In the study, researchers examined 30,108 participants aged 35 to 69 in a baseline survey of the Japan Multi-Institutional Collaborative Cohort Study.
They assessed dietary intake with a 46-item food frequency questionnaire (FFQ), and used BMI instead of waist circumference to diagnose metabolic syndrome.
They also extracted three nutrient patterns by applying factor analysis to the participants' energy-adjusted intake of 21 nutrients: Factor 1 (fibre, potassium and vitamins), Factor 2 (fats and fat-soluble vitamins) and Factor 3 (saturated fatty acids, calcium and vitamin B2).
After adjusting for sex, age and other potential confounders using multiple logistic regression analysis, the researchers found that Factor 1 scores were linked to a significantly lower odds ratio of metabolic syndrome.
They noted that Factor 1 scores were "positively correlated with the intake of folate, insoluble and soluble dietary fibre, carotene, iron, vitamin C and potassium", seven nutrients abundant in fruits and vegetables.
This observation was consistent with common wisdom that dietary patterns characterised by a high intake of fruits, vegetables, legumes and cereals were inversely associated with metabolic syndrome.
Among the seven nutrients, potassium and insoluble dietary fibre had the strongest link to a lower odds ratio of metabolic syndrome, with the latter being known for its ability to prolong intestinal transit time, increase post-meal satiety, and reduce insulin resistance.
Previous studies have also drawn an inverse association between metabolic syndrome and high dietary potassium intake, attributing this to potassium's ability to help lower blood pressure by combating the effects of sodium intake.
The researchers hypothesised that the other nutrients may have "contributed through mechanisms such as the gel-forming effects of soluble dietary fibre to reduce postprandial blood glucose, and the anti-oxidative effects of vitamin C".
In contrast, Factor 2 scores were linked to a significantly higher incidence of metabolic syndrome, obesity and high blood pressure. The scores were positively correlated with the intake of monounsaturated fatty acids, omega-3 and omega-6 polyunsaturated fatty acids (PUFAs), and vitamin E, which were linked to the prevalence of metabolic syndrome, obesity and high blood pressure.
The researchers wrote: "We think that Factor 2 reflects the overall consumption of a high-fat diet because this pattern was positively correlated with deep-fried foods, stir-fried foods, mayonnaise, beef / pork, ham / sausage / salami / bacon, and fish."
The high energy density of a high-fat diet is likely to result in long-term excessive energy consumption, and the researchers found this was associated with high blood pressure, with the link partially mediated by obesity.
A high intake of refined grains, red meat, processed meat and fried foods has been reported to be positively associated with metabolic syndrome in previous research. However, the positive correlation between metabolic syndrome and omega-3 PUFA and fish intake, as well as the negative correlation between saturated fatty acids and metabolic syndrome, were unexpected.
Factor 3 scores were associated with a reduced odds ratio of metabolic syndrome (apart from obesity and high blood glucose), as well as high serum triglyceride and low HDL cholesterol levels.
Of the three nutrients closely linked to Factor 3 — calcium, saturated fatty acids and vitamin B — calcium had the strongest association with a lower odds ratio of metabolic syndrome, and after adjusting for calcium, the researchers found that vitamin B and saturated fatty acid intake were "not independently associated with metabolic syndrome".
In terms of food, milk, yoghurt, beef, pork and eggs had a positive correlation with Factor 3 scores, which the researchers considered a reason for the "high factor loading of saturated fatty acid on Factor 3".
Previous studies have found an inverse link between calcium intake and metabolic syndrome, with dairy consumption reported to be associated with a lower incidence of metabolic syndrome.
The study attributed the beneficial effects of calcium on lipid profiles to the "formation of soaps with fatty acids and increased faecal fat excretion". In addition, calcium was said to have preventive effects on high blood pressure.
Analysis and association
The researchers acknowledged that the use of an FFQ to assess nutrient intake meant that some degree of measurement error could not be avoided.
Furthermore, due to the use of BMI instead of waist circumference to diagnose metabolic syndrome, some participants with high muscle mass or excess subcutaneous fat could have been wrongly classified as having abdominal obesity, while those with normal BMI but high abdominal circumference may have been considered healthy.
The Japanese population assessed in the study also meant that it the results were not necessarily generalisable to other ethnic populations.
In conclusion, the researchers wrote: "The fibre, potassium and vitamins pattern, and saturated fatty acids, calcium and vitamin B2 pattern, were associated with a reduced odds ratio of metabolic syndrome, while a fats and fat-soluble vitamins pattern was associated with an increased odds ratio of metabolic syndrome.
"Analysis of nutrient patterns may be useful to assess the overall diet and its association with metabolic syndrome, and to set dietary guidelines for metabolic syndrome prevention."
"Associations of Nutrient Patterns with the Prevalence of Metabolic Syndrome: Results from the Baseline Data of the Japan Multi-Institutional Collaborative Cohort Study"
Authors: Yuki Iwasaki, et al.