The present aggregate analyses of 13 trials in 260 predominantly young, male participants, who were overweight/obese or otherwise healthy, investigated the effect of fructose on markers of NAFLD under two different types of trial conditions: one where fructose in beverage form was isocalorically exchanged for other carbohydrates and the other where fructose in beverage form supplemented control diets with excess energy (+21–35% energy) at extreme doses (104–220 g/day) relative to the same control diets without the excess energy. These two types of trial conditions produced different results. Although there was no effect of fructose in isocaloric trials, fructose increased both IHCL and ALT in hypercaloric trials.

Relation of findings to other lines of evidence

Our finding of a lack of effect of fructose on NAFLD markers in isocaloric trials contradicts evidence from animal models and observational studies. The ability of fructose to induce a metabolic syndrome phenotype and NAFLD is thought to lie in its ability to act as an unregulated substrate for de novo lipogenesis, bypassing the major rate-limiting step of glycolysis at phosphofructokinase.7, 27 This mechanism contributes significantly to de novo lipogenesis in rodent models, in which fructose fed at supraphysiological doses under isocaloric (∼60% energy) or hypercaloric (+30% excess energy) conditions induces steatosis and steatohepatitis.8, 9, 10, 11 Small cross-sectional and retrospective case–control studies have also shown an association between fructose-containing sugar intake and NAFLD.12, 13, 14 Clinical translation of these data, however, has several limitations. Rodent models are complicated by supraphysiological doses and excess energy,28 and marked differences exist in the metabolic fate of fructose between animals and humans. Although de novo lipogenesis from fructose accounts for 60–70% of fatty acids in rodents,28 its contribution in humans is quantitatively insignificant.29, 30 Two carefully conducted reviews of the available isotopic tracer studies showed that de novo lipogenesis from fructose contributes <1% of fatty acids, whereas glucose (∼50%), lactate (∼25%) and glycogenesis (>15%) synthesis remain the major pathways of hepatic fructose disposal in humans.29, 30 Cross-sectional and retrospective case–control studies do not provide evidence of causation and have found positive associations with many other factors that might be equal or better predictors of NAFLD, such as increased intake of energy, total fat, total carbohydrate, animal protein, cholesterol and the n-6:n-3 ratio of polyunsaturated fatty acids and decreased intake of dietary fiber.6 No large prospective observational studies have evaluated the relationship between fructose and NAFLD.

Energy represents an important confounding factor in the effect of fructose. Overfeeding of a ‘fast food’ diet has been shown to relate to an increase in ALT in healthy paticipants.31 Randomized trials of energy-restricted diets focusing on total energy reduction and exercise to promote weight loss have also shown reversal of NAFLD markers in people with NAFLD.6, 32 In the present analyses, we observed increases in IHCL and ALT only in hypercaloric trials. The lack of effect in the isocaloric trials was seen even under conditions of positive energy balance. Six of the isocaloric trials (three of four trials assessing IHCL19, 20, 26 and five of six trials assessing ALT19, 25, 26) used excess energy diets in both the fructose and comparator arms, so permitting the effect of fructose to be isolated from that of energy under matched, yet excess energy-feeding conditions. Restricting our analyses to these trials did not show an effect of fructose on NAFLD markers. We made similar observations for the lack of effect of fructose on both body weight33 and uric acid34 in two earlier systematic reviews and meta-analyses. These data suggest that the effect of fructose on NAFLD markers may not be different from that of other carbohydrates as long as energy remains matched.

Previous meta-analyses have identified subgroup effects on related metabolic end points. A dose threshold was observed for a triglyceride-raising effect of fructose: 100 g/day for fasting and 50 g/day for postprandial triglycerides across different participant groups35 and >60 g/day for fasting triglycerides in type 2 diabetes.36 A fasting triglyceride-raising effect of fructose was also seen where starch was the comparator and follow-up was 4 weeks in type 2 diabetes,36 whereas a weight-loss effect of fructose was seen in overweight/obese individuals and where fructose was in fruit form.33 None of these subgroup analyses were significant in the present analysis. Although the number of trials was small, the lack of effect modification across a priori subgroup analyses was consistent with that seen in our earlier meta-analyses for blood pressure37 and uric acid.34

Limitations

Our analyses have several limitations. First, the available trials had small sample sizes and narrow participant demographics. Combining the seven isocaloric and six hypercaloric trials, our median sample size was 16 participants, the majority of whom were young, male, and either overweight/obese (without any comorbidities) or otherwise healthy. Although the baseline IHCL values in the overweight/obese participants were >95th percentile for the general population (>5.56%),38 the data generated from such a generally healthy group may not be truly reflective of the disease physiology in people with or at risk for NAFLD, especially given that in patients with histologically established NAFLD, fructose may be associated with worse disease.39 Second, none of the trials in our meta-analysis had a follow-up period exceeding 10 weeks. The isocaloric and hypercaloric trials had a median follow-up of 4 weeks and 3 weeks, respectively. It is unclear whether the changes in IHCL and ALT seen in hypercaloric trials or the null effects seen in isocaloric trials are sustainable over the longer term. Third, study quality was poor (MQS<8) in 46% of the trials. Most of the low-quality scores were attributable to a lack of or poor description of randomization, nonconsecutive or poorly described patient selection and absence of blinding. However, no effect modification by study quality was seen in subgroup analyses. Fourth, none of the available trials assessed NAFLD by histological analysis of liver biopsies. This analysis remains the gold standard assessment for NAFLD, as ALT is quite insensitive, while IHCL by 1H-magnetic resonance spectroscopy cannot detect inflammation and/or fibrosis.1 The two measurements, however, showed good agreement among the trials. Finally, given the small number of available trials, publication bias remains unclear, although no small study effects were detected.

Implications

Although our results bear on the question of whether fructose-containing sugar-sweetened beverages have a unique role in the development of NAFLD, their translation to ‘real-world’ intake patterns is complicated. The median level of fructose exposure was >95th percentile U.S. intake (87 g/day)40 across all trials: 2.5-fold greater than this threshold (+215 g/day providing +35% excess energy) in the hypercaloric trials, in which there was an effect, and 1.4-fold greater than this threshold (115 g/day) in the isoclaloric trials, in which there was no effect. Also, no trials used non-beverage grain or fruit sources of fructose, which together account for >30% of fructose in the U.S. diet40 and have been linked (as whole grains and fruits) to weight loss and improved metabolic outcomes in large prospective cohort studies41, 42 and randomized trials.43, 44 Dietary trials of more representative sources of fructose at more representative levels of exposures remain a research priority.