Lifestyle, medical, hormones & other risk factors
There are several non-genetic factors, including lifestyle and constitutional factors and medical interventions that have been linked to increased risk for RA. Included among lifestyle factors are socioeconomic class and diet.
Diet and smoking are the two most studied lifestyle factors related to risk for developing RA. Smoking, which is perhaps the most well established risk factor for development of RA, is discussed in the section that covers environmental risk factors.
There is significant body of research showing that what you eat may affect your risk of developing RA.
Benefits of the ‘Mediterranean’ diet
Several studies have shown that consuming a diet high in vegetables, olive oil, and fish, and low in red meats (known as the ‘Mediterranean’ diet) may protect against the development of RA.1
One key population-based study found that people who consumed a diet high in red meat and with lower amounts of fruit and vegetables had significantly increased risk for developing inflammatory arthritis. For those who consumed low amounts of vitamin C (found in many fruits and vegetables) the increased risk for inflammatory arthritis was almost 3 times higher than in the general population. Consumption of high amounts of red meats doubled the risk of developing arthritis.2
Another study found that the risk of developing RA decreased as the consumption of cooked vegetables and olive oil increased.3 Not all studies have found evidence to support the notion that a diet high in red meat and low in fruit and vegetables increases risk for RA. For instance, one study failed to find any association between RA risk and a range of food and vitamins, including red meat, olive oil, vitamins A, E, C, D. This same study did find that consumption of fish oil protected against development of RA.4
There is some evidence suggesting that consumption of antioxidants (β-cryptoxanthin and zeaxanthin) found in certain fruits and vegetables may lower risk for developing RA. Results from a few studies have suggested that increased overall consumption of antioxidants, including selenium, alpha-tocopherol, and beta-carotene, is associated with decreased risk of developing RA. The Women’s Health Study, a large research effort that included over 150,000 post-menopausal women in the US, examined relationships between antioxidant consumption and risk for development of RA. The study found that only consumption of the antioxidant β-cryptoxanthin was associated with reduced RA risk. No associations were found between RA risk and consumption of other antioxidants, including alpha- or beta-carotene, total carotenoids, lycopene, or lutein/zeaxanthin.1
Results from one large study failed to find any association between daily consumption of caffeine and risk of developing RA.5 However, results from the Iowa Women’s Health Study did show that consumption of 4 or more cups of decaffeinated coffee per day increased the risk of developing RA, while consumption of 3 or more cups of tea per day protected against development of RA.6
Socioeconomic status and occupation
Several studies have examined risk of RA from the perspective of socioeconomic status. One large study conducted in Sweden found that people without a University degree were at somewhat higher risk for developing RA than those with such a degree.7 Similarly, a study conducted in Denmark found that the more time a person spent receiving a formal education, the less likely they were to develop RA.8 However, other studies have failed to find any association between level of education and increased risk. A study of 687 adults with RA from the Norfolk Arthritis Register failed to find any association between education and other socioeconomic status indicators and risk of developing RA.9
While the link between socioeconomic status and risk for RA is uncertain, there are occupations associated with increased risk for developing RA. In general, it has been found that occupations involving manual labor are associated with higher risk for developing RA than those involving non-manual labor. Occupations associated with increased RA risk typically involve exposures to substances known to increase risk for RA, including farming, mining, and occupations involving chemical exposures.1
One of the highest occupational risks associated with RA is mining involving silica dust exposure, typically from the crushing of stone and drilling of rock. One Swedish study found that men who were exposed to high levels of silica dust were at about three times more likely to develop RA than the general public.10 Other occupations involving exposure to mineral oils have been found to be associated with increased RA risk. One study that examined risk for RA among women working within the agriculture industry failed to find an association between pesticide exposure and increased risk of RA.1
A number of studies have examined whether taking an oral contraception pill, typically containing the hormones estrogen and/or progesterone, may increase risk of developing RA. Most of these studies have found that oral contraception actually has a protective effect against development of RA.1
There have been conflicting results from studies examining whether blood transfusion is associated with increased risk for RA. Results from a Norfolk Arthritis Register study found a quite high increase in RA risk associated with blood transfusion.11 However, another study conducted in a group of elderly women found that risk for RA actually decreased among women who had received a blood transfusion.12
Hormonal factors in women
The fact that RA is more common in women and the tendency for RA to first appear during childbearing years suggests that hormones may play a role in triggering the disease. Many studies have examined possible connections between hormones and RA.1
Several studies have examined whether pregnancy protects against RA with conflicting results. One study found increased risk for RA among women who had never been pregnant. Women who had been pregnant were 2 times less likely to develop RA than their counterparts who had never been pregnant.13,14 Other studies have failed to confirm a clear link between pregnancy and risk for RA, however, results from some of these studies suggest that there may be a small protective effect associated with pregnancy.1
The Nurses Health Study, which started in 1976 and is one of the longest running and largest studies of factors that impact women’s health (the three groups of nurses taking part in the study since its start total over 300,000), examined the role of hormones in development of RA. The study found associations between RA and high irregularity in menstrual cycles and early age at first menstrual period, but failed to find any link between risk for RA and other hormonal factors, including whether woman had ever been pregnant, age at first birth, and use of oral contraceptive medications.15 The Iowa Women’s Health Study, another large population-based study focusing on a group of elderly women, also found that the majority of factors related to fertility and hormones had no link to increased risk for RA.16 These included age at first menstrual period (this finding contrasted with the Nurses Health Study), history of oophorectomy or hysterectomy, oral contraceptive medication use, history of pregnancy, infertility, history of miscarriage, and total number of pregnancies.
Possible links between breast-feeding and risk of RA have been examined in a number of studies. However, results from these studies have been inconsistent. Results from two large population-based studies (one of which was the Nurses Health Study) suggest that increased lactation time may have a protective effect against development of RA.15,17 However, another study found the opposite, that increased lactation was linked to development of more severe RA.18 Still another study linked breast-feeding after a first pregnancy with increased risk for RA, with risk decreasing during a second pregnancy and disappearing altogether during a third pregnancy.19
Obesity and body shape
There is some evidence that obesity may be associated with increased risk for RA. This was the finding from the Norfolk Arthritis Register, in which risk of RA was increased in people with a body mass index of 30 or greater.11 Body mass index or BMI, for short, is calculated using measurements of height and weight. A BMI of 30 or greater is considered obese. For example, a person who is 5 feet 9 inches and weighs 203 lbs or more has a BMI of 30 and is considered obese. Results from the Norfolk Arthritis Register have been confirmed by other researchers. However, other studies have failed to find a link between obesity or distribution of body fat (body shape) and increased risk for RA.1