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Journal of Endocrinology & Metabolism

Review

Volume 8, Number 4, August 2018, pages 57-61

Effects of Glycemic Index and Intake of Dietary Fiber on Serum HDL-Cholesterol Levels

High-density lipoprotein (HDL) plays a role in reverse cholesterol transport from the peripheral tissues to liver, suppressing cholesterol accumulation in the peripheral tissue. Therefore, serum low HDL-cholesterol (HDL-C) level was strongly associated with development of atherosclerotic diseases [1, 2].

We previously studied effects of glycemic index (GI) and intake of dietary fiber on serum HDL-C levels to make “Dietary Reference Intakes for Japanese 2015”, by using data obtained by clinical trials which evaluated effects of GI and intake of dietary fiber on HDL-C in Asian populations [3]. We found that low GI and an increased intake of dietary fiber may be beneficially associated with HDL metabolism [3]. Here we review meta-analyses on the effects of GI and intake of dietary fiber on serum HDL-C levels, to make “Dietary Reference Intake for Japanese 2020”.

To make “Dietary Reference Intake for Japanese 2020”, we searched meta-analyses of randomized controlled trials (RCTs). A search was conducted by using PubMed, Embase and Google Scholar, with the following keywords: glycemic index and HDL and meta-analysis or dietary fiber and HDL and meta-analysis. The search period was comprised up to May 2018.

Meta-analyses evaluated effects of GI on HDL-C were shown in Table 1. We found five meta-analyses which studied effects of GI on HDL-C [4-8]. All meta-analyses denied a significant effect of GI on HDL-C. However, three meta-analyses showed that compared with high GI, low GI reduced total cholesterol (TC) and low-density lipoprotein (LDL)-C [5-7]. Only one meta-analysis showed that low GI significantly reduced triglyceride (TG) [4]; however, other four studies challenged a significant effect of low GI on TG.

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Table 1. Meta-Analyses Evaluated Effects of Glycemic Index on HDL-C

Meta-analyses evaluated effects of dietary fiber intake on HDL-C were shown in Table 2. Eleven meta-analyses were eligible. In three meta-analyses, data on effects of dietary fiber intake on HDL-C were not available [9-11]. Seven meta-analyses reported that intake of dietary fiber is not significantly associated with changes in HDL-C. Two meta-analyses described that HDL-C was significantly reduced, but only by a small amount [12, 18].

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Table 2. Meta-Analyses Evaluated Effects of Dietary Fiber Intake on HDL-C

Regarding other serum lipids, all meta-analyses demonstrated a significant reduction of LDL-C due to dietary fiber intake [9-19]. Three meta-analyses showed a significant reduction of non-HDL-C by intake of dietary fiber [9-11]. Only two meta-analyses reported that dietary fiber intake reduced serum TG [13, 17]; however, six studies challenged a significant influence of dietary fiber intake on serum TG levels [12, 14-16, 19].

To make “Dietary Reference Intakes for Japanese 2015”, we mainly adopted clinical studies performed in Japanese or Asian populations. Regarding effects of GI on HDL-C, we adopted and used three cross-sectional studies performed in Korea and Japan. Odds ratios for having low HDL-C in the highest quintile was 1.54 (CI: 1.17 - 2.03) for glycemic load (GL) which is calculated indirectly as GI times the weight of available carbohydrate, in Korean men as compared with the second quintile as a reference [20]. In the cross-sectional study on the association between dietary GI and GL and metabolic risk factors in healthy Japanese women (n = 1,354), dietary GL was independently negatively correlated with HDL-C [21]. In the study investigating the correlation between dietary GI, GL and cardiovascular risk factors in 32 Japanese women aged 52.5 ± 7.2 years old, the highest concentration of HDL-C was observed in the lowest GI tertile (P < 0.01), and the highest concentration of HDL-C was observed in the lowest GL tertile (P < 0.05) [22].

However, to make “Dietary Reference Intakes for Japanese 2020”, we mainly adopted meta-analyses which have high evidence level. In spite of the existence of a significant association between low GI and reduction of LDL-C, the present study failed to show a significant effect of GI on HDL-C.

Several clinical trials to study effects of dietary fiber on HDL-C were performed in Asian populations. Zhang et al investigated the impact of oat consumption on cholesterol levels in Chinese adults with mild-to-moderate hypercholesterolemia [23]. Dietary fiber intake increased significantly in the oat group compared to the control group. HDL-C decreased significantly in the control group versus the oat group. Singh et al performed a RCT to examine the effects of a soluble fiber and a potassium-rich diet containing daily 0.5 - 1.0 kg of guava intake in patients with essential hypertension [24]. In their study, increased intake of soluble dietary fiber was associated with an insignificant increase in HDL-C (4.6%) [24]. In the study performed in China, a total of 110 elderly people with hyperlipidemia were randomly assigned to the experimental group who consumed an ordinary diet plus foods containing refined konjac meal, and the control group who consumed only the ordinary diet for 45 days [25]. At the end of the trial, HDL-C significantly elevated (P < 0.01) in the experimental group.

The present study showed a significant association between dietary fiber intake and reduction of LDL-C; however, it did not demonstrate a significant influence on HDL-C.

In spite of significant associations of low GI and dietary fiber intake with reduction of LDL-C, we could not observe any significant influences of low GI and dietary fiber intake on HDL metabolism.

Conflict of Interest

The authors declare that they have no competing interests.

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