Introduction

Garlic (Allium sativum) and onion (Allium cepa) are a rich source of several phytonutrients recognized as important elements of Mediterranean diet [11]. During the last three decades, several studies have reported beneficial effects of garlic and onion intake on prevention and treatment of selected diseases, including coronary heart disease, obesity, hypertension, hypercholesterolemia, platelet mediated thrombosis and cancer [3, 6]. Several in vitro studies and clinical trials on garlic supplementation suggested that garlic intake might protect from cardiovascular disease risk by reducing serum cholesterol concentration and blood pressure [2]. As for onion intake, to our knowledge, only two epidemiological studies considered the relation between dietary intake of onions and cardiovascular disease. A cohort study conducted in Finland, based on 473 deaths for coronary heart disease, found a relative risk of 0.74 [95% confidence interval (CI): 0.53–1.02) in men and 0.50 (95% CI: 0.30–0.82] in women for the highest quartile of onion intake, compared to the lowest one [9]. A cross-sectional analysis based on the SU.VI.MAX study and including 1,286 women and 1,005 men, found that women in the highest tertile of flavonoid-rich food consumption were at lower risk for cardiovascular disease [odds ratio (OR) = 0.31; 95% CI: 0.14–0.68], whereas a direct trend was observed in men (OR = 1.38; 95% CI: 0.96–2.00) [12]. The results were similar for various foods separately, including onions.

To provide further information on the role of dietary intake of allium vegetables the risk of coronary heart disease, we analysed the relationship between onion and garlic intake and the risk of non-fatal acute myocardial infarction (AMI), using data from a case–control study conducted in Italy.

Methods

The data derives from a case–control study of non-fatal AMI, conducted in the greater Milan area, Italy, between 1995 and 2003. Cases were 760 patients [580 men and 180 women; median age 61 (range 19–79 years)] with a first episode of non-fatal AMI, defined according to the World Health Organization criteria, admitted to a network of general hospitals in the area. Controls were 682 patients [439 men and 243 women; median age 59 (range 16–79 years)] from the same geographic area, admitted to the same hospitals as cases for a wide spectrum of acute conditions not related to known AMI risk factors nor diet. We excluded subjects with history of major cardiovascular events. Among controls, 30% had traumas, 25% non-traumatic orthopedic disorders, 18%, acute surgical conditions, 18% eye, nose, throat disorders, and 9%, miscellaneous other illnesses. Less than 5% of the cases and controls approached refused to participate.

Interviews were conducted in the hospital using a structured questionnaire, including information on socio-demographic factors, anthropometric variables, smoking, alcohol and coffee consumption, physical activity, other lifestyle habits, a problem-oriented medical history, and history of AMI in first-degree relatives. Cholesterol levels were obtained from clinical records. Information on diet referred to the 2 years preceding diagnosis and was based on a food-frequency questionnaire (FFQ), tested for reproducibility and validity [7, 8]. The FFQ included 78 foods, food groups or recipes, and allowed an estimation of energy intake. Among the items in the FFQ, two questions referred specifically to consumption of onion and garlic. For onion intake, we asked for the weekly frequency of consumption and usual portion size (small, intermediate, large), where an intermediate portion corresponded to 80 g of onion. A small portion was considered as 0.7-times an intermediate portion, and a large portion was considered as 1.3-times an intermediate portion. Frequencies of less than once per week, but at least once per month, were coded as 0.5 portions per week. For frequency of garlic use, we asked for the customary consumption as a qualitative variable, scored as 1 for non-use or low use (when garlic was used only for flavouring foods but it was not eaten), 2 for intermediate use (when garlic was used for flavouring foods and it was eaten occasionally) and 3 for high use (when garlic was used in many recipes and always eaten). No information on the type of garlic and onion consumed (fresh, powders or garlic supplements) and on manner of using was available (raw or cooked).

The ORs of AMI, and the corresponding 95% CIs, for different levels of onion intake (non-users, >0 to <1 portion per week, ≥1 portion per week) and garlic use (none or low, intermediate, high) were derived using unconditional multiple logistic regression models, including terms for age, sex, education, tobacco smoking, coffee, alcohol drinking, total energy intake, fish intake, vegetable intake, body mass index, physical activity, cholesterol levels, history of hypertension, diabetes and family history of AMI in first-degree relatives.

Results

Table 1 reports the distribution of cases of AMI and controls and the ORs and 95% CIs for subsequent levels of onion intake and garlic use. Compared with non-users, the ORs of AMI, adjusted for sex and age, for the subsequent categories of onion intake were 0.87 (95% CI: 0.67–1.13) for less than 1 portion of onion per week and 0.69 (95% CI: 0.54–0.90) for 1 or more portions per week, with a significant trend in risk. After allowance for major confounding factors, the corresponding ORs of AMI were 0.90 (95% CI: 0.69–1.21) and 0.78 (95% CI: 0.56–0.99) with a significant trend in risk. For garlic, as compared to none or low use, the ORs were 0.84 (95% CI: 0.66–1.09) for intermediate and 0.94 (95% CI: 0.68–1.32) for high use.

Table 1 Distribution of 760 cases of acute myocardial infarction (AMI) and 682 controls, with corresponding odds ratios (ORs) and 95% confidence intervals (CI), according to onion intake (portions/week) and garlic use
Full size table

Discussion

Several epidemiological studies indicated a protective effect of a variety of plant-based foods on the risk of cardiovascular disease [5]. Possible constituents in vegetables accounting for this protection are bioactive compounds, such as phenolic and organosulfur compounds. Onions are particularly rich in both, while garlic is rich in organosulfur, but not in phenolic compounds [4]. Phenolic compounds, including their subcategory, flavonoids, have antithrombotic and endothelial protective activity [10], which might explain the protective effect against coronary mortality found in several epidemiological studies [9, 10]. Consistently, the finding that onion consumption was protective on AMI risk could be explained by these biological mechanisms, as onions are one of the major sources of flavonoids in the Italian diet [14]. In the same Italian population, we found a favourable role of high intake of flavonoids, and in particular of anthocyanidins, on AMI risk [15].

We found no significant inverse relation between garlic intake and AMI risk. Several trials suggested possible small short-term benefits of garlic on the lipid and antiplatelet factors [1]. Much variability has been observed between different studies because of differences in duration of treatment with garlic, total quantity of garlic consumed, and lack of consistency when preparing garlic [1]. We collected information on garlic use by means of a score, and thus we could not estimate quantitatively the intake of garlic that, however, will certainly be lower than the dosage in clinical trials. Moreover, we have no information on the modalities of consumption, and there are important varietal differences in the composition, concentration, and beneficial activities of these bioactive compounds, for example by modalities of cooking, which could explain the inconsistent findings. We also have no information on intake of supplements, which however is uncommon in Italy.

A limitation of this study is that onion and garlic intake in Italy could be considered markers of a healthier lifestyle, which may include complex aspects of quantity and quality of diet, and in particular of a diet rich in cooked vegetables, that has been inversely associated with AMI [13]. In fact, in the Italian diet, onion and garlic are often eaten or cooked in combination with other foods, such as tomatoes and olive oil in salads and tomato sauces for pasta.

In this study, cases and controls were interviewed in the same hospitals, and they came from the same geographical area. We excluded from the comparison group all patients admitted for chronic conditions or diseases related to known or potential risk factors for AMI, diet-related conditions and long-term modifications of diet. The FFQ was satisfactorily valid and reproducible [7, 8]. Furthermore, the findings of our study cannot be due to a selectively higher response rate of health conscious control subjects, because participation was practically complete for both cases and controls. The potential confounding effect of several covariates associated with AMI risk in this study was allowed for in the analysis. Therefore, the current study, the first from Mediterranean countries, suggests that a diet rich in onions may have a favourable effect on the risk of AMI. However, additional epidemiological studies are needed to assess the association between allium vegetables intake and risk of cardiovascular disease.