Effects of Low-fat Diet on Serum Lipids in Women

Abstract and Introduction

Abstract

Objective. This study aims to conduct a meta-analysis to evaluate the effects of a low-fat diet, in comparison with participants’ usual diet, on serum lipids in premenopausal and postmenopausal women.

Methods. Ten electronic databases were searched for relevant articles reporting randomized controlled trials through August 31, 2012, including PubMed/Medline, EMBASE, Cochrane Library, CINAHL, Web of Science, SCOPUS, ProQuest Dissertations and Theses, SinoMed, CNKI (Chinese database), and WanFang (Chinese database). This systematic review and meta-analysis, which evaluated the effects of a low-fat diet, in comparison with the participants’ usual diet, was conducted according to the guidelines of the Cochrane Handbook for Systematic Reviews of Interventions. All analyses were performed using RevMan version 5 (Cochrane Collaboration).

Results. From 512 potentially relevant publication citations reviewed, 8 randomized clinical trials were included in the meta-analysis, representing 22 groups (11 intervention groups and 11 control groups). A total of 1,536 women (900 in the intervention group and 636 in the control group) met the inclusion criteria. Total cholesterol (TC), highdensity lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C) values changed more favorably in participants randomized to low-fat diets than in participants randomized to their usual diets. Low-fat diet was found to induce significant reductions in TC (random-effects model: mean difference [MD], −0.49 mmol/L; 95% CI, −0.69 to −0.29; I = 42%; P_effect < 0.00001), HDL-C (MD, −0.12 mmol/L; 95% CI, −0.20 to −0.05; I = 49%; P_effect = 0.00006), and LDL-C (MD, −0.24 mmol/L; 95% CI, −0.38 to −0.09; I = 42%; P_effect = 0.001) for two groups. For subgroup analysis, low-fat diet was efficacious in reducing TC, HDL-C, and LDL-C in premenopausal women but did not significantly reduce the same outcomes in postmenopausal women. However, there were also no statistically significant differences in triglycerides (TG) and TC-to-HDL-C ratio between a low-fat diet and the participants’ usual diet (TG: MD, 0.04 mmol/L; 95% CI, −0.02 to 0.11; I = 0%; P_effect= 0.16;TC-to-HDL-C ratio: MD, 0.08 mmol/L; 95%, CI −0.21 to 0.36; I = 0%; P_effect = 0.59) in two groups.

Conclusions. Overall results suggest that a low-fat diet is efficacious in reducing the concentrations of TC, HDL-C, and LDL-C but not in reducing TG and TC-to-HDL-C ratio in women. A low-fat diet is efficacious in reducing TC, HDL-C, and LDL-C in premenopausal women. Additional studies are needed to further address its effects on postmenopausal women.

Introduction

Cardiovascular diseases (CVDs) are the leading cause of deaths globally. Every year, more people die of CVDs than of any other cause. According to the World Health Organization, an estimated 17.3 million people died of CVDs in 2008, representing 30% of all global deaths, followed by infectious and parasitic diseases (15.3%) and malignant neoplasms (13.3%). By 2030, almost 25 million people will die of CVDs—mainly heart disease and stroke—annually.

Women experience a lower rate of CVDs before menopause in comparison with men. The incidence and severity of CVDs among premenopausal women are lower than among men of comparable age, even after correction for various risk factors. The cause of these differences is not clear. Before 2000, estrogen seemed to be the obvious candidate to explain these differences. Accumulating evidence indicates the cardiovascular-protective effects of estrogens. But in the past decade, a number of studies have been unable to prove a direct causal effect. Regardless, young or premenopausal women are protected from CVDs by cardioprotective mechanisms that may include estrogens. Mounting evidence indicates that, as women age and enter menopause, they undergo significant changes in cardiovascular risk factors.

One of the major risk factors for CVDs in women is lipid profile. Reviews on the effects of lipid level alterations on cardiovascular morbidity and mortality suggested that changes in lipids affect cardiovascular risk. A decrease in estrogen levels in postmenopausal women is associated with unfavorable changes in lipid profiles, including total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and TC-to-HDL-C ratio. Women experience higher blood cholesterol levels than men after their fifth decade of life and decrease their levels of HDL-C after menopause. In women, HDL-C is more closely related to CVDs than is LDL-C.

There are several approaches to improving serum lipid. Although hyperlipidemia is often treated with cholesterol-lowering drugs such as statins, these drugs are not only associated with certain risks such as liver damage and myalgias but are also costly.

Diet is another approach to improving women’s lipid profiles. A woman’s diet can play a key role in the development of many CVD risks, including dyslipidemia, obesity, and hypertension. Low-fat diets are defined as diets aiming at energy intake with 30% or less of energy derived from fat (saturated fat, unsaturated fat, or both). A low-fat diet is a simple low-cost lifestyle change that has been recommended for improving lipid concentrations in women.

Randomized controlled trials (RCTs) investigating the impact of low-fat diet on women have led to conflicting findings because of differing populations or dietary fat levels. Furthermore, existing meta-analyses of low-fat diets have not analyzed outcomes based on sex because many studies did not report separate data for men and women. Based on the existing Cochrane Review, lower dietary fat levels reduced TC and LDL-C in adults but had no significant effects on HDL-C and TG. A meta-analysis of aerobic exercise and prudent diet resulted in improved selected lipids (TC, TC-to-HDL-C ratio, LDL-C, and TG) but not HDL-C concentrations in adults. A prudent diet is defined as a diet in which saturated/trans-fat intake constitutes less than approximately 10% of the total calories, cholesterol is less than 300 mg/day, and/or fiber intake is 25 g/day or more in women and 35 g/day or more in men. Furthermore, meta-analyses of the effects of low-fat diet on serum lipids have focused on women, but a subgroup analysis of premenopausal and/or postmenopausal women is needed.

This study aims to use the aggregate-data meta-analytic approach to evaluate the effects of a low-fat diet, in comparison with participants’ usual diet, on serum lipids in women reported in RCTs. Another aim is to perform a subgroup analysis for the same objective in premenopausal and postmenopausal women.