1. Introduction
The influence of dietary fat and cholesterol intake on cardiovascular risk has been analyzed for years [
1]. That a general association exists is well known, and the American Heart Association (AHA) [
2] recommends to reduce the intake of saturated fatty acids (SFAs) and, instead, to increase the intake of monounsaturated and polyunsaturated fatty acids (MUFAs and PUFAs, respectively). However, a number of studies indicate a lack of a statistically significant influence, and even systematic reviews and meta-analysis prove that the influence may not be strong enough to be statistically significant. In a meta-analysis of prospective epidemiologic studies by Siri-Tarino et al. [
3], it was observed that the association between SFAs and increased risk of coronary heart disease or cardiovascular disease cannot be concluded and more data are needed. Similarly, in the systematic review by Clifton and Keogh [
4], it was reported that intake of SFA is associated with neither coronary heart disease events nor cardiovascular disease mortality. Furthermore, in the systematic review and meta-analysis of observational studies by de Souza et al. [
5], it was observed that intake of SFAs was not associated with coronary heart disease, cardiovascular disease, and all-cause mortality.
The indicated lack of a significant association is attributed to the fact that when reducing intake of fats or SFAs, patients simultaneously change their general dietary habits and these should be also monitored [
4]. However, the lack of association is attributed primarily to the methodological limitations related to a lack of sufficient numbers of studies conducted [
3] and, as a result, the heterogeneous groups analyzed and heterogeneous studies included in the analysis [
5]. Taking this into account, it may be surmised that the effect of fat intake on cardiovascular risk may differ in various population groups, whereas specific groups at increased risk should be especially taken into account.
At the same time, a quantitative meta-analysis of metabolic ward studies by Clarke et al. [
6] indicated that replacing 60% of SFAs by other fats and removing 60% of cholesterol from the diet reduces the blood cholesterol level by 10–15% and, especially, reduces low-density lipoprotein (LDL) cholesterol blood levels (80% of reduction). However, despite this conclusion, the other observations often did not prove the association in practice in specific groups of patients. In particular, in a group of overweight and obese adults, a recent systematic review and meta-analysis of randomized control trials by Hannon et al. [
7] indicated that, in eight of the studies that were included and conducted in a group of 663 patients, there was no significant influence of the replacement of SFAs by unsaturated fatty acids on the correction of the blood lipid profile.
Based on the indicated results, one could decide about the necessity to abandon recommending changes of dietary fat intake, as there is a little evidence that it may benefit lipid profiles in this population and there is no evidence that it may reduce cardiovascular risk. However, as previously indicated, the heterogeneity of studies may influence the observed results [
5]. In the systematic review and meta-analysis by Hannon et al. [
7], it was concluded that studies should be conducted for groups of various ages, ethnicities, or genders; the heterogeneous groups analyzed so far may be considered the reason for inconclusive results. Moreover, obese individuals, while compared with overweight ones, may be characterized by more serious cardiovascular disturbances, as subclinical vascular disease is commonly reported [
8], so combining overweight and obese ones may also contribute to obtaining heterogeneous groups.
The aim of the study was to analyze the associations between dietary fat intake and lipoprotein cholesterol fractions, as well as atherogenic blood properties, in young and middle-aged overweight Caucasian women. The objective was to analyze the influence of intakes of fat, SFA, MUFA, PUFA, and cholesterol, as well as the PUFA/SFA ratio, on the blood levels of total cholesterol (TC), low-density lipoprotein (LDL), high-density lipoprotein (HDL), and triglyceride, as well as the TC/HDL ratio, HDL/LDL ratio, and Atherogenic Index of Plasma (AIP).
3. Results
In the analyzed group of overweight women, no differences of dietary fat intake were observed between women from various age groups (
p > 0.05; Student’s
t-test, Mann–Whitney
U test). Moreover, neither were differences of lipoprotein cholesterol fractions and atherogenic blood properties detected between age groups (
p > 0.05; Student’s
t-test, Mann–Whitney
U test), nor were differences in the percentage of individuals characterized by the recommended level of lipoprotein cholesterol fractions observed between groups (
Table 4).
The analysis of the correlation between TC blood level and dietary fat intake in overweight women, stratified by age, is presented in
Table 5. No association between factors associated with dietary fat level and TC blood level was observed for overweight women, independent of the age group.
The analysis of the correlation between LDL cholesterol blood levels and dietary fat intake in overweight women, stratified by age, is presented in
Table 6. No association between factors associated with dietary fat and LDL cholesterol blood levels was reported for overweight women, independently of the age group.
The analysis of the correlation between HDL cholesterol blood level and dietary fat intake in overweight women, stratified by age, is presented in
Table 7. No association between factors associated with dietary fat and HDL cholesterol blood level was reported for the younger group of overweight women. At the same time, for middle-aged overweight women, the positive correlation between PUFA intake (% of energy value of diet) and HDL cholesterol blood level was statistically significant (
p = 0.049;
R = 0.3056).
The analysis of the correlation between TC/HDL blood cholesterol ratio and dietary fat intake in overweight women, stratified by age, is presented in
Table 8. No association between factors associated with dietary fat level and TC/HDL blood cholesterol ratio was identified for the younger group of overweight women. However, for middle-aged overweight women, a negative correlation between the dietary PUFA/SFA ratio and TC/HDL blood cholesterol ratio was found to be statistically significant (
p = 0.017;
R = −0.3680).
The analysis of the correlation between the HDL/LDL blood cholesterol ratio and dietary fat intake in overweight women, stratified by age, is presented in
Table 9. No association between factors associated with dietary fat level and HDL/LDL blood cholesterol ratio was observed for the younger group of overweight women. However, for middle-aged overweight women, a positive correlation between the dietary PUFA/SFA ratio and HDL/LDL blood cholesterol ratio was found to be statistically significant (
p = 0.049;
R = 0.3062).
The analysis of a correlation between triglyceride blood level and dietary fat intake in overweight women, stratified by age, is presented in
Table 10. No association between factors associated with dietary fat level and triglyceride blood level was observed for overweight women, independently of the age group.
The analysis of the correlation between AIP and dietary fat intake in overweight women, stratified by age, is presented in
Table 11. In a group of young overweight women, a positive correlation between SFA intake (g) (
p = 0.040;
R = 0.4223) and SFA intake as a percentage of the energy value of the diet (
p = 0.031;
R = 0.4417) with AIP was observed; moreover, a negative correlation between the dietary PUFA/SFA ratio and AIP (
p = 0.019;
R = −0.4757) was found to be statistically significant. However, in a group of middle-aged overweight women, a negative correlation between the dietary PUFA/SFA ratio and AIP (
p = 0.011;
R = −0.3901) was found to be statistically significant.
4. Discussion
A number of studies so far have indicated the role of blood cholesterol level monitoring in the prevention of cardiovascular diseases. Among others, it was proven in a meta-analysis of individual participant data by Paige et al. [
28] that the repeated measurements of blood cholesterol level and blood pressure are significantly associated with cardiovascular risk prediction. The meta-analysis by Robinson et al. [
29] indicated that non-HDL cholesterol blood level must be an important target in the treatment and prevention of coronary heart disease. Furthermore, a systematic review and meta-analysis by Peters et al. [
30] indicated that total blood cholesterol level is a strong risk factor for coronary heart disease.
However, not only the blood lipid profile, but also other factors may influence cardiovascular risk. Besides, there is increasing evidence that body mass is a strong independent factor. Despite BMI being indicated as a factor significantly associated with all-cause mortality, which was confirmed from a meta-analysis of individual participants’ data comprising 239 prospective studies in four continents [
31], a specific association with cardiovascular incidents and related mortality is emphasized. A general association between body mass and cardiovascular events has been reported [
32], whereas BMI assessment combined with waist circumference measurement is indicated to have significant clinical benefits in cardiovascular risk assessment [
33]. This results from the role of excessive body mass combined with abdominal adiposity in the increasing risk of heart failure, which was proven in a systematic review and dose–response meta-analysis of prospective studies by Aune et al. [
34].
Taking it into account, in overweight and obese individuals, cardiovascular events’ prevention should include not only blood cholesterol monitoring, but also body mass reduction. It is accordingly recommended by the AHA, as indicated for the prevention of cardiovascular diseases, that a BMI lower than 25 kg/m
2 should be maintained [
2]. Moreover, it must be emphasized that the analyzed overweight patients may be treated as a group characterized by increased cardiovascular risk, independently from their blood cholesterol level. Furthermore, although only lipoprotein cholesterol fractions level parameters were analyzed, various cardiovascular risks were indicated depending on the parameter that was applied—the risk was highest for total cholesterol, LDL cholesterol, and AIP levels (atherogenic potential for 60–90% of respondents in both groups), whereas the lowest risk was noted for HDL cholesterol and the HDL/LDL ratio (atherogenic potential for 5–20% of respondents in both groups).
Due to the cardiovascular risk stated in the presented group, action necessary to decrease the elevated risk should be indicated in such patients. However, when the dietary factors were considered, various roles of diet were observed in the age-dependent subgroups of overweight women. Because the association between fatty acid intake and blood lipid profile was indicated in the meta-analysis of randomized control trials by Hannon et al. [
7] as not significant in a group of excessive body mass individuals, and on the basis of the presented study conducted in a homogenous group, it may be hypothesized that such an association is significant, but only in older, and not younger, female respondents. In our study, it was observed that for middle-age overweight Caucasian female individuals, the PUFA share in the energy value of diet was positively correlated with HDL cholesterol blood level, whereas the PUFA/SFA ratio was negatively correlated with the TC/HDL ratio and positively with the HDL/LDL ratio—both proving the positive effect of increasing the proportion of PUFA in the total fat intake. However, the correlations denoted for middle-age respondents were not observed for younger respondents and were not even close to statistical significance.
The beneficial effect of PUFA fatty acids is well known, especially when their influence on the HDL cholesterol blood level is stated [
35]. Moreover, the effect of PUFA increasing the HDL level is stated independently from the lipid profile [
36]. It was observed in the study of Sanders and Hochland [
37] that a supplement of a fish oil concentrate causes an increase of the HDL cholesterol levels in healthy individuals. Similarly, in a study of Simons et al. [
38], the same observations were indicated for patients with hyperlipidaemia.
It is indicated that PUFA fatty acids have an impact on the reverse cholesterol transport mechanism, by which excessive cholesterol is transported from peripheral tissues to the liver for hepatobiliary excretion; this inhibits foam cell formation and reduces the development of atherosclerosis [
39]. Furthermore, in the meta-analysis of control trials by Mensink et al. [
40], an identical influence was observed to that in the present study because a replacement of other fatty acids by cis-PUFA had the most beneficial effect as it caused the largest decrease of the TC/HDL cholesterol ratio.
Moreover, a specific influence of n-3 PUFA supplementation on HDL cholesterol levels is in general observed [
41], but it is may be secondary to a triglyceride level decrease [
42] that is also commonly stated to be caused by n-3 PUFA intake [
43]. The indicated effect of increasing HDL level is especially strong for DHA, but not for ALA or EPA [
44]. Similarly, the DHA, but not EPA, is observed to increase HDL and LDL particle sizes [
45], but the clinical significance of this fact is stated to be unclear [
42]. However, in the presented study, such an effect of specific n-3 fatty acids was observed for none of the analyzed n-3 fatty acids, ALA, EPA, and DHA, and the influence was also not stated for the triglyceride blood level, only for total PUFA.
Nonetheless, the question arises as to why the indicated influence was not observed in the case of younger respondents and, in the present study, only for women older than 40 years was a beneficial effect of PUFA observed. It may be associated with the general decrease of PUFA blood level observed as age progresses, as was observed on the basis of observations from 160,000 patients for red blood cell membrane fatty acids [
46]. In the study of Harris et al. [
46], the influence of age was especially significant for LA for which, on the basis of the systematic review and meta-analysis of prospective cohort studies by Farvid et al. [
47], an especially important dose–response inverse association with coronary heart disease risk was confirmed. Thus, the significant influence of PUFA intake on blood cholesterol in older respondents may be attributed to a natural decrease of blood PUFA levels and a resulting higher susceptibility to the influence of dietary PUFA. As a result, with increasing age, PUFA may become more important to counteract this natural decrease and regulate the levels of lipoprotein cholesterol fractions.
The other important observation from the present study is the lack of influence of dietary fat levels on lipoprotein cholesterol fractions in younger respondents, accompanied by the influence on the calculated AIP. Because the AIP is a strong marker for predicting the risk of atherosclerosis and coronary heart disease [
48], and given its correlation with BMI [
49], its role is also important. Moreover, it is calculated as a comprehensive factor, including both triglyceride and HDL cholesterol levels; therefore, it may be considered to be a better predictor than a single lipid parameter than is, in general, indicated for comprehensive lipid indexes [
50,
51] and specifically for AIP [
52,
53].
The presented study, conducted in a population of young, Caucasian, overweight women, showed that the dietary SFA intake and SFA share in the energy value of diet were positively correlated with AIP, whereas the PUFA/SFA ratio was negatively correlated with it, indicating the potential atherogenic influence of SFA intake and a beneficial influence of PUFA that was not observed when only single lipid parameters were analyzed. In the middle-aged population of Caucasian overweight women, an identical effect of the PUFA/SFA ratio was reported, confirming the previously indicated important influence of PUFA.
However, the results observed for AIP are especially important for the younger population. Although the influence was not observable when single lipid parameters were assessed, the influence of dietary fatty acid intake and its atherogenic potential cannot be denied. Therefore, it could be recommended, especially for younger women populations, to not only assess single lipid parameters, but to also calculate the AIP to analyze the potential influence of diet, which may not be observed with single parameters alone due to the potential influence of a natural higher PUFA blood level [
46]. Such an approach may be an important aim for public health to obtain better cardiovascular risk prevention. Moreover, as for young women with no additional risk factors, the formal cardiovascular disease risk assessment may not be conducted [
15], the control of such an additional parameter may be an easy way to monitor the risk in the most efficient way. Such an approach is in agreement with the statement that tailored health education for specific communities is needed to reduce coronary heart disease risk [
54].
Despite important observations not indicated so far in other publications, the limitations of this study must be mentioned. The main limitation is the sample size; although the aim of the study was to assess diet in a homogenic group (Caucasian women; age groups: 20–40 and 40–60; BMI: 25–30 kg/m
2), the sample size was supposed to be smaller than in a more heterogeneous group of respondents. Moreover, excluding all women considered as under-reporters was the other factor that significantly reduced the sample size, as every second participant was excluded; this is typical because under-reporting is commonly noted for excessive body mass participants [
55]. At the same time, excluding all respondents considered as under-reporters must be confirmed as an important strength of the study, as the group was not only homogeneous, but also their dietary observations must be considered reliable. Moreover, the important purpose of the study was to conduct it not only in a homogenous group, but also for women (as majority of similar studies were so far conducted for the population of male respondents) and overweight individuals (as majority of conducted so far ones were performed for obese patients). Further studies should include larger groups of respondents, but the essential aim should be conducting analysis for homogenous groups.