, objective measures (e.g. accelorometers) are preferred to less reliable self-reports (25). Achieving adequate statistical power is considered essential to detecting true relationships among study variables.
There were 12 experimental studies (11 adult and one child) and 11 longitudinal studies (seven adult and four children).
Adult experimental studies
Of 11 adult experimental studies, eight showed higher FV intake was related to weight loss (4,26–32) while three did not (increased FV consumption, but no change in adiposity (33,34); significant change in adiposity without change in FV consumption (35), Table S1). Studies that found the expected inverse association more specifically tested the relationship than studies that did not. For example, the primary focus of the three studies not showing the expected inverse association (33–35), and only one that did (32), was to test an intervention's ability to change behaviours (e.g. dietary intake, PA levels); adiposity was a secondary outcome. Conversely, studies showing the expected association were more likely aimed at testing the effect of diet changes (i.e. increased FV) on adiposity. To do this they used more intensive interventions, such as frequent face-to-face counselling sessions and frequent adiposity assessments, to enhance compliance (4,26–31). All studies detecting the expected relationship recruited participants with excess adiposity (lowest mean BMI at baseline among these studies was 26.0 (32)). Two of three studies not showing the expected relationship recruited participants in healthier weight ranges without excess adiposity (average BMI's at baseline: 23–24 (34) and 25–26 (33)).
Most of the intervention studies did not report the FV classification nor controlled for over- or under-reporting. Four studies that found the expected relationship provided inconsistent FV classification systems: one excluded juices and found no difference in results whether fried and dried FV were included or not (26), one included dried fruit and fruit juice (31) and one excluded potatoes (28). One study detecting the expected association controlled for under-reporting by comparing reported EI with WHO EE estimates (27), and one that did not find the expected association controlled for under-reporting with a similar method, but measured EE with indirect calorimetry (30).
No differences between studies finding and not finding the expected association were detected on statistical power, method of group assignment, method of measuring adiposity, length of interventions, control of possible confounders or diet assessment method.
Mechanisms of effect: energy intake
None of the 11 adult experimental studies tested whether changes in EI mediated the relationship of FV intake to weight changes (36). However, eight studies found weight loss occurred when increased FV intake was accompanied by reduction in total EI (4,26–31) or increased PA (32) (Table 1). In seven of these eight studies multiple behaviours including increased FV consumption were targeted for change, including decreasing overall EI (4,27), decreasing energy-dense food consumption (26,27,29–31), increasing PA (32), decreasing alcohol consumption (31) and modifying carbohydrate consumption (30,31). One targeted only FV consumption (increased intake by 700 grams) and resulted in decreased weight and EI (28) (Table 1). Two studies not showing the expected association instructed participants to increase FV intake without instruction on EI or energy-dense food consumption. In these studies, EI and FV intake significantly increased (by 229 and 395 g d−1 in each group (34); by 1.5 servings per day (33)) without significant changes in weight (34).
Table 1. Summary of energy intake, FV and weight changes in each adult experimental study
|Detected expected relationship|| || || || |
| de Oliveira et al. 2008 (4)||↑ 3 servings per day||↓ 104.88 kJ d−1d|
↓ 82.31 kJ d−1f
|↓ 0.93 kgd,e|
↓ 0.84 kgf
| Sartorelli et al. 2008 (29)||↑ 123 g d−1||↓ 1331 kJ d−1g||↓ 1.4 kg||6 months|
| Ello-Martin et al. 2007 (26)||↑ 52.3 g d−1||↓ 2090 kJ d−1||↓ 7.9 kg||1 year|
| Svendsen et al. 2007 (28)||↑ 500 g d−1||↓ 1463 kJ d−1||↓ 3.4 kg||3 months|
| Howard et al. 2006 (30)||↑ 1.4 servings per day||↓ 1509 kJ d−1||↓ 2.2 kg|
↓ 0.8 kg
| Ortega et al. 2006 (27)||↑ 4.69 servings per day||↓ 2117 kJ d−1||↓ 2 kg||6 weeks|
| Stamler and Dolecek 1997 (31)||↑2.2–4.4% EI from F|
↑1.0–2.0% EI from V
|↓2135–2736 kJ d−1||↓ 3.0 lb||6 years|
|Did not detect expected relationship|| || || || |
| Ely et al. 2008 (35)||NSh change||Not reported.||↓ 9.4 lb||6 months|
| Whybrow et al. 2006 (34)||↑229 g d−1|
↑395 g d−1
|↑400 kJ d−1|
↑ 659 kJ d−1
|NSh change.||8 weeks|
| John et al. 2002 (33)||↑1.5 servings per day||Not reported.||0.6 kg(ns)h||6 months|
Mechanisms of effect: fibre
No adult experimental studies examined whether the expected relationship was mediated by fibre intake or whether relationships between fibre consumption and weight loss were mediated by changes in hunger or satiety. However, three studies found participants who achieved significant weight loss also significantly increased both FV and fibre consumption (27,28,31). Two studies found increased fibre intake was significantly related to weight loss (29,30); another showed the intervention led to simultaneous significant increases in FV and fibre and decreases in EI and weight, which was predicted by FV intake and satiety ratings (26). No adult experimental studies instructed participants to increase fibre intake specifically; but two instructed participants to increase whole-grain consumption (30,31).
Adult longitudinal studies
Three of the seven longitudinal studies found the expected inverse association (2,3,37). Three obtained mixed results (i) an expected association between adiposity and V (but not F) among women, and neither (FV) among men (38); (ii) an expected association between adiposity and F (but not V) consumption (39) and (iii) an expected association with FV consumption among women, but not men (40). One study found no relationship between FV consumption and adiposity (41) (Table S2).
Studies that found the expected association or mixed results tended to be stronger than the one not showing the expected relationship. For example, studies that found the expected inverse association, including those with mixed results, followed participants over longer periods of time (10 years (2,37); 12 years (3); 24 years (38); 6 years (39); 5 years (40)) than the one not showing an association (6 months (41)); and quantified FV intake with more detailed measures (i.e. quartiles of FV intake [g d–1](2,38), quintiles of FV consumption [servings per day](37), change in FV intake [servings per day](3) and EI [MJ d–1 from FV sources of carbohydrate](40)). The study not finding the expected association (41) placed participants in ‘adequate’ FV intake (≥11 servings of FV per week) or ‘low’ FV intake (<11 servings of FV per week (41)) categories based on a food frequency questionnaire (FFQ). This cut-off may not have been sensitive enough to detect an effect or there may have been substantial misclassifiction error from a FFQ (42). Simplistic classification may have also been a problem for one study with mixed results (39) which also quantified FV intake with a single-item categorical variable (i.e. whether participants believed they consumed the same, less or more FV daily at follow-up than at baseline). Studies showing the expected association tended to use larger samples (74 063 (3); 79 236 (37); 206 (2)) than those with mixed (42 696 (40); 248 (39); 168 (38)) or null results (193 (41)). Three studies showing the expected relationship (2,3,37) and two out of three showing mixed results (39,40) recruited participants with greater adiposity (average baseline BMI ≥ 25) than the one not showing the expected relationship (average baseline BMI = 23 (41)). One study showing mixed results recruited participants as adolescents (age 13 years at baseline without age and gender adjusted BMI provided) (38) and followed them for 24 years. The three studies showing the expected association (2,3,37) and two with mixed results (38,40) controlled for more possible confounding variables (i.e. gender, age, EI and PA) than the study not showing an association (i.e. only controlled for gender and age (41)) or the other with mixed results (i.e. only controlled for age (39)).
There were no differences between studies that found the expected inverse association and those that did not on measures of adiposity or diet (all but one (39) used FFQ). However, measurement limitations were common to all longitudinal adult studies. For example, only two studies calculated BMI from height and weight measured by trained technicians (one found the expected association (2) and one did not (41)); the rest used self-reported height and weight (3,37–40). Only two studies controlled for dietary reporting error (3,40): one that found the expected relationship eliminated outliers (i.e. extreme values) (3), and one with mixed results adjusted for over-reporting at the individual level using global food group summary questions (40). Most adult longitudinal studies (4/7) did not provide the list of foods classified as FV (3,37–39). One that found the expected association (2) and one that found mixed results (40) did not include potatoes and the one that found mixed results included juice and nuts (40).
Child experimental studies
One child experimental study did not find the expected inverse association (43) (Tables 2 and S3). This study (among normal weight school-age children with at least one overweight/obese parent) tested an intervention aimed at modifying multiple behaviours to decrease energy-dense food consumption without specific direction to lower EI or induce negative energy balance (43). The intervention failed to increase children's FV intake to five servings per day (43) (Table 2), which minimized the ability to test the relationship of interest.
Table 2. Estimated change in FVa intake (child experimental studies)
|Epstein et al. 2001 (43)||Not reported||I1e↑0.72 ± 1.1|
I2f↓0.55 ± 1.3
|NSg↓ in % OWh change in both groups||1 year|
The child experimental study was of relatively long duration (i.e. 12 months), and tested a high-intensity intervention involving eight weekly 1.5-h counselling sessions, four biweekly sessions and two monthly meetings over 1 year (43). This study laudably used random assignment to groups, used height and weight measured by trained technician, and defined adiposity as BMI %ile using Center for Disease Control and Prevention reference growth curves (43), but did not discuss power. Diet was assessed by FFQ (self-reported by children 6–11 years of age) (43). Unfortunately, EI, EE and social desirability of response were not controlled (43).
Children's longitudinal studies
Of the four children's longitudinal studies, only one found the expected inverse association between FV intake and adiposity (44). Specifically, overweight children (6–13 years of age living in China) with high FV consumption at baseline were less likely to remain overweight at 2-years follow-up than overweight children who had a lower FV diet at baseline (44). One longitudinal study among children and adolescents ages 9–14 years in the USA found mixed results: an inverse association between V intake and BMI z-score change among boys, not girls (45), when EI was not controlled. However, when EI was statistically controlled, this association did not remain, suggesting change in EI accounted for the observed effect (45). Alternatively, when EI was controlled among girls and boys, F consumption was positively associated with BMI z-score change suggesting EI was suppressing this relationship, and F was influencing BMI z-score by other than caloric means (45). The remaining two studies were conducted in the USA among low-income pre-school children (46,47). One found no relationship between FV consumption and adiposity (46) and one found a positive relationship between V consumption and adiposity (47) (Table S4).
Few patterns suggestive of research quality appeared to differentiate the studies finding vs. not finding the expected relationship. The basis for this statement follows. Two longitudinal studies that found expected (44) or mixed results (45) recruited elementary to middle school age children (9–14-year-olds (45); 6–13-year-olds (44)), while two that found no or a positive association recruited pre-school age children (1–5-year-old (46); 2–5-year-old (47)). The single study that found the expected association used 24-h recall to assess diet (44); while those not finding the expected association used FFQ (45–47). One study not finding a significant inverse association did not include juice, carrots, potatoes and salad (46); the other three did not report FV classification. Across all child longitudinal studies, the lengths of follow-up were shorter than those among adults: 2 years for the study showing the expected association (44), 3 years for the study with mixed results (45) and 6 months (47) to 2 years (46) for the null studies. No consistent differences were found among studies with regard to methods used to assess height and weight; methods to operationalize adiposity; adjustment for misreporting of EI; control for possible confounding variables; or sample size.
Most experimental and longitudinal studies among adults found either the expected inverse relationship between FV consumption and adiposity (8/11 experimental and 3/7 longitudinal studies) or mixed results (3/7 longitudinal studies). However, in these studies, especially the experimental studies, it was unclear whether this relationship was due to higher FV consumption alone or multiple behaviour changes (e.g. lower intake of energy-dense/low-nutrient foods, changes in PA) including higher FV consumption aimed at negative energy balance among overweight and obese individuals. The only experimental study and two longitudinal studies among children did not find the expected association; however, one of the children's longitudinal studies found the expected relationship, and one found mixed results.
Evidence for association between FV intake and adiposity and possible mechanisms: adults
The patterns found among adult experimental studies suggest the relationship between FV intake and adiposity was due to multiple weight-related behaviours. The patterns found among adult longitudinal studies suggest the association between FV consumption and adiposity is weak.
The relationship between increased FV consumption and decreased adiposity may have been accounted for by reduced consumption of energy-dense foods. In the adult experimental studies showing the expected relationship, weight loss occurred when EI was reduced in conjunction with increased FV consumption and decreased energy-dense food consumption (26–31). When FV consumption increased without change in EI, weight loss did not occur (33,34). Of the four studies that instructed participants to increase FV consumption only, without instruction to lower EI or energy-dense foods (4,28,33,34), EI increased in two studies as FV were added to participants' usual diets (33,34) and decreased in two studies as FV possibly displaced energy-dense foods in the diet (4,28). In one adult longitudinal study those who increased F consumption had less increase in EI over time, suggesting increases in FV intake without intentional EI or energy-dense food restraint may have a weak displacement effect on energy-dense foods (39). Unfortunately, no studies tested whether EI mediated the change relationship directly. Some adult experimental studies also showed increases in fibre intake (without instruction to do so) co-occurred with increases in FV consumption and losses in weight. Taken together, FV consumption may lead to weight loss or lower weight gain as part of a larger dietary change pattern that includes increases in fibre content and/or lowers energy density of the diet.
Evidence for association between FV intake and adiposity and possible mechanisms: children
The experimental study conducted among normal weight children did not show the expected association between FV consumption and adiposity. Despite using a relatively long (1-year (43)) and intensive (8-weekly, 4-biweekly, and 2-monthly 1.5-h counselling sessions) (43) intervention, FV dietary outcome goals were not achieved. Lack of change in percent overweight possibly reflected an inability of the intervention to influence FV consumption rather than a lack of relationship between increased FV consumption and adiposity.
The two children's longitudinal studies that found the expected inverse relationship or mixed results, recruited older children (elementary age) than those that found no association (pre-school children). Compared with the adult longitudinal studies, the children's longitudinal studies, showing the expected relationship (i) had smaller sample sizes (95 (44), 1379 (47), 971 (46) and >10 000 (45) for one that had mixed results); (ii) did not control for EE (except one (45) that found mixed results) and (iii) had shorter durations of follow-up (≤3 years). Because the association between FV consumption and adiposity is likely weak, these study limitations may have prevented the association from emerging. The relationship of FV intake to adiposity among children remains unclear.
Effects primarily among overweight/obese participants: adults and children
The expected inverse relationship between FV intake and adiposity in both experimental interventions and longitudinal studies was obtained mainly among overweight adults and children. It is possible that those with higher weight at baseline were more highly motivated to reduce weight. Alternatively, higher weight might have allowed more room for change, or diet changes may have had a greater impact with higher weight participants. Whether these differences were due to biological, dietary or motivational factors remains to be explored.
Implications for future research
Research assessing the relationship between FV intake and adiposity in cross-sectional or even longitudinal studies is not likely on its own to shed more light on this relationship. Future research requires better measures of dietary intake with clearer and more consistent FV classification systems; controls for other aspects of diet and EE; explanatory mechanism mediation tests; technician measurement of body composition; larger samples followed over longer durations and clear specification of intervention procedures (i.e. theory-based methods, contact frequency and duration). Intensive dietary counselling will likely be needed to obtain weight loss or maintain healthy weight. There is inadequate evidence to support targeting FV intake alone among children for obesity prevention. With regard to treatment of overweight, future research should target overweight or obese children, older than pre-school age, with intensive long duration interventions to elucidate potential effects of increased FV intake and mechanisms for effects.
Only published English-language studies were reviewed; there may be unpublished studies or studies published in other languages that tested the FV to adiposity relationship.
Conclusions and implications for clinical and public health practice
Higher levels of FV intake were weakly associated with weight loss among overweight or obese adults (but not children) in multi-component experimental studies that promoted several behaviours to induce negative energy balance, including increased FV consumption. In longitudinal studies, high FV consumption was associated with less or slower weight gain over lengthy time intervals among adults, but to a lesser degree among children. More, high-quality research is needed to clarify the mechanisms of effect and identify the types, durations and intensities of procedures needed to produce these dietary changes.
Conflict of Interest Statement
No conflict of interest was declared.
The completion of this review was incentivized by the Robert Wood Johnson Foundation. In addition, this work is a publication of the USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine and Texas Children's Hospital, Houston, Texas. This project has been funded in part by federal funds from the USDA/ARS under Cooperative Agreement 58-6250-6001. The contents of this publication do not necessarily reflect the views or policies of the USDA nor mention of trade names, commercial products or organizations imply endorsement by the US Government. We appreciate the comments of Theresa Nicklas, DrP.H., R.D. on an earlier draft of this manuscript.