SOCIOECONOMIC POSITION, CLUSTERING OF RISK FACTORS, AND THE RISK OF MYOCARDIAL INFARCTION

We read with great interest the article by Kivimäki et al. on socioeconomic position (SEP), clustering of behavioral risk factors, and the risk of coronary heart disease published in the May 2007 issue of the Journal.1 To contribute to the sparse discussion on SEP and risk factor clustering, we propose the use of the wording of between-group clustering to describe the clustering of risk factors in different socioeconomic groups, as seen in Table 1 ▶ of the cited paper, and within-group clustering to describe the clustering of risk factors within a socioeconomic group, taking the group-specific prevalence of risk factors into account, as seen in Table 2 ▶ in that same article. (There may be a possible error in the table: Men, 3–4 rf, low SEP, Exp. No., reads 283, should probably read 213). We acknowledge the large study sample and the clarity of the article.

TABLE 1—

Risk of Myocardial Infarction by Socioeconomic Position (SEP) and Exposure to Increasing Number of Risk Factors: Stockholm Heart Epidemiology Program, Stockholm, Sweden, 1992–1994

	Childhood Nonmanual Worker SEP		Childhood Manual Worker SEP
	Adult Nonmanual Worker SEP, OR (95% CI)	Adult Manual Worker SEP, OR (95% CI)	Adult Nonmanual Worker SEP, OR (95% CI)	Adult Manual Worker SEP, OR (95% CI)
Total, No.	405	87	404	285
No. of risk factors
0	Ref	1.27 (0.49, 3.29)	1.12 (0.68, 1.85)	0.96 (0.52, 1.76)
1	1.18 (0.75, 1.86)	2.13 (1.07, 4.22)	1.54 (1.00, 2.39)	2.64 (1.66, 4.20)
2	2.03 (1.26, 3.27)	2.71 (1.32, 5.55)	2.25 (1.43, 3.54)	3.21 (2.01, 5.12)
≥ 3	3.39 (2.02, 6.69)	4.12 (2.23, 7.61)	4.55 (2.74, 7.58)	4.79 (3.03, 7.57)

Note. OR = odds ratio; CI = confidence interval.

TABLE 2—

Age-Adjusted Observed and Expected Prevalence of Exposure to Increasing Number of Risk Factors and Observed-to-Expected Comparison, by Socioeconomic Position (Manual or Nonmanual Worker), in Childhood and Adulthood Among the Male Control Group: Stockholm Heart Epidemiology Program, Stockholm, Sweden, 1992–1994

				Childhood Nonmanual Worker SEP								Childhood Manual Worker SEP
	All Men (N = 1498)			Adult Nonmanual Worker SEP (n = 405)				Adult Manual Worker SEP (n = 87)				Adult Nonmanual Worker SEP (n = 404)				Adult Manual Worker SEP (n = 285)
	Observed Prevalence, %	Expected Prevalence, %	Within-Group Comparison, %	Observed Prevalence, %	Expected Prevalence, %	Within-Group Comparison, %	Between-Group Comparison, %a	Observed Prevalence, %	Expected Prevalence, %	Within-Group Comparison, %	Between-Group Comparison, %a	Observed Prevalence, %	Expected Prevalence, %	Within-Group Comparison, %	Between-Group Comparison, %a	Observed Prevalence, %	Expected Prevalence, %	Within-Group Comparison, %	Between-Group Comparison, %a
No. of risk factors,
0	27	22	122	33	29	115	150	18	12	154	82	27	25	108	123	21	15	146	95
1	34	39	89	36	41	88	92	30	32	95	77	38	40	95	97	30	34	87	77
2	24	27	89	20	23	89	74	20	33	59	74	25	26	97	93	26	32	80	96
≥ 3	15	12	122	11	7.6	138	92	33	24	138	275	11	10	107	92	24	20	121	200
P			< .001			.01	< .001			.01	< .001			.72	.18			< .001	< .001
λ			32.1			10.6	26.2			10.7	17			1.3	4.9			16.3	16.7

Note. SEP = socioeconomic position. Childhood SEP refers to the SEP of the parent (foremost father) of the study participant during the participant’s childhood. Risk factors were diabetes, hypertension, low life control, low social network, obesity, physical inactivity, and smoking. P and λ values derived from the χ² test with 3 degrees of freedom testing the null hypothesis of no difference in observed and expected frequencies across all the number of risk factor categories.

^a The between-group comparison used the expected value from the total number of control participants as comparison.

We analyzed trajectories of SEP, clustering of 7 risk factors (diabetes, hypertension, low life control, low social network, obesity, physical inactivity, and smoking) and risk of acute myocardial infarction in the Stockholm Heart Epidemiology Program, a population-based case-control study. Accumulation of SEP over the entire life course, risk factor exposure, and acute myocardial infarction incidence have previously been analyzed in the same data.2 Our results confirm the findings by Kivimäki et al., with a larger risk factor burden on lower SEP trajectories, a greater than expected prevalence of null- and multiex-posed individuals in most trajectories of SEP, and an increased risk of acute myocardial infarction with exposure to an increasing number of risk factors. There was also no systematic socioeconomic difference in risk of acute myocardial infarction in the group unexposed to all 7 risk factors (Tables 1 ▶ and 2 ▶). The larger prevalence of multiexposed individuals among low SEP groups and the larger prevalence of null-exposed individuals in higher SEP trajectories does not seem to be the effect of differential individual clustering among different SEPs, because the within-group clustering is similar in all SEPs. Rather, it seems to be the effect of risk factor accumulation correlated to the socioeconomic career on the group level (i.e., between-group clustering). This is important for future attempts to reveal this part of the etiology of health inequalities, because it leads us to look for determinants of risk factor clustering that are distributed differentially between socioeconomic groups.