Does the Level of Physical Activity in University Students Influence Development and Progression of Myopia?—A 2-Year Prospective Cohort Study

Abstract

purpose. To study whether physical activity has a protective effect on the development and progression of myopia in medical students.

methods. In a 2-year longitudinal cohort study, 156 Caucasian first-year medical students from the University of Copenhagen were enrolled. The baseline examination included visual acuity, subjective refraction, Maddox Wing test (Clement Clarke International Ltd., Harlow, UK), partial coherence interferometry, slit lamp examination, automated refraction in cycloplegia, an oral questionnaire, and a cycle ergometer test. Measurements were repeated at the follow-up. A total of 151 (97%) participants completed the study.

results. The prevalence of myopia (spherical equivalent [SE] ≤ −0.5 D) increased from 37% (95% confidence interval [CI]: 29.1–44.9) to 43% (95% CI: 34.6–50.8, P < 0.001). The incidence rate of myopia was 6.1/100 person years of observation. The mean refractive error (SE) decreased from −0.50 (1.81) to −0.74 (1.95) D (P < 0.001), and the mean axial length increased from 23.81 (1.06) to 23.94 (1.09) mm (P < 0.001). In a multiple regression analysis, time spent reading scientific literature (P = 0.024) and younger age (P = 0.022) were associated with a refractive change toward myopia, whereas physical activity was inversely associated with a refractive change toward myopia (P = 0.015). Myopic eyes progressed significantly more than did emmetropic and hyperopic eyes (P = 0.002).

conclusions. An association between physical activity and myopia was observed, suggesting a protective effect of physical activity on the development and progression of myopia in university students. The results confirm that intensive studying is a risk factor of myopia and that myopic progression or development is more likely in medical students in their early 20s than in their late 20s.

In describing different traits and characteristics of people with myopia, Parssinen et al. ¹ observed that myopic young males reported less physical exercise in childhood than did their age peers. Likewise, a trend of lower rates of myopic progression was observed in schoolchildren reporting more time spent on sports and outdoor activities, ² and an association between high levels of outdoor activity and a more hyperopic refractive error has recently been described in children. ³ A cross-sectional study describing the association between juvenile-onset myopia and parental myopia, near work, and school achievement, revealed that people with myopia spent significantly less time engaged in sports activities and that the amount of time engaged in sports was independently associated with myopia. ⁴ Most recently, Jones at al. ⁵ have reported that greater weekly participation in sports and outdoor activities in third-grade children is associated with reduced odds of having myopia by the eighth grade. 

Although the influence of physical activity on the development and progression of myopia in children has been studied to some extent, the published data on this issue in young adults and in prospective studies are sparse. Previous studies have shown that university students and especially medical students are at risk of the development of adult-onset myopia during their education, ^{6 7} possibly as a result of extensive studying. ⁸ Thus, we found it relevant to study whether physical activity has a protective effect on the development and progression of myopia in medical students. 

Methods

All Caucasian, Danish-raised students enrolled at the University of Copenhagen in the fall of 2004 and another randomly selected 33 Danish students enrolled at the university in January 2005 were invited to participate in the study (n = 185). Names, addresses, and national service registration numbers were provided by the Faculty of Medicine, University of Copenhagen, and subjects were contacted by letter. The subjects could reply by e-mail or phone to the study coordinator. Subjects who did not respond were contacted by phone, and if contact was not established, a new letter was sent. As an attempt to minimize genetic and environmental differences, only Caucasian students raised in Denmark were included in the study. Diabetes mellitus and ocular diseases were exclusion criteria, but no subjects were excluded because of these. Twenty-nine students did not respond or refused to participate. A total of 156 medical students were enrolled from February to May 2005. Five students were not re-examined because they had left medical school, and no contact could be established. Thus, 151 (96.8%) students were re-examined in 2007. Of these, eight were excluded from the analyses: seven were no longer attending medical school, and one had had LASIK due to high myopia. Hence, statistical analyses are based on data from 143 (91.7%) students. 

A temporary eye clinic was established at the department of Physiology, University of Copenhagen. The 2005 examination of the students included the following measurements and procedures: visual acuity with habitual and best correction, subjective and automated refraction, Maddox Wing test, instillation of tropicamide 1% in the right eye, partial coherence interferometry, and slit lamp examination, including examination of the posterior fundus, an oral questionnaire, and a cycle ergometer test. One student with a recent knee injury was unable to complete the cycle test but was included in the other statistical analyses. The 2007 examination included the same measurements except the cycle ergometer test. All eye examinations were performed by the same person (NJ). 

Information

Before the examinations, the students were informed in writing and orally of the purpose of the study and the possible side effects. All students gave informed consent. The study was approved by the Danish Data Protection Agency and by the local ethics committee. The study protocol adhered to the tenets of the Declaration of Helsinki. 

Visual Acuity

Monocular visual acuity was measured at 3 m with the Logarithmic Visual Acuity Chart 2000. Visual acuity was measured with the students’ habitual correction. Best corrected visual acuity was also registered. 

Subjective Refraction

Subjective refraction was measured at a distance of 3 m in each eye separately, by using the fogging method. Trial frames were used in steps of 0.25 D. The subjective refraction was measured to check the glasses of the students and was not used in the statistical analyses. 

Maddox Wing

Near heterophoria was measured by using a Maddox Wing (Clement Clarke International Ltd., Harlow, UK). The phoria groups were classified as follows: (1) greater than 6 prism diopters of exophoria; (2) 0 to 6 prism diopters of exophoria; and (3) any esophoria. This classification system is based on normal values obtained by Chung and Chong ⁹ and Morgan. ¹⁰ Exophoria greater than 6 prism diopters and any esophoria were considered to be outside the normal range. 

Instillation of Eye Drops

Two drops of 1% tropicamide were instilled into the right eye, separated by 5 minutes. We used tropicamide because it has few side effects, a short duration of action, and effectiveness almost equal to that of cyclopentolate. ^{11 12} As the students were dependent on the ability to read on the day of the examination, only the right eye was treated. 

Cycloplegic Autorefraction

Automated refraction (Retinomax2; Nikon, Tokyo, Japan) was measured in the right eye 30 minutes after instillation of the first drop of tropicamide. The same apparatus was used in 2005 and 2007. 

Oral Questionnaire

During the visit, the subjects answered questions about whether they wore glasses and, if so, when they first started wearing glasses and/or contact lenses; parental myopia; weight (if in doubt, the subjects were weighed); height; and amount of time spent reading scientific literature (not including classes and lectures at the university), reading a newspaper or other literature, and using a computer. Furthermore, the subjects were asked how many hours per week, on average, they had spent exercising during the past 6 months. Exercising was considered to include riding a bicycle, since most Danish students use bicycles to get around. The oral questionnaire was validated after the examination in 2007. 

Partial Coherence Interferometry

Axial length and corneal curvature were measured in both eyes after cycloplegia (IOLMaster; Carl Zeiss Meditec AG, Jena, Germany). The mean value of three consecutive measurements was noted. 

Slit Lamp Examination

The anterior segment and posterior fundus of both eyes were examined by slit lamp (Haag-Streit, Köniz, Switzerland) and a +90-D lens. 

Cycle Ergometer Test

The index of physical fitness, defined as maximum power output per kilogram (W_max · kg⁻¹), was determined with a cycle ergometer test with progressively increasing workload until exhaustion on an electronic cycle ergometer (Monark 839; Ergomedic, Sunny Isles, FL). This test has been validated in young males and females (15–28 years) with a test–retest correlation of 0.95 and a high correlation (r = 0.88) with directly measured V_O2max . ¹³ The cycle ergometer was electronically calibrated every test day and mechanically calibrated at the beginning of the study. 

Statistical Methods

Quantitative observations have been summarized as the mean ± SD, and the median and range are given in refractive values. Prevalence data are reported with 95% CI. t-Tests were used when comparing quantitative data between myopia and parametric data. Categorical data were compared by χ² test. One-way ANOVA was used to compare more than two groups. A paired t-test was used when comparing baseline data with follow-up data. A multiple linear regression analysis with backward elimination was used to analyze the association between the refractive change and potential risk factors. P < 0.05 was considered significant. Sample size calculation: We expected that 95% of all autorefractive measurements in one individual would be within ±1 D and that the difference in refractive change during the 2 years would be 0.3 D between two subgroups of students. ¹⁴ The sample size of our study aimed at 90% power and a 5% significance level. According to these assumptions, the size of the study population should be 150. Myopia was defined as spherical equivalent (SE) ≤ −0.5 D, emmetropia as SE > −0.5 D and < 0.5 D, and hyperopia as SE ≥ 0.5 D. The cycloplegic autorefraction result in the right eye was used in all analyses, after the refractive error was converted to SE. There was no significant difference between the subjective SE (P = 0.747) between right and left eyes. 

Calculations were performed with commercial software (SPSS ver. 12.0, SPSS, Chicago, IL; and the SAS ver. 9.1, SAS Institute Inc., Cary, NC). 

Results

Characteristics

Baseline values in the 143 medical students are given in Table 1 . No statistically significant differences were observed in sex, age, weight, height, or refraction between the participating students and the 13 students excluded from the follow-up analyses. 

As seen in Table 1 , the males and females differed significantly in some of the parameters. Whereas the females on average spent more time reading scientific literature, the males used the computer more and spent more time being physically active than did the females. Moreover, the males had better VA and tended to have flatter corneas than did the females. The average refraction in the females showed 0.5 D more myopia than in the males (not statistically significant)—the most likely explanation of why the axial length did not differ between the males and the females. 

In the following analyses, data from the males and females were analyzed together, since sex was not significantly related to refractive change in the multiple regression analysis 2 3 (Table 4) . 

To evaluate whether there was any relation between time spent studying and time spent being physically active, we compared physical activity in students spending more than the median (3 hours) time studying with that of the remainder of the students. There was no significant difference (P = 0.163). 

We compared baseline values in myopic students and nonmyopic students to demonstrate possible differences (Table 2) . Myopic students had longer eyeballs (P < 0.001) and poorer vision (P = 0.005) than did nonmyopic students and spent less time being physically active than nonmyopic students (P = 0.049). The index of physical fitness (standard deviation [SD]) was 44 (7) W_max · kg⁻¹ in myopic students and 45 (8) W_max · kg⁻¹ in nonmyopic students (P = 0.321). Myopic students did not spend more time reading scientific literature than did nonmyopic students, although there was a trend (P = 0.091). 

Of the 90 nonmyopic students at baseline, myopia developed in 11 students, corresponding to an incidence rate of 6.1/100 person years of observation. The prevalence of myopia increased from 37.0% (95% CI: 29.1–44.9) to 42.7% (95% CI: 34.6–50.8, P < 0.001), the refractive error decreased on average −0.25 D (95% CI: −0.18 to −0.31) and the axial length increased (P < 0.001). Only the corneal curvature remained stable. Students spent more time reading scientific literature (P = 0.001), using a computer (P = 0.035), and being physically active (P = 0.002) in 2007 than in 2005 (Table 3) . 

Myopic students had significantly larger refractive changes than did emmetropic students and hyperopic students (P = 0.002). Whereas myopic students had a mean refractive change of −0.40 (0.46) D, emmetropic students had −0.14 (0.35) D and hyperopic students −0.21 (0.28) D. The difference in refractive change between emmetropic students and hyperopic students was not significant. At baseline, the mean refraction of the myopic students was −2.24 ± 1.74 D (median, −2.00; range, −9.38 to −0.5); of the emmetropic students, 0.12 ± 0.28 D (median, 0.13; range, −0.38 to 0.5); and of the hyperopic students, 1.26 D ± 0.95 (median, 0.88; range, 0.63–4.25). 

A multiple regression analysis was generated, demonstrating that time spent reading scientific literature and younger age were both associated with a refractive change toward myopia (Table 4) . Time spent being physically active was inversely associated with a refractive change toward myopia (estimate, 0.175 D per hour of physical activity per day). The index of physical activity was not associated with refraction in a univariate regression analysis and therefore was not included in the multiple regression analysis. The correlation coefficient between time spent being physically active and the index of physical activity was 0.397 (Pearson, P < 0.001). Interaction between baseline ocular refraction and refractive change was tested by including baseline ocular refraction in the multiple regression analysis. No interaction was observed, and the previously mentioned variables were still significantly associated with refractive change. 

Oral Questionnaire

The oral questionnaire was validated after the examination in 2007, in 21 (14%) randomly selected students who completed the questionnaire a second time after 2 to 3 weeks. The Pearson correlation coefficient on physical activity was 0.97 and on reading scientific literature, 0.96. 

Phoria

At baseline, 100 (69.9%) students were within the normal range of the Maddox Wing test, whereas 26 (18.2%) were exophoric and 16 (11.2%) esophoric. One student had no stereopsis. Comparing the distribution of the phorias in myopic, emmetropic, and hyperopic students there was no significant difference (P = 0.646), and there was no significant difference in refractive change between students with no phoria and esophoria (P = 0.648). 

Discussion

The main finding in this prospective cohort study was that physical activity appeared to be protective against development and progression of myopia in university students. 

Bias

In a prospective cohort study, a high degree of participation is important to reduce the risk of biased estimates. A total of 96.8% (151/156) of the students attended the examination in 2007, and 91.7% (143/156) were included in the statistical analyses. 

The 29 students who did not respond or agree to participate in the study did not differ from the participants in age and sex. Other baseline characteristics were not available in these 29 students. 

Results of data based on interviews should be interpreted cautiously. Thus, potential bias, such as recall bias or interviewer bias, was minimized by posing objective, closed-end questions, and the questions were read out loud to avoid misinterpretation. As the hypothesis that the level of physical activity influences myopia has not been proven, we believe that the students were not biased in self-reporting a higher or lower amount of time spent being physically active. The questionnaire was validated with a high correlation coefficient (0.96–0.97), suggesting reliability and consistency. 

The Study Population

First-year medical students were chosen as study population because they represented a relatively homogenous group regarding age, education, and environment and because they were likely to develop myopia or experience progression during the study period. The results of the study are not applicable to the general population, but they are relevant to those who, being engaged in intensive studying, are prone to the development of myopia. 

It is arguable whether it is reasonable to use the change in refraction in the total study population as an outcome, because the expected change is different in initially emmetropic, myopic, and hyperopic students. However, since there was no interaction between baseline ocular refraction and refractive change in the multiple regression analysis, this conclusion seems reasonable. The results of the study show that all refractive subgroups changed toward myopia, although several individuals were stable or changed toward hyperopia. 

Prevalence of Myopia

The prevalence of myopia in the present study (37.0%) was comparable with those previously reported among university students in Denmark ¹⁵ and Norway. ¹⁶ In a recent publication, the prevalence of myopia (≤ −0.5 D) among Danish conscripts (mean age, 19.3 [1.6] years) was 12.8%. ¹⁷ The present observation is in accord with the well-known association between myopia and level of education. ^{18 19 20}  

Physical Activity

The daily amount of time spent being physical active was inversely associated with a refractive change toward myopia. The estimates of the multiple regression analysis suggest that the protective effect of 1 hour of physical activity per day is equal in magnitude to the detrimental effect of 3 hours of study per day. Other researchers have described an association between high levels of outdoor activity in children and a more hyperopic refractive error in cross-sectional data. ³ Recently, Jones et al. ⁵ reported that a lower number of hours spent in sports and outdoors activities per week in third grade was associated with future myopia. Limitations to physical activities due to the wearing of glasses or an actual protective effect of physical activity are some of the possible explanations of these observations, although the common use of contact lenses makes the first explanation less likely. It is not possible to conclude whether the association between physical activity and myopia is causal or merely represents an association. Thus, the association could be confounded by factors related to physical activity. As most university students in Denmark ride the bicycle to get around, it constitutes a considerable amount of the person’s physically active time. Since riding the bicycle implies being outside, the protective effect may be related to the outdoor environment rather than physical activity. On the other hand, it is possible that the association between physical activity and myopia is causal. It has been suggested that the association could be caused by some effect of distant gazing, or just being away from reading. ² However, our results indicate that the latter does not explain the effect of physical activity, as the impact of physical activity seems stronger than the impact of studying. Another possibility is that physical activity influences the refraction by nonvisual means. Physical activity influences the body and brain via several different mechanisms, including effects on body fatness, blood pressure, lipid and lipoprotein metabolism, vascular function, central and peripheral growth factors, inflammation, oxidative stress, and insulin resistance. ^{21 22} Thus, growth hormones or other systemic mechanisms induced as a result of physical activity may be involved in the regulation of the growth of the eyes. 

Although it could be suspected that students spending more time studying consequently spent less time being physically active, this was not the case. In fact, the trend was the reverse. 

Index of Physical Fitness

As we were well aware that there was a risk of bias in the reported amount of time being physically active, it was important to include an objective measure of the level of physical activity. However, this is both time consuming and demanding for the study population. An alternative option was to measure the index of physical fitness which to some extent corresponds with the level of physical activity, and was feasible to measure. Therefore, the index of physical activity was measured by a cycle ergometer test. 

The positive correlation between the reported amount of time spent in physical activity and the index of physical fitness strengthens the assumption that the reported time of physical activity is reliable. However, as the correlation coefficient was 0.397, it is obvious that the index of physical fitness is determined by other factors. Regular physical activity is a major contributor to the index of physical fitness, but there are marked interindividual differences in the trainability of cardiorespiratory endurance after exposure to an identical training program. ²³ Hence, a certain level of physical activity does not correspond with a specific index of physical activity in different individuals. Thus, the observed association between physical activity and refractive error does not imply an association between the index of physical fitness and refractive error. 

Correlation of Time Spent Reading Scientific Literature with Myopia

The amount of time spent reading scientific literature was significantly associated with a refractive change toward myopia in the regression analysis. This observation is in accord with conclusions by most researchers. ^{24 25 26 27}  

Myopic Versus Nonmyopic Students

In agreement with the literature, myopic students had longer ocular axes than did nonmyopic students. ^{8 28} Furthermore, the two groups did not differ in height or weight, which is consistent with most published data. ^{29 30} However, height and weight were self-reported. Students uncertain about their weight were weighed at the examination. Myopic students and nonmyopic students did not differ in sex, in accordance with some reports ^{16 31} but opposed to others. ^{32 33} Myopic students tended to spend more time reading scientific literature, although not significantly more, and less time on physical activity, which agrees with results of studies in children. ⁴  

Changes from 2005 to 2007

An increasing prevalence and degree of myopia have been reported in university students ⁶ and would be expected from the higher prevalence of myopia previously observed in fifth-year Danish medical students. ¹⁵ Students spent more time reading scientific literature and being physically active in 2007 than in 2005. The latter could indicate that the students were aware of the study hypothesis, but based on statements from the students, we suppose that the students, initiating their medical studies, spent much time getting acquainted with the new environment at the expense of physical activity. It is probable that this changing behavior influenced the refractive change during the study, thus generating a less accurate estimate of the association with the refractive error. 

Conclusions

The observed inverse association between physical activity and a refractive change toward myopia in university students suggests a protective effect of physical activity on myopia. 

 Supported by grants from the Danish Eye Research Foundation, the Bagenkop Myopia Foundation, The Danish Eye Health Society, and The Velux Foundation.

 Submitted for publication September 3, 2007; revised October 24, November 22, and December 5, 2007; accepted February 11, 2008.

 Disclosure: N. Jacobsen, None; H. Jensen, None; E. Goldschmidt, None

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Table 1.

 

Baseline Characteristics of All 143 Students, and by Sex

	All (n = 143)	Females (n = 87)	Males (n = 56)	P
Age (y)	23.1 (3.3)	22.8 (3.1)	23.6 (3.6)	0.163
Weight (kg)	67.0 (11.5)	60.5 (7.7)	77.1 (8.6)	0.000
Height (cm)	175.4 (9.6)	169.8 (6.4)	184.0 (7.0)	0.000
Myopia n (%)*	53 (37.0%)	34 (39.1%)	19 (33.9%)	0.534
SE (D), †	−0.50 (1.81)	−0.68 (1.88)	−0.18 (1.67)	0.104
BCVA (logMar), †	−0.15 (0.07)	−0.13 (0.07)	−0.16 (0.07)	0.022
Corneal radius (mm), †	7.79 (0.24)	7.76 (0.24)	7.84 (0.25)	0.053
Axial length (mm), †	23.81 (1.06)	23.72 (1.09)	24.00 (1.01)	0.192
Studying (min/d)	197 (70)	210 (69)	177 (67)	0.005
PC (min/d)	48 (42)	43 (36)	57 (50)	0.041
Physical activity (min/d)	57 (28)	51 (25)	65 (30)	0.006
VO2max (Wmax · kg−1)	44 (7)	42 (7)	48 (6)	0.000

 Data are expressed as the mean (SD). BCVA, best corrected visual acuity.

*  Myopia: SE ≤ −0.5 D

†  Right eye, median SE (range): 0.00 D (4.25 to −9.38)

Table 2.

 

Baseline Characteristics of 143 Students by Refractive Error

	Myopic Students (n = 53)	Nonmyopic Students (n = 90)	P
Sex (F:M)	34:19	53:37	0.534
Age (y)	23 (3)	23 (3)	0.924
Weight (kg)	67 (11)	67 (12)	0.924
Height (cm)	176 (9)	175 (10)	0.795
BCVA (logMar)*	−0.13 (0.07)	−0.16 (0.06)	0.005
Corneal radius (mm)*	7.80 (0.26)	7.78 (0.24)	0.785
Axial length (mm)*	24.65 (1.01)	23.32 (0.73)	0.000
Studying (min/d)	210 (72)	190 (68)	0.091
PC (min/d)	55 (53)	45 (35)	0.166
Physical activity (min/d)	51 (24)	60 (30)	0.049
VO2max (Wmax · kg−1)	44 (7)	45 (8)	0.321

 Data are expressed as the mean (SD).

*  Right eye.

Table 3.

 

Comparison of Characteristics in 2005 and 2007

	2005 (n = 143)	2007 (n = 143)	Mean Change	P
Myopia, n (%)*	53 (37.0%)	61 (42.7%)		0.000
SE, †	−0.50 (1.81)	−0.74 (1.95)	−0.25 (0.39)	0.000
BCVA (logMar), †	−0.15 (0.07)	−0.14 (0.07)	0.01 (0.05)	0.005
Corneal radius (mm), †	7.79 (0.24)	7.79 (0.24)	0.00 (0.04)	0.461
Axial length (mm), †	23.81 (1.06)	23.94 (1.09)	0.13 (0.13)	0.000
Studying (min/d)	197 (70)	225 (84)	28 (95)	0.001
Computer (min/d)	48 (42)	56 (44)	8 (43)	0.035
Physical activity (min/d)	57 (28)	65 (30)	9 (32)	0.002

 Data are expressed as the mean (SD).

*  Myopia: SE ≤ −0.5 D.

†  Right eye.

Table 4.

 

Multiple Linear Regression Analysis Evaluating the Association between Personal Characteristics and the Refractive Change from 2005 to 2007

Covariates	Estimate	95% CI	P
Studying (h/d)	−0.063	−0.117 to −0.008	0.024
Physical activity (h/d)	0.175	0.035 to 0.315	0.015
Age (y)	0.023	0.003 to 0.043	0.022
Sex	−0.087	−0.278 to 0.105	0.373
Computer (h/d)	0.018	−0.074 to 0.110	0.703
Reading, e.g., newspaper (h/d)	0.017	−0.147 to 0.181	0.835
Weight (kg)	0.003	−0.003 to 0.009	0.268
Height (cm)	0.001	−0.012 to 0.013	0.921

 Dependent variable: refractive change from 2005 to 2007. n = 143; R = 0.298 R ² = 0.0891 adjusted R ² = 0.069; SE = 0.380. Statistically significant associations are shown in bold.