Is the quality of drinking water a risk factor for self-reported forearm fractures? Cohort of Norway
Compared to pH ≥7.0 in Norwegian municipal drinking water, pH <7.0 increased the risk of forearm fractures in the population-based Cohort of Norway (CONOR; n = 127,272). The association was attenuated (p > 0.05) after adjustments for indicators of bacteria and organic matter, which may signify an association between poor drinking water and bone health.
The Norwegian population has the highest rate of fractures ever reported. A large variation in fracture rate both between and within countries indicates that an environmental factor, such as the quality of drinking water, could be one of the causes of the disparities. Our aim was to investigate a possible association between pH (an important parameter for water quality) and self-reported forearm fracture and to examine whether other water quality factors could account for this association.
Using Geographic Information Systems, information on the quality of drinking water was linked to CONOR (n = 127,272; mean age, 50.2 ± 15.8 years), a database comprising ten regional epidemiological health surveys from across the country in the time period 1994–2003.
The highest risk of forearm fracture was found at a pH of around 6.75, with a decreasing risk toward both higher and lower pH values. The increased adjusted odds of forearm fracture in men consuming municipal drinking water with pH <7.0 compared to water with pH ≥7.0 was odds ratio (OR) = 1.19 (95 % CI, 1.14, 1.25), and the corresponding increased odds in women was OR = 1.14 (95 % CI, 1.08, 1.19). This association was attenuated (p > 0.05) after further adjustments for other water quality factors (color grade, intestinal enterococci, and Clostridium perfringens).
Our findings indicate a higher risk of fracture when consuming water of an acidic pH; however, the risk does not only seem to be due to the acidity level per se, but also to other aspects of water quality associated with pH.
KeywordsEpidemiology Forearm fracture pH Water microbiology Water supply
The incidence of forearm fractures in the adult Norwegian population is among the highest ever reported [1, 2]. Forearm fracture is one of the strongest predictors of later hip fractures, with serious consequences for the individual and substantial expenses for society [3, 4]. Fracture risk has been shown to vary by degree of urbanization both in Norway and elsewhere, with more fractures in urban areas compared to rural areas [5, 6, 7]. Personal level risk factors (e.g., age, bone mineral density, height, body mass index [BMI], and smoking habits) and area level exposures (e.g., climatic conditions) cannot fully explain these differences [2, 5, 7]. One possible culprit could be an environmental variable to which all inhabitants in a certain area are exposed, such as the quality of drinking water. To our knowledge, no previous study has examined the association between drinking water quality and fractures in a large population-based study.
Although generally meeting most quality standards, Norwegian drinking water is more corrosive towards the water distribution network (pipes, plumbing, etc.) than typical drinking water in most other countries, mainly due to low pH and low contents of carbonate and other minerals [8, 9]. Surface water is the dominant source of drinking water in Norway, whereas ground water supplies around 10 % of the population . There are large variations across the country both in the chemical composition of water and in water treatment practices [8, 9, 10, 11, 12].
Bone is the main reservoir for minerals and plays a large role in regulating the body’s mineral homeostasis, and it has been proposed that drinking water with a low pH could affect bone health by disturbing this homeostasis [13, 14, 15]. A small decrement in plasma bicarbonate and a small increment in blood acidity have been reported to contribute to age-related bone loss, possibly through low-grade metabolic acidosis [15, 16].
pH is one of the most important parameters for determining drinking water quality . The pH level is related to several aspects of drinking water, including (1) the level of carbonate (alkalinity), which is important for the corrosivity of the water [17, 18], (2) the concentration of potentially toxic metals [8, 9, 19], and (3) the concentration of microorganisms in the water [11, 12, 20].
The standard for pH in drinking water in Norway is 6.5–9.5 [21, 22]. The guidelines are to maintain a stable pH, preferentially in the range 7.5–8.5, mainly to minimize corrosion of the distribution network [11, 12]. However, many Norwegian waterworks supply drinking water with pH below this range, sometimes well below, whereas only few waterworks supply water with pH above 8.5 [8, 9, 11].
Materials and methods
Cohort of Norway
CONOR is a collaborative project between the four medical/health university faculties in Norway and the Norwegian Institute of Public Health. It is a national database containing health data from ten regional epidemiologic surveys conducted during 1994–2003 [7, 23]. In all surveys, the data collection followed a standard protocol including measurements of weight and height and one or more questionnaires on health status and health behaviors.
The questionnaires comprised approximately 50 common questions on self-reported health and selected diseases, various risk factors, sociodemographic factors, use of medications, and reproductive history (women). The location of the study sites and information on each study, including a description of the participants, have been published previously  and may also be seen at http://www.fhi.no/CONOR.
The Norwegian Waterworks Register
The Norwegian Waterworks Register contains information on all waterworks in Norway supplying more than 50 persons or 20 households . Measures of water quality for 87 % (approximately 4.3 million) of the total Norwegian population are reported to this register. Because the locations of some of the smaller waterworks were difficult to identify, persons being served by these waterworks were linked to their neighboring waterworks. We have used information from 1993 to 2008 because data were manually recorded before this time period. Because not all included waterworks provided information every year during the time period, the information from each waterworks was averaged for all the years with provided reports. The register database contains information on the number of persons/households supplied, distribution system (pipelines), water sources (surface or ground water), water purification processes (chlorination, UV irradiation, ozone/biofiltration, membrane filtration, coagulation, and corrosion control), and quality for a large set of water components. Samples are collected from both raw water and tap water.
Geographic Information Systems (ArcGIS 9.3, ESRI 2008) were used to link the water register information to each participant in the CONOR database. We identified the coordinates of the individual waterworks using information from the Norwegian Geological Survey, the Norwegian Water Resources and Energy Directorate, and the municipal administrations in the biggest cities. Larger municipalities encompassing several waterworks were divided into smaller waterworks areas, and smaller municipalities sharing one waterworks were combined, so that each area in the study was served by only one waterworks. To divide the municipalities, we produced a Voronoi polygon based on the coordinates of the waterworks, so that each Voronoi polygon was closer to its generating point than to any other point . The numerical address of each CONOR participant was then geocoded (placed onto the map within its respective waterworks area), enabling the linkage of each CONOR participant to information on water quality. Because it was not possible to obtain the numerical address of every single participant during the time period of our study, we were not able to geocode or place all the participants in CONOR (see “Subjects” section below) within their respective waterworks area (Online Resource).
Altogether, 173,236 subjects, 20 years and older, participated in CONOR from 1994 to 2003 . Of these, 22,420 could not be geocoded because their numerical address was unknown. The analyses were conducted among the 127,272 participants that had complete information on forearm fractures, pH in tap water, and background variables (age, gender, BMI, and degree of urbanization). The number of observations used varied according to which water quality factor in addition to tap water pH was included (Online Resource).
Information on forearm fractures was ascertained from participants’ self-report to the following question: “Have you ever broken (fractured) your wrist/forearm? (Yes/no).”
pH was measured by the individual waterworks and reported to the Norwegian Waterworks Register on an annual basis. Before averaging individual values over the time period, the pH values were converted to hydrogen ion concentrations [H+] because pH is measured on a logarithmic scale. The logarithm of the average [H+] was then taken to give the average pH value of four groups.
As potential confounders or intermediate factors, we considered available variables that might be associated with pH and possibly or knowingly affect fractures (Fig. 1).
Demographic characteristics including age, gender, marital status, and place of residence were taken from the Norwegian Person Registry. Body weight (in kilograms, one decimal) and height (in centimeters, one decimal) was measured according to a standard protocol with the participants wearing light clothing without shoes (manually recorded until 2000 and thereafter with an electronic height and weight scale). BMI was calculated as weight in kilograms divided by height in square meters. Marital status was dichotomized into married vs. not married (i.e., cohabitants, widow/widowers, separated, divorced), and smoking was classified as current daily smoker vs. previous/never. The participants were asked about years of education, and this variable was dichotomized into “12 years and less” and “more than 12 years.” Degree of urbanization was determined based on the participant’s home address. Participants were divided into three population density groups: rural (municipalities with <10,000 inhabitants), suburbia (municipalities with 10,000–19,999 inhabitants), or cities (municipalities with 20,000 or more inhabitants) .
Because there is very limited knowledge of the associations between drinking water components and bone health, all available water quality factors were investigated. In addition to the main exposure variable, pH in tap water, the investigated variables were metals (iron, manganese, and aluminum), bacterial indicators (Escherichia coli, coliform bacteria, intestinal enterococci, Clostridium perfringens, and total bacteria content), sensory indicators (turbidity and color grade), and others (pH in raw water, calcium, nitrate, nitrite, ammonia, chloride, and conductivity). The Norwegian Waterworks Register does not obtain information on fluoride. With the exception of pH in raw water, all factors used in this paper were measured in tap water.
The factors that had an asymmetrical distribution were categorized (pH in raw water, aluminum, iron, manganese, conductivity, E. coli, coliform bacteria, intestinal enterococci, C. perfringens, and color grade) or log transformed (total bacteria content, turbidity, calcium, nitrite, ammonia, and chloride). pH in raw water was dichotomized to <6.5 and 6.5 and above, on the basis of the Norwegian Drinking Water Regulation . Metal factors were divided into two groups, using half the permissible limits  of aluminum and iron concentration in tap water as cutoffs (<0.1 and 0.1 mg/l and above). For manganese, one fourth of the permissible limit was used (<0.0125 and 0.0125 mg/l and above) due to few observations of high concentration in the sample. According to the Norwegian Drinking Water Regulation , bacterial indicators should not be present at all in the water; therefore, these factors (except for total bacteria content) were dichotomized into “zero detected bacteria per 100 ml”, vs. “more than zero detected bacteria per 100 ml.” Color grade was categorized into four groups: 0–5, >5–15, >15–20, and >20 mg/l Pt, where milligrams per liter of platinum is the official unit for measuring the color grade of water (distilled water has 0 mg/l Pt).
To determine if the mentioned water quality factors could be intermediate factors in the association between pH in tap water and fracture, we investigated the effect of each water quality factor in separate univariate (age-adjusted) analyses. Factors found to have an effect (p < 0.25, two-sided) on the odds of fracture and a significant association (p < 0.05) with pH were included for further analyses. The correlations between pH in tap water (continuous) and other continuous water quality factors were assessed by Spearman’s correlation coefficients, and t tests were used to examine differences in mean pH between groups of the bacterial indicators. Mean pH between groups of color grade was compared using analysis of variance.
The odds of having suffered a fracture of the forearm among participants whose tap water had an acidic pH (<7.0) was compared to those whose tap water had a neutral or alkaline pH (≥7.0) in multivariate logistic models, adjusting for age, BMI, and urbanization and stratifying on gender. Then, the various water quality factors found to have an association with pH and fracture were included one by one in separate models, where the aim was to see whether the association between pH and fracture changed substantially when adding the new water quality factor. Adjusted effects were compared to models of crude effects with the same number of observations (results not shown). Only three factors were found to substantially weaken the association between pH and fracture, and these are presented in Table 4. In model 1, the effect of pH (<7.0 vs. >7.0) on forearm fracture is adjusted for age, BMI, and urbanization, while in model 2, we adjusted for the three water quality factors in separate analyses. pH is repeated three times for each gender because the number varies according to which water quality factor other than pH is included in model 2. The effect of each of these water quality factors on fractures are also adjusted for background variables and pH in model 2.
Interactions between pH and metals (aluminum, iron, and manganese) were assessed by looking at the effect of pH within stratified groups of metal concentration. Gender and other background variables were added as covariates in these models. We considered p values <0.05 as statistically significant; all tests were two-sided. The analyses were done in STATA 11 (StataCorp 2009).
All participants in the regional health surveys comprising CONOR provided written informed consent. Each individual study was approved by the Norwegian Data Inspectorate and evaluated by the Regional Committees for Medical Research Ethics, including the linkage between CONOR and the Norwegian Waterworks Register. The studies were conducted in accordance with the World Medical Association Declaration of Helsinki.
Mean pH (SD) in drinking water by background variables in men and women, Cohort of Norway
Mean pH (SD)*
Degree of urbanization
The pH of raw water was 6.54 ± 0.55 (mean ± SD), range 4.5–8.5, substantially lower than the pH of treated water (tap water), which was 7.15 ± 0.50, range 4.5–8.6. pH in the two water types were significantly correlated (r = 0.39, p < 0.001). Of the 127,272 subjects included in our study, 49,524 (39 %) were consuming water with acidic pH, i.e., pH <7.0.
OR for self-reported forearm fracture in men and women by four groups of pH adjusted for background variables, Cohort of Norway
OR (95 % CI) adjusted for age
OR (95 % CI) adjusted for age and BMI
OR (95 % CI)a adjusted for age, BMI, and urbanization
0.84 (0.70, 0.98)*
0.84 (0.71, 0.98)*
0.94 (0.79, 1.10)
1.25 (1.18, 1.32)***
1.25 (1.19, 1.32)***
1.19 (1.12, 1.25)***
0.94 (0.88, 0.99)*
0.94 (0.88, 0.99)*
0.95 (0.90, 1.01)
0.81 (0.70, 0.95)**
0.82 (0.71, 0.96)*
0.92 (0.79, 1.08)
1.16 (1.10, 1.22)***
1.16 (1.10, 1.22)***
1.12 (1.07, 1.19)***
0.93 (0.88, 0.99)*
0.94 (0.88, 0.99)*
0.95 (0.90, 1.0)
Age-adjusted associations between self-reported forearm fracture and water quality factors other than pH, Cohort of Norway
Water quality factor
Men, OR (95 % CI)
Women, OR (95 % CI)
0.79 (0.68, 0.92)**
0.88 (0.77, 1.01)
0.79 (0.67, 0.94)**
0.80 (0.68, 0.95)**
0.90 (0.81, 1.0)
0.80 (0.72, 0.90)***
Total bacteria content
CFU/ml (log transformed)
1.02 (1.00, 1.03)*
1.00 (0.98, 1.01)
0.88 (0.72, 1.08)
0.93 (0.77, 1.11)
0.75 (0.68, 0.82)***
0.78 (0.71, 0.85)***
1.48 (1.42, 1.55)***
1.42 (1.35, 1.49)***
1.49 (1.42, 1.57)***
1.41 (1.34, 1.48)***
<5 mg/l Pt
5–15 mg/l Pt
1.03 (0.96, 1.10)
0.87 (0.81, 0.93)***
15–20 mg/l Pt
1.42 (1.35, 1.49)***
1.30 (1.23, 1.37)***
>20 mg/l Pt
0.93 (0.79, 1.09)
0.91 (0.77, 1.06)
FNU (log transformed)
1.13 (1.10, 1.16)***
1.11 (1.08, 1.14)***
mg/l (log transformed)
0.97 (0.93, 1.0)
0.92 (0.89, 0.95)***
mg/l (log transformed)
1.11 (1.08, 1.15)***
1.09 (1.06, 1.13)***
mg/l (log transformed)
0.98 (0.92, 1.04)
0.99 (0.93, 1.04)
mg/l (log transformed)
1.10 (1.04, 1.18)***
1.08 (1.01, 1.15)*
mg/l (log transformed)
0.86 (0.83, 0.89)***
0.86 (0.83, 0.90)***
0.83 (0.79, 0.86)***
0.87 (0.83, 0.91)***
Multivariate models with water quality factors
Men consuming acidic tap water (pH <7.0) had a higher age-adjusted odds of fracture compared with men consuming water with pH ≥7.0 (odds ratio [OR] = 1.26; 95 % CI, 1.20, 1.32), whereas the corresponding OR in women was 1.17 (95 % CI, 1.11, 1.22). Adjusting for BMI and urbanization reduced the ORs to 1.19 (95 % CI, 1.14, 1.25) and 1.14 (95 % CI, 1.08, 1.19) in men and women, respectively (results not shown in the tables).
OR of reported forearm fracture according to pH in tap water and other water quality factors found to be associated with pH and fracture
Water quality factor
Model 1a, OR (95 % CI)
Model 2b, OR (95 % CI)
1.19 (1.13, 1.25)***
0.98 (0.90, 1.05)
1.37 (1.25, 1.5)***
1.23 (1.16, 1.29)***
0.98 (0.91, 1.06)
1.39 (1.28, 1.52)***
1.20 (1.14, 1.26)***
1.04 (0.96, 1.13)
<5 mg/l Pt
5–15 mg/l Pt
15–20 mg/l Pt
1.25 (1.14, 1.38)***
>20 mg/l Pt
0.92 (0.78, 1.09)
1.15 (1.09, 1.20)***
0.98 (0.91, 1.05)
1.31 (1.19, 1.42)***
1.16 (1.10, 1.23)**
0.99 (0.91, 1.06)
1.30 (1.19, 1.41)***
1.11 (1.06, 1.17)***
1.04 (0.96, 1.13)
<5 mg/l Pt
5–15 mg/l Pt
0.96 (0.89, 1.03)
15–20 mg/l Pt
1.17 (1.07, 1.28)***
>20 mg/l Pt
0.91 (0.77, 1.08)
The effects of each water quality factor in the separate multivariate models described above are also presented in Table 4. The strongest effect was seen for C. perfringens (men). Participants drinking water that contained measurable levels of this bacterial indicator had a 39 % (men) and 30 % (women) higher odds of reporting a distal forearm fracture, compared with those without C. perfringens in their water, after adjusting for pH and background variables. Similar effects were seen for intestinal enterococci (Table 4). The effect of color grade 15–20 mg/l Pt was also strong, with 25 and 17 % increased odds of fracture in men and women, respectively, compared with those who had drinking water with 0–5 mg/l Pt color grade (Table 4). The correlations between these three variable pairs were C. perfringens and color grade (r = 0.71), intestinal enterococci and C. perfringens (r = 0.93), and intestinal enterococci and color grade (r = 0.62).
Interactions between pH and metals
None of the three metals (aluminum, iron, and manganese) significantly altered the association between pH and forearm fracture in either gender. There was, however, an effect modification seen by different concentration levels of the three metals on the association between pH and forearm fracture: When the aluminum level was below 0.1 mg/l, those consuming tap water with pH <7.0 had a significantly increased risk of reporting a fracture for both genders, compared with the participants consuming water with pH ≥7.0 (OR = 1.17, 95 % CI, 1.11, 1.23; results not shown in the tables). No effect of pH was seen when the concentration of aluminum was above 0.1 mg/l. The pattern was similar for iron and manganese. The OR for pH when the iron level was <0.1 mg/l was 1.19 (95 % CI, 1.14, 1.24), and the OR when the manganese level was <0.0125 mg/l was 1.21 (95 % CI, 1.16, 1.26), with no effect of pH on distal forearm fracture above these concentrations.
In this large, population-based cohort, we saw higher odds of reported forearm fractures in men and women consuming drinking water with a slightly acidic pH (6.0–6.99), compared to neutral/alkaline pH (7.0–7.5). It is interesting to note that the risk of forearm fracture across the pH range was not distributed linearly, as previously hypothesized . Rather, the association was an inverted U-shape with the highest risk around pH 6.75. Water with pH above 7.5 had a protective effect on fractures when adjusting for age and BMI, but with further adjustment for urbanization, the association was no longer significant (p > 0.05). The observed association could partly be explained by the other water quality factors such as intestinal enterococci, C. perfringens, and color grade, while background factors such as age, BMI, degree of urbanization, marital status, education, and smoking did not explain the association.
Acidic water and bone health
All Norwegian waterworks are required to supply drinking water of a pH above 6.5 , and it is recommended that the pH is above 7.5 . However, the large majority of CONOR participants were consuming neutral or acidic water with a pH <7.5. Norwegian surface water, the main source of drinking water in the country, is often acidic partly due to anthropogenic acid precipitation . Low-pH water generally contains little hydrogen carbonate, calcium, and magnesium , which are important components of bone. Only 36 of the participants in the present study were supplied hard water (calcium concentration >35 mg/l), and over 90 % of the participants was drinking water classified as extremely soft (<15 mg/l). An acidic environment low in minerals has been shown to adversely affect bone cells in vitro, leading to increased bone resorption and causing a net expulsion of minerals from bone cells to the exterior growth medium [26, 27]. In the body, even a small decrease in blood pH leads to components like calcium, magnesium, sodium, and bicarbonate being expelled from bone and an increased urinary excretion of calcium, magnesium, and bone resorption markers [13, 15, 16, 26, 28]. Nutritional acid load [14, 26] and the intake of cola drinks [29, 30] have been seen to contribute to bone loss, possibly through low-grade metabolic acidosis . The potential effects on health of drinking acidic water are not known, although one speculation has been that the risk of fracture is higher towards the lower end of the pH scale . Contrary to this argument, we found the highest risk to be around pH 6.75, with a decreasing risk of fracture toward both the higher and the lower ends of the pH scale.
In multivariate models, the color grade of the water and two bacterial indicators were found to explain parts of the association between pH and forearm fracture. A substantial proportion of CONOR participants were exposed to water of a high color grade, which indicates a high content of organic matter. This provides better growth opportunities for potentially harmful microorganisms and reduces the effectiveness of water disinfection . About one third of the participants were exposed to water with a measurable concentration of intestinal enterococci, and almost half to water containing C. perfringens. These bacteria should, in theory, not be able to multiply in water or cause harm if ingested, but they could be indicators of related organisms that have these abilities . Many (mostly small) waterworks in Norway have only low-grade water treatment, and some do not disinfect the water at all [9, 12]. In addition, the pH level by itself is known to influence the effectiveness of the disinfectants (e.g., chlorine is most effective in the range of pH 6.5–8.0) . Acidic water with a high corrosive potential may also damage the water distribution network, possibly leading to harmful organisms from sewage, soil, and agricultural runoff leaking into the drinking water . It is possible that microorganisms in the drinking water could lead to or exacerbate chronic inflammation in the body (e.g., inflammatory bowel disease), which has been found to adversely affect bone health [31, 32]. Several species of bacteria cause inflammatory processes that both increase bone resorption and decrease bone formation [32, 33]. Endotoxin produced by gram-negative bacteria (e.g., E.coli) has been found to be a potent osteolytic factor, activating immune cells and cytokines, leading to increased osteoclastic activity .
An interaction was found between pH and the metals iron, manganese, and aluminum. Higher metal concentrations seemed to explain parts of the association between pH and fractures. The pH of the water is a key factor determining the dissolution of metals from the soil into raw water or from the water pipes into drinking water [8, 9, 19, 35]. In Norway, the permissible level for iron and aluminum in drinking water is 0.2 mg/l, and for manganese, it is 0.05 mg/l [21, 22]. To our knowledge, no studies have investigated the possible effects of high levels of iron and manganese. Other metals, such as aluminum, have been found to have a negative effect on bone under certain conditions [19, 35].
Strengths and weaknesses
The strengths of this population-based study is the large number of participants, the representation from several areas of the country (both rural and urban), and the standardized procedures and questions used at all sites. In addition, the linkage between the register of water quality factors and information from the CONOR health surveys is unique.
Self-report of fractures
Several studies have compared self-reports with official hospital registries/medical records and found quite high validity, between 80 and 85 %, with very few cases of overreporting [36, 37, 38]. There are, however, substantial variations in the validity of self-report by the fracture site . Parts of the self-reported forearm fractures in the CONOR database were compared to a radiographic archive. Of those actually answering the questionnaire in the Tromsø Study in 1994/1995, 220 forearm fractures were found in the archive—of which 158 (72 %) were reported correctly. However, another 40 (18 %) reported a fracture, but did not state the correct time of the event . Assuming the same result in the rest of the CONOR database, this slight underreporting could have led to an underestimation of effects in this present study.
The association found between pH and fracture may be underestimated due to the uncertainties in the pH determination. pH was measured after water treatment at an arbitrary consumer in the water distribution network. The pH of the water can vary, depending on the retention time in the water distribution system, the materials used in the pipelines, and the accuracy of the pH meter used. Without these measurement errors, it is possible that we may have seen an even stronger effect of pH on fracture.
Participants may have changed place of residence during the time period of exposure, or they may have consumed most of the water at a different location than at their home address (e.g., at the workplace). However, as most people move within municipalities (in general not switching to a new waterworks) and work fairly close to their home, we believe that this is not a large source of error in our study.
The amount of tap water ingested varies between individuals. Nevertheless, drinking is not the only means of exposure to tap water; it is used in preparation of foods, for tea and coffee, etc., and most Norwegians are exposed to it on a daily basis. Although bottled water may be an important source for drinking water in many countries, in Norway, there was only on average 18 l of water (carbonated and noncarbonated) sold per capita per year during the time period of our study (calculated from numbers provided by Statistics Norway and the Brewery and Beverage Association), signifying that tap water is the main source for drinking water in Norway.
Our results do not necessarily indicate a causal connection between drinking water quality and forearm fracture. Because this is a cross-sectional study, the exposure may not have preceded the outcome, as the outcome (forearm fracture) may have occurred at any time during the lifetime of the participant. Furthermore, we used average measurements of the entire time period 1994–2008 as exposure; however, there has been a slight increase in pH over time. We do not know for certain what effect this may have had on the present study, but we assume that the increase has happened equally in all areas and is independent of fracture. This could have caused some nondifferential misclassification, attenuating the observed effect of pH on fracture.
We excluded almost all waterworks that provide water to <100 people, and most of these waterworks are located in the rural areas of the country. In addition, more participants in the rural areas than in the cities could not be geocoded due to missing numerical addresses. It is generally known that rural waterworks usually supply water of poorer quality because of the lack of water treatment. Excluding participants supplied by these waterworks could have overrated the water quality in the rural areas, explaining the higher pH found in this group. In addition, not all waterworks report on all the water quality indicators. For example, only the largest waterworks measure the concentration of calcium in the water, which means that this factor may have been prone to selection bias. This could be the reason why we did not see an effect of calcium on the association between pH and fracture in the present study.
Unfortunately we did not have any observations to look at the effect of components such as fluoride, magnesium, phosphorous, boron, and carbonate that could have a positive effect on bone. The fluoride concentration in Norwegian waterworks is generally very low . Although fluoride could be an intermediate variable in the association between pH and fracture, we believe that this factor would have little influence on our results. We also did not have information on cadmium, lead, or other heavy metals that could have a negative effect on bone.
We found a significant association between drinking water pH and the prevalence of forearm fracture in a large population-based study. However, this association may be overshadowed by other factors, for example, a high content of organic matter, the presence of possibly harmful microorganisms, and/or high concentrations of metals in acidic drinking water. It is worrying that the peak in risk was seen to be above the required minimum level of pH 6.5, indicating that tap water with a pH above this permissible level may in fact increase the risk of fractures of the population. Assuming a causal connection between acidic (pH <7.0) drinking water and forearm fracture, we calculated attributable fractions of 16 % in men and 12 % in women, meaning that approximately one out of six forearm fractures in men and one out of eight forearm fractures in women could have been avoided if the drinking water pH had been higher. Currently, legal and recommended guidelines for drinking water alkalinity and acidity are set to minimize damage to the water distribution pipelines and to household appliances, rather than being based on empirical evidence of health effects. Thus, we need more studies which, together with our findings, may contribute in elucidating the effects of components in drinking water on bone health.
This study was supported by grants from the Norwegian Research Council. The authors wish to acknowledge the services of Cohort of Norway (CONOR), the contributing research centers delivering data to CONOR, and all the study participants. Carl Fredrik Nordheim and Liliane Myrstad (Norwegian Waterworks Register) have contributed greatly in the collection and validation of data on water quality.
Conflicts of interest
Professor Tell did not report receiving fees, honoraria, grants, or consultancies. The Department of Public Health and Primary Health Care, University of Bergen, Bergen, Norway is, however, involved in studies with funding from a pharmaceutical company as a research grant to (and administered by) the university. This study has no relation to the present study. All other authors declare that they do not have any financial conflicts of interest. In addition, all authors report that they did not have any limitations on access to data or other materials critical to the work being reported.
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