Results
The study flow diagram of the literature search and screening as per
Preferred Reporting Items for Systematic Reviews and Meta-Analyses
(PRISMA) is showed in Figure 1. A total of 2331 journals were identified
when using keywords and various databases (Pubmed, Cochrane, Embase) to
perform the literature search. In addition, 8 other journals were
identified through a reference list of other related journals. After
duplicates were removed, 2312 records were left to screen and we
identified 2060 non-related journals that were not clinical trials,
randomized controlled trials of humans and animals, unavailable full
text, or other vegetable oils that were not relevant. The remaining 72
full-text articles were further accessed for eligibility through reading
and 41 of them were removed due to the absence of control groups and
unavailable data. In the end, a total of 31 articles fulfilled our
criteria and was included in the meta-analysis. In this review, we
observed 5 health complications which were obesity, atherosclerosis,
hypertension, diabetes, and oxidative stress from consuming heated palm
oil (HPO), heated corn oil (HCO), heated canola oil (HCNO), heated extra
virgin olive oil (HEVOO), heated sunflower oil (HSFO), and heated
soybean oil (HSBO). A total of 8 human studies and 23 in vivo studies
involving rats and rabbits were included in this meta-analysis. For
obesity, data were taken from either their weight gain, BMI
(kg/m2), and adipose tissue; meanwhile, for
atherosclerosis, we extracted data from low-density lipoprotein (LDL)
cholesterol and thickness of the artery wall as it is caused by an
abnormally high concentration of LDL‐cholesterol that deposits on the
artery. Data of vasodilation and blood pressure were taken for
hypertension while oxidative stress includes oxidative markers such as
liver thiobarbituric acid reactive substances (TBARS), malondialdehyde
(MDA), and the ratio of glutathione to oxidized glutathione (GSH/GSSG);
and lastly, for diabetes, data of homeostatic model assessment of
insulin resistance (HOMA-IR) was used. Table 1 shows the study design
and baseline characteristics of these 31 studies.
Figure 2 shows the comparison between heated (inclusive of multiple
reheating) palm oil and control on atherosclerosis, hypertension, and
obesity only. Seven parts of data (from five reported journals)
(Famurewa et al., 2017; Kamisah et al., 2016; Kamisah et al., 2015;
Nkanu et al., 2018; Siti et al., 2017) were statistically significant at
the study level and did not overlap the line of null effect. The results
were also statistically significant at the meta-analysis level (P
< 0.00001), therefore there were no health complications with
the HPO consumption. This set of studies showed a high heterogeneity
(I2 = 100%) with the random-effects model as the
studies had different study designs. The overall mean difference was
0.18 (95% CI: 0.11, 0.25; P < 0.00001). Figure 3 shows the
effect of HCNO on obesity and both studies (Bautista et al. 2014a and
Bautista et al. 2014b) showed statistically significant and the overall
effect estimate was statistically significant (P < 0.00001)
which meant there was no adverse effect of HCNO on human health. The
heterogeneity of this forest plot was high (I2 = 95%)
and the total mean difference was 5.50 [95% CI: 4.12, 6.87]. Figure
4 shows the comparison between HCO and control of hypertension. The
total mean difference was 0.62
[95% CI: -4.71, 5.95] with P = 0.82 (not significant). This set of
studies had a low heterogeneity (I2 = 0%) as they
were of the same paper. The comparison between HEVOO and control can be
seen in Figure 5. Four studies (Battino et al., 2002; Rueda-Clausen et
al., 2007; Sayon-Orea et al., 2013) which were Battino et al. 2002,
Rueda-Clausen et al. 2005a; 2005band Sayon-Orea et al. 2013, respectively were statistically significant
on the right side of the forest plot. The overall effect was also
statistically significant (P = 0.003) which meant HEVOO was not cause
any adverse effects mentioned above. The mean difference was 1.13
[95% CI: 0.39, 1.87] with a relatively high heterogeneity of
I2 = 96%. Figure 6 shows a comparison of HSFO and
control of obesity and oxidative stress. Three out of five studies
(Aruna et al., 2005; Garrido-Polonio et al., 2004; Srivastava e al.,
2010) were statistically significant which were Aruna et al. 2005,
Garrido-Polonio et al. 2004a, and Srivastava et al. 2010. The overall
effect estimate was statistically significant (P < 0.00001) on
the left side of the forest plot which meant the effects of consuming
HSFO include obesity and oxidative stress. This analysis had a
relatively high heterogeneity (I2 = 99%) and the mean
difference is -3.79 [95% CI: -5.61, -1.97]. The comparison between
HSBO and control is shown in Figure 7 above with 9 statistically
significant studies including (Rueda-Clausen et al., 2007, Das et al.,
2017; Leong et al., 2010; Yen et al., 2010) Adam et al. 2009b, Awney
2001, Chiang et al. 2011, Chuang et al. 2013, Leong et al. 2010b; 2010c;
2010d, Rueda-Clausen et al. 2005a and Yen et al. 2010. The heterogeneity
was relatively high for this set as well (I2 = 98%)
as they included various study designs and intervention method and the
mean difference was 0.46 [95% CI:0.29, 0.64]. However, the overall
effect was statistically significant (P < 0.00001) which means
HSBO will not cause any health implications.