Effects of Cardiorespiratory Fitness on Serum Ferritin Concentration and Type 2 Diabetes
Although some researchers studied the association between serum ferritin and diabetes in other countries, very few studied evaluated serum ferritin and diabetes in the United States. In addition, there were very few studies on the effects of physical activity or cardiorespiratory fitness (CRF) on the association of serum ferritin and incident diabetes. Previous epidemiological studies examining the association of serum ferritin with type 2 diabetes have been inconsistent.
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Recent studies reported that physical activities play an important role in reducing serum ferritin concentration (Furqan et al.,2002; Lakka, Nyyssonen, & Salonen, 1994; Liu et al., 2003). An increase in physical activity decreased serum ferritin concentration (Furqan et al.,2002), and a decrease in serum ferritin concentration was shown to depend on duration and frequency of physical activity (Lakka, Nyyssonen, & Salonen, 1994). Naimark et al. (1996) found a similar pattern for performance and ferritin concentration. The mean serum ferritin decreased significantly after 24 weeks in those who walked 5 days per week, but not in those who walked 3 days per week (Naimark et al., 1996). Furqan at al. (2002) reported moderate physical activity to be more important in lowering serum ferritin than vigorous activity. Bartfay et al. (1995) demonstrated regular exercise could decrease serum ferritin concentrations. Lakka, Nyyssonen, and Salonen (1994) reported mean ferritin concentration to be 16.8% lower in individuals with the highest quartile of physical activity (>2.6 hours per week) as compared to those with the lowest duration of activity (<0.4 hours per week), and to be 19.9% lower in individuals with the highest category of physical activity frequency (>3 sessions per week) as compared to those with the lowest activity frequency (<1 session per week).
Many researchers have found an association between physical activity and diabetes (CDC, 1999; Church et al., 2004, Church, LaMonte, Barlow, & Blair, 2005; Wei et al., 1999; Wei, Gibbons, Kampert, Nichaman, & Blair, 2000a), serum ferritin and diabetes (Acton et al., 2006; Forouhi et al., 2007; Jehn et al., 2007; Jiang et al., 2004; Lecube et al., 2004, Oba et al., 1997; Salonen et al., 1998), and physical activity and serum ferritin (Bartfay et al., 1995; Furqan et al., 2002; Lakka, Nyyssonen, & Salonen, 1994; Naimark et al., 1996). However, the relationships among different levels of cardiorespiratory fitness and serum ferritin on type 2 diabetes have not been investigated. Therefore, in this study, I investigated the association between cardiorespiratory fitness and serum ferritin concentration on type 2 diabetes.
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Results
There were 6,230 subjects including 1,995 females (32%) and 4,235 males (68%) in the study. The mean (± SD) age was 47 (± 9) years and ranged from 20 to 88 years of age. Demographic characteristics, cardiorespiratory fitness, and diabetes status at baseline appear on Table I. Most of subjects were white (95%). Among the participants in the study, the proportions of participants being overweight and obese were 46.0 and 24.6%, respectively. Approximately 12% of the subjects consumed 5 or more drinks per week, 13% were current smokers, and 17.2% were former smokers.
Among all participants in this study, more than 25% were in the highest CRF quintile level and only 10.24% were in the lowest CRF quintile level (Figure 1). Only 6.36% of the subjects reported having diabetes and 3.66% of the subjects reported a family history of diabetes.
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Table 3 shows the results of the association between risk factors and type 2 diabetes after adjustment for age, sex, and ethnicity. Individuals with a high ferritin concentration were 1.89 times more likely to have type 2 diabetes (OR: 1 89; 95% Cl: 1.31,2.73) compared to those with normal ferritin. Overall, CRF levels were inversely associated with type 2 diabetes [p < 0.0001) and diabetes was strongly associated with CRF levels, especially for those in the highest quintile level who reduced their risk of getting type 2 diabetes by 55 % as compared to the lowest quintile level. Similarly, other CRF quintile levels showed inverse associations with reducing the risk of getting type 2 diabetes by 14% (OR: 0.86; 95% CI:0.60, 1.23), 38% (OR: 0 62; 95% Cl: 0 42,0.91), and 29% (OR: 0.71; 95% Cl: 0.51, 1.01) comparing with fitness levels II, 111, and IV to the lowest fitness (level I), respectively.
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Among participants without diabetes, the median range for serum ferritin concentrations in the lowest and highest CRF levels were 76.0 ng/ml and 47.5 ng/ml respectively for normal weight participants and 131.5 ng/ml and 96.0 ng/ml respectively for obese participants. Among participants with diabetes, the median ferritin concentration was 148 ng/ml for the lowest CRF level group compared with 104.0 ng/ml for the highest CRF level group. Correlations between each CRF level across BMI status were determined with Spearman’s correlations and showed significant differences in those with and without diabetes. The median ferritin levels significantly decreased with increased CRF quintile levels and correlated directly with increasing BMI levels. Among non-diabetic individuals, the median serum ferritin decreased by 37.5%, 20.6%, and 27.0% among normal weight, overweight, and obese individuals, respectively, when participants in the highest CRF' quintile level were compared to those in the lowest CRF quintile level.
Among participants with diabetes, ferritin levels were associated with CRF quintile levels across BMI status as shown by the Spearman’s correlation coefficients in Table 5. Particularly, the median ferritin concentrations in obese diabetic participants at the highest fitness levels decreased 29.7%, compared to those at the lowest fitness levels.
Discussion
In this study, an inverse association between serum ferritin concentration and CRF was found. The findings from this analysis were similar to other studies in which ferritin concentration was significantly correlated with physical exercise (Furqan et al., 2002; Malczewska, Stupnicki, Blach, & Turek-Lepa, 2004; Wilkinson, Martin, Adams, & Liebman, 2002). Participants having high ferritin concentrations were 48% more likely to have type 2 diabetes when compared to those with normal ferritin levels. Similarly, recent studies have focused on elevated serum ferritin concentrations that contributed to increased risk of diabetes (Acton et al., 2006; Canturk et al., 2003; Forouhi et al., 2007; Ikeda et al., 2006; Jehn et al., 2007; Jiang et al., 2004; Lecube et al., 2004). Serum ferritin concentration can be an independent predictor for development of type 2 diabetes (Forouhi et al., 2007). Wilkinson et al. (2002) found that mean (± SD) serum ferritin decreased significantly from 55.9 (± 9.7) to 42.2 (± 8.0) ng/ml after a 6-week high-intensive cycle training program. In another longitudinal study on the effect of a running-based training program on serum ferritin and other serum parameters, Kaiser, Janssen, and van Wersch (1989) found a significant inverse association between running and serum ferritin. Similarly, in this study, I found serum ferritin concentrations among normal weight, overweight, and obese non-diabetic participants in the highest quintile CRF level were reduced by 37.5%, 20.6%, and 27.0%, respectively, as compared to similar participants at the lowest CRF level. For obese diabetic participants in the highest CRF quintile level, serum ferritin concentrations were reduced by 29.7% as compared to similar participants in the lowest CRF level.
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Rescarchers studied the effects of running and swimming on ferritin, haptoglobin, and red cell indexes and found that serum ferritin and hemoglobin were lower in runners even though the runners had an adequate dietary iron intake compared with the baseline (Pizza, Flynn, Boone, Rodriguez-Zayas, & Andres, 1997).
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