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Does the Amount of Carbohydrate in the Diet Affect Metabolic Syndrome Risk?

Whether the carbohydrate content of the diet affects metabolic risk or not is hotly debated. A recent systematic review and meta-analysis sheds some light on this topic. In this research commentary, Cliff Harvey PhD delves into this study and its implications.

Study Highlights:

  • Carbohydrate intake is associated with more likelihood of having metabolic syndrome.
  • Linear dose-response relationships were found, with a 2.6% increase in the risk of metabolic syndrome per 5% energy intake from carbohydrates.
  • Association between carbohydrates intake and risk of metabolic syndrome appears to be affected by region.

Research Commentary: Carbohydrate intake and risk of metabolic syndrome: a dose-response meta-analysis from observational studies

Ya-shu Liu, Qi-Jun Wu, Yang Xia, Jia-Yu Zhang, Yu-Ting Jiang, Qing Chang, Yu-Hong Zhao

Nutrition, Metabolism and Cardiovascular Diseases

https://doi.org/10.1016/j.numecd.2019.09.003

Abstract

Background and Aims

Epidemiological association studies have shown inconsistent findings between carbohydrate intake and the risk of metabolic syndrome. Therefore, we aim to conduct the first dose-response meta-analysis to investigate this effect.

Methods and Results

A systematic search in PubMed and Web of Science databases throughout June 01, 2019, together with relevant literature scrutiny, was performed to identify related studies for inclusion into the study. We calculated the odds ratios (ORs) with 95% confidence intervals (CIs) using a random effect model. Furthermore, subgroup, sensitivity, heterogeneity, and publication bias analyses were performed. This meta-analysis included 14 cross-sectional and four cohort studies, totalling 284,638 participants and 69,554 metabolic syndrome cases. The highest versus the lowest carbohydrate intake values were associated with increased risk of met. syndrome (OR: 1.253, 95% CI: 1.147-1.368), with moderate heterogeneity (I2=54.5%). Using dose-response analysis, we found a linear association between carbohydrate consumption and met. syndrome risk with a corresponding OR of 1.026 (95% CI, 1.004-1.048) and with significant heterogeneity (I2=82.0%) at 5% energy from carbohydrate intake. We have found similar results using subgroup analyses for major study characteristics and adjustment for confounders. Sensitivity analysis further enhanced the robustness of the results, and no publication bias was detected.

Conclusion

Carbohydrate intake is associated with an increased risk of developing metabolic syndrome. Therefore, additional large prospective cohort studies are warranted to confirm our findings.1

Comment

In first reading the title of this study, you might be forgiven for thinking, ‘yeah, no shit…’ but this is important and informative to the scientific debate around carbs vs fat and protein.

In first reading the title of this study, you might be forgiven for thinking, ‘yeah, no shit…’

As a systematic review and meta-analysis, it combines many studies featuring hundreds of thousands of participants, and thus, has significant strength to show robust associations between dietary inputs and health outcomes. But, like all observational data, it does suffer from shortcomings as there was no intervention being compared to a control or placebo, and no control over what the participants did and didn’t do. This means that there is a large potential for confounding and other lifestyle factors could influence the findings. For example, those who eat the most carbohydrate might be consuming sugary drinks, or other processed foods that happen to be higher in carbohydrate content (but are comparatively nutrient-poor and are independently associated with poor health outcomes.) It is also possible that those choosing to eat higher carbohydrate diets have poorer health habits and might smoke and drink more, or exercise less. It would be unlikely if this were the case though. Typically, those with healthier habits have been more inclined towards a higher carbohydrate intake because that has been the prevailing dietary guidance.

Typically, those with healthier habits have been more inclined towards a higher carbohydrate intake because that has been the prevailing dietary guidance

In fact, up until very recently, dietary guidelines specifically for those with diabetes and metabolic syndrome were high-fat, low-carbohydrate diets. So, we most often see in cohort studies, that those who tend to eat fewer carbohydrates (and more meat in particular) are those who are change-resistant, and they concomitantly have higher rates of smoking, drinking, and obesity.

[See this previous issue of CARR]

So, the findings of this study seem to be stronger than those that suggest we should reduce meat for very small improvements in risk (<10%).

In this research, the highest vs lowest carbohydrate intakes resulted in a 25% increased risk of metabolic syndrome and ~3% increase in risk per 5% increase in total energy from carbohydrate (i.e. moving from a 30 to 35% carbohydrate diet).

Overall, the  ‘state of the nation’ remains generally the same though. Excessive amounts of processed and refined foods (which are generally those high in sugar and refined carbohydrate) are the factor most associated with worse health outcomes. Eating unrefined foods, on the other hand, is the proven way to reduce disease risk.

References

1.         Liu Y-S, Wu Q-J, Xia Y, Zhang J-Y, Jiang Y-T, Chang Q, et al. Carbohydrate intake and risk of metabolic syndrome: A dose&#x2013;response meta-analysis of observational studies. Nutrition, Metabolism and Cardiovascular Diseases. 2019;29(12):1288-98.

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