Associations of food consumption, serum vitamins and metabolic syndrome risk with physical activity level in middle-aged adults: the National Health and Nutrition Examination Survey (NHANES) 2005–2006

The metabolic syndrome (MetS) is a cluster of CHD risk factors: diabetes and elevated fasting blood glucose, abdominal obesity, low HDL cholesterol (HDL-C) and high blood pressure( 1 – 3 ). It is estimated that 20–25 % of the world’s adult population and approximately 34 % of the US population have MetS( 4 ). Those with elevated MetS risk factors are twice as likely to die from MetS and three times as likely to have a heart attack or stroke compared with people without MetS. In addition, people with MetS have a fivefold greater risk of developing type 2 diabetes( 5 ). According to the American Heart Association( 6 ), the following factors have been reported as being directly associated with incident MetS in prospective or retrospective cohort studies: age, low socio-economic status, smoking, low levels of physical activity, intake of soft drinks, Western dietary pattern, heavy alcohol consumption, long-term stress at work, obesity or BMI. Four behavioural risk factors targeted in primary care to prevent and manage non-communicable diseases, including MetS and its sequelae, type 2 diabetes and CVD, are tobacco use, physical inactivity, unhealthy diet and alcohol( 7 ). One of the four behavioural risk factors, physical inactivity, is not only a leading risk factor for mortality but also a significant contributor to rising health-care costs. Each year, approximately 3·2 million deaths and 32·1 million disability-adjusted life years, representing about 2·1 % of global disability-adjusted life years, are attributable to insufficient physical activity( 8 ). People who are insufficiently physically active have a 20–30 % increased risk of all-cause mortality compared with those who engage in at least 30 min of moderate-intensity physical activity on most days of the week( 9 ). Participation in 150 min of moderate physical activity each week or equivalent is estimated to reduce the risk of IHD by approximately 30 %, the risk of diabetes by 27 % and the risk of breast and colon cancer by 21–25 %( 8 , 9 ). In the National Health and Nutrition Examination Survey (NHANES) 2007–2008, 32 % of US men and 36 % of US women were obese and an additional 40 % of men and 28 % of women were overweight. About one in twenty Americans had a BMI of >40 kg/m2 (class III obesity)( 10 ). Compared with adults with a BMI of 20·0–24·9 kg/m2, those with a BMI of 30·0–34·9 kg/m2 and ≥35·0 kg/m2 had 25 % and 44 % higher mean annual total (inpatient and outpatient) health-service costs, respectively( 11 ). Although a large number of studies have reported associations between MetS and physical activity behaviours using questionnaires, little research has been conducted about the relationship of MetS risk with objectively measured physical activity. A literature search using the keywords ‘objectively measured physical activity’, ‘accelerometer’ and ‘MetS’ in PubMed yielded only fourteen articles. Studies reporting associations between daily steps and MetS yielded only two articles( 12 , 13 ). The remaining articles reported associations between intensity-related physical activity and MetS. Furthermore, to our knowledge, no studies have been published about the associations of food consumption and serum vitamins with MetS risk stratified by levels of physical activity measured by daily steps taken. However, diet is indispensible in every day of our lives and one of the four major risk factors for non-communicable diseases( 7 ). Also diet can be an immediate factor affecting the action of other factors. This means that diet is a key element in maintaining or increasing physical activity and in disease prevention or treatment. In addition, serum vitamin concentrations are closely connected with physical activity, usual food consumption and health status. For example, low levels of serum vitamins are closely associated with insufficient physical activity, insufficient consumption of fruit and vegetables, and poor health status such as overweight and MetS as well as undernutrition. In this respect, it is valuable to measure these three factors (food consumption, serum vitamin status, MetS risk) by objectively measured physical activity level. Therefore the objective of the present study was to examine the associations of food consumption, serum vitamins and MetS risk with physical activity level in middle-aged adults.