Drop each label into the appropriate box, indicating whether it is a vitamin or a mineral.

Available literature demonstrates that serum 25OHD levels increase in response to increased vitamin D intake, although overall it can be concluded that the relationship is non-linear rather than linear. Factors that may affect the relationship between vitamin D intake and serum 25OHD levels are not entirely clear, and the reliability of such measures may be less than desirable. Moreover, there remains debate over the equivalence of vitamins D2 and D3 in the diet (Armas et al., 2004; Rapuri et al., 2004; Vieth, 2004), although it has been assumed that they are 25-hydroxylated at similar rates (see previous discussion of functions and physiological actions of vitamin D).

As part of the 2007 Agency for Healthcare Research and Quality (AHRQ) systematic review (Cranney et al., 2007, 2008), referred to hereafter as AHRQ-Ottawa, exploratory meta-regression analysis was conducted of 16 trials in adults, which suggested an association between vitamin D dose and serum 25OHD concentrations. The analysis found that for each additional 100 IU of vitamin D3, serum 25OHD concentrations rose by 1 to 2 nmol/L. Trials varied in their use of vitamin D2 or vitamin D3. A few of the studies reported different effects of vitamin D2 and vitamin D3 on serum 25OHD levels. Although the later AHRQ-Tufts analysis (Chung et al., 2009) did not identify newer (compared with AHRQ-Ottawa) randomized controlled trials related to intake of vitamin D and serum 25OHD levels, it did graphically evaluate the net changes in serum 25OHD concentrations against the doses of vitamin D supplementation using data from the trials in adults. The analysis confirmed the relationship between increasing doses of vitamin D and increasing net change in serum 25OHD concentrations in both adults and children, but it also concluded that the dose–response relationships differ depending upon study participants' baseline serum 25OHD levels (≤ 40 vs. > 40 nmol/L) and duration of the supplementation (≤ 3 vs. > 3 months).

In a recent study conducted by Smith et al. (2009), personnel stationed in the Antarctic in winter months (and thus presumed to obtain vitamin D from food and supplements only) were given graded doses of 400, 1,000, or 2,000 IU of vitamin D3 per day for 5 months. Baseline levels of serum 25OHD rose from approximately 44 nmol/L to 57, 63, and 71 nmol/L, respectively, representing a change in 25OHD levels of 13, 19, or 27 nmol/L. Evident in this study is the continuing fall in 25OHD level in the “no pill” group (to 34 nmol/L) of men who were deprived of sunlight and received approximately 250 to 350 IU of vitamin D (which included foods and any non-study supplements) per day. A possible complicating factor in interpreting these data is that the subjects were consuming diets with a vitamin D content that ranged from 241 to 356 IU/day, in addition to the graded doses of vitamin D from administered supplements, although the amounts of vitamin D obtained from foods (or non-study supplements) were not significantly different between treatment groups. Therefore, any effect of these sources of vitamin D on serum levels would have been consistent between treatment groups.

Another study of serum 25OHD response to total intake under conditions of minimal sun exposure used two populations based in Cork, Ireland (51°N), and Coleraine, Northern Ireland (55°N). The study estimated the dose of vitamin D required to maintain 25OHD levels above certain chosen cutoff values (i.e., 25.0, 37.5, 50.0, and 80.0 nmol/L) during the winter months (Cashman et al., 2008). The researchers found that serum 25OHD levels that ranged between 65.7 and 75.9 nmol/L in late fall fell in all groups receiving 200, 400, and 600 IU of vitamin D3 per day, as well as in the placebo group, in winter. The decrease in the 600 IU/day group was minimal, from 75.9 to 69.0 nmol/L. Cashman et al. (2008) went on to plot the serum 25OHD levels attained after 5 months versus the estimated total vitamin D intake (approximately 0 to 1,400 IU/day) in 215 individuals, as shown in Figure 3-4. They concluded that, in these two populations at 51°N and 55°N, the wintertime intake required to achieve 25OHD cutoff levels of 37.5, 50.0, and 80.0 nmol/L were 796, 1,120, and 1,644 IU/day, respectively.

Importantly, the relationship between vitamin D intake and serum 25OHD response appears not to be linear, given evidence that increasing serum 25OHD level above 50 nmol/L requires more vitamin D intake than does increasing serum 25OHD levels when the starting point is less than 50 nmol/L (Aloia et al., 2008). Factors such as baseline serum 25OHD level in the population may be relevant. Further, there have been reports that the rise in serum 25OHD levels for a given dose tends to stabilize by week 6 (Harris and Dawson-Hughes, 2002; Holick et al., 2008) and that it does not vary with age at least up to 80 years of age (Harris and Dawson-Hughes, 2002; Cashman et al., 2008, 2009).

The cutaneous synthesis of vitamin D, and in turn its contribution to the concentration of serum 25OHD, is initially dependent upon the presence of 7-dehydrocholesterol in the skin. However, many variables can affect the cutaneous synthesis of vitamin D, making it difficult to estimate an average amount of vitamin D and, in turn, serum 25OHD levels that are produced by sun exposure in North America. There is, however, agreement that sun exposure is a significant source of the circulating serum 25OHD in summer for many North Americans, and is notably reduced as a contributor in the winter months. Early work from Webb et al. (1988, 1989) as well as a letter from Holick et al. (1989) have outlined the role of sunlight in regulating cutaneous synthesis of the vitamin and have implicated factors in vitamin D3 synthesis in the skin to include aging, melanin pigmentation, season of the year, latitude, and use of sunscreen. Matsuoka et al. (1992) has discussed the role of clothing in preventing synthesis.

The 2007 AHRQ-Ottawa systematic review (Cranney et al., 2007) noted that the few available randomized clinical trials conducted between 1982 and the time of the analysis, which focused on the effect of UVB radiation on serum 25OHD level revealed little information about the impact of age, ethnicity, skin pigmentation, BMI, or latitude on serum 25OHD levels. However, UVB exposure increased serum 25OHD levels in vitamin D–deficient and –sufficient subjects with mean increases ranging from 15 to 42 nmol/L.

The difference in seasonal contributions to serum 25OHD level from sun exposure is discussed first. Next, factors affecting the synthesis of vitamin D—and, in turn, the levels of 25OHD in serum—are outlined.

Effect of season on circulating serum 25OHD level Sunlight exposure as a source of cutaneous synthesis of vitamin D is subject to a number of limitations. For example, excess exposure can lead to photo-degradation as a regulatory mechanism to avoid toxicity (Chen et al., 2007). In addition, latitude, time of exposure, and season all affect cutaneous synthesis, depending on the ability of UV rays to stimulate vitamin D production. Differences in seasonal exposure can vary by as much as 6 months at extreme northern and southern latitudes (Lucas et al., 2005; Kull et al., 2009).

A number of studies have examined serum 25OHD levels in different seasons. Van der Mei et al. (2007) compared cross-sectional data from men and women less than 60 years of age living in Australia and concluded that season was a strong determinant of serum 25OHD concentrations. Berry et al. (2009) examined white adults ages 20 to 60 years living in the United Kingdom (UK) at latitude 53°N. In this study, women were found to have higher average serum 25OHD levels than men in both summer and winter (9 and 20 percent higher, respectively). In a study of young adults of diverse ethnic backgrounds living in Toronto, Gozdik et al. (2009) found that in winter, serum 25OHD levels of individuals with East and South Asian backgrounds were significantly lower than those of individuals with European ancestry. Kull et al. (2009) measured seasonal variance of 25OHD levels in adults ages 25 to 70 years living in Estonia in northern Europe, where dairy products are not fortified with vitamin D. During the winter, 73 percent of the study population had 25OHD levels that were below 50 nmol/L, and 8 percent had levels that were below 25 nmol/L, compared with 29 percent and 1 percent, respectively, during the summer. Rapuri et al. (2002) examined white, black, and Hispanic women 65 to 77 years of age in Omaha and reported mean serum 25OHD levels of 68 nmol/L in February and 86 nmol/L in August. These studies, despite variations in age, gender, and ethnicity, all suggest that seasonal change can affect cutaneous vitamin D synthesis. The winter low serum 25OHD concentrations and the summer high serum 25OHD concentrations from these studies are summarized in Table 3-1.

Although there are variations in the available data as well as a number of unknowns that may influence such values, they suggest a seasonal change in serum 25OHD concentrations between the winter nadir and the summer zenith of approximately 25 nmol/L. Free-living individuals in the latitudes studied appear to experience an approximately one-third seasonal increase in their circulating serum 25OHD levels as they moved from the winter months to the summer months.

The 25 nmol/L change in serum 25OHD level would appear to be similar in magnitude to change experienced by subjects given 2,000 IU/day in the Antarctic study (Smith et al., 2009). Although these data suggest that average cutaneous synthesis during the summer in northern latitudes equates to 2,000 IU/day, this may be a questionable conclusion given the many variables that come into play, ranging from feedback mechanisms to skin pigmentation to baseline levels of 25OHD. For example, recent work from Olds et al. (2008) suggested a curvilinear relationship between sun exposure and serum 25OHD levels, as well as variation depending upon initial concentrations of 7-dehydrocholesterol levels. At lower doses, vitamin D3 production rises immediately in response to UV exposure, whereas at higher doses, the rate of production is lower and reaches an earlier plateau.

Effect of skin pigmentation on synthesis The presence of melanin in the epidermal layer is responsible for skin pigmentation. A number of recent studies have reinforced the relationship of skin pigmentation to the capacity to produce vitamin D3 after UV exposure, but the results are not all consistent. Armas et al. (2007) studied the incremental change in serum 25OHD levels in individuals with average 25OHD baseline levels of 52 nmol/L and with different skin pigmentation who were exposed to different daily doses of 20 to 80 mJ/cm2 of UVB light three times per week for 4 weeks on 90 percent of their skin surface area. The work suggested that for individuals at the lighter pigmentation scale,7 exposed to similar UVB doses (20 to 80 mJ/cm2) resulted in twice the increase in serum 25OHD concentration compared with individuals at the opposite extreme (i.e., 62 vs. 32 nmol/L change) (Armas et al., 2007) (see modeled representation in Figure 3-5). However, other studies have shown that the response to UV dose is non-linear and dependent on genetic factors (Snellman et al., 2009), duration and dose rate of UV exposure, and baseline serum 25OHD levels (Bogh et al., 2010).

Another population study of 237 subjects at a single geographical location (Toronto, 43°N) also explored the effect of skin pigmentation on 25OHD synthesis (Gozdik et al., 2009). The mean serum 25OHD levels in Canadians of European, East Asian, and South Asian ancestry were 71.7, 44.6, and 33.9 nmol/L, respectively, in late fall; and 51.6, 28.1, and 26.5 nmol/L, respectively, in the winter. The authors found that differences between the three subgroups were strongly associated with skin pigmentation as well as amount of time spent outdoors and total vitamin D intake. A recent study of 182 individuals in Denmark, screened in January and February and selected to reflect wide ranges of baseline 25OHD levels and skin pigmentation, found that the increase in 25OHD levels after UVB exposure was inversely correlated with skin pigmentation as well as with baseline 25OHD (Bogh et al., 2010).

A number of small studies have reported serum 25OHD levels to be consistently lower in persons with darker skin pigmentation, and data from NHANES suggest that serum 25OHD levels are highest in whites, lowest in non-Hispanic blacks, and intermediate in Hispanic groups (Looker et al., 2008). Overall, there is considerable evidence that darker skin pigmentation is associated with a smaller increase in serum 25OHD concentration for a given amount of UVB exposure.

Effect of latitude on synthesis Early on, in vitro methods, such as exposure of sealed vials of 7-dehydrocholesterol to UVB radiation under “idealized conditions” at various geographical locations, were used to assess the effect of latitude, time of day, and season on the rate of vitamin D production (Webb et al., 1988). However, this approach cannot completely simulate the in vivo conditions in the body, where many factors serve to regulate this process. Furthermore, although the measurement of vitamin D concentrations in a mixture of irradiation products is analytically simple, vitamin D3 levels in human serum are rarely used to estimate cutaneous synthesis of the vitamin, in part because of the transient nature of this blood parameter and the difficulty of measuring the low levels in serum (Holick, 1988; Hollis, 2008). Nevertheless, Holick and colleagues used a serum vitamin D3 assay to augment in vitro methodology and suggested that, at latitudes above 43°N, cutaneous synthesis contributes little serum 25OHD to the system in the winter months between October and March in North America (Webb et al., 1988; Matsuoka et al., 1989).

More recent data may call into question current assumptions about the effect of latitude. In fact, Kimlin et al. (2007), using computer modeling, concluded that it may no longer be correct to assume that vitamin D levels in populations follow latitude gradients. Indeed, the relationship between UVB penetration and latitude is complex, as a result of differences in, for example, the height of the atmosphere (50 percent less at the poles), cloud cover (more intense at the equator than at the poles), and ozone cover. The duration of sunlight in summer versus winter is another factor contributing to the complexity of the relationship. Geophysical surveys have shown that UVB penetration over 24 hours, during the summer months at Canadian north latitudes when there are many hours of sunlight, equals or exceeds UVB penetration at the equator (Lubin et al., 1998). Consequently, there is ample opportunity during the spring, summer, and fall months in the far north for humans (as well as animals that serve as food sources) to form vitamin D3 and store it in liver and fat. These factors may explain why latitude alone does not consistently predict the average serum 25OHD level of a population.

Effect of sunscreen on synthesis Sunscreens are used to protect the skin from ultraviolet A (UVA) and UVB waveband exposure that is associated with deoxyribonucleic acid (DNA) damage—the same UVB exposure that is needed for vitamin D synthesis. Experimental studies suggest that sunscreens can decrease cutaneous vitamin D synthesis (Misra et al., 2008). However, emerging evidence suggests that although sunscreens are effective, many may not actually be blocking UVB because they are improperly or inadequately applied. Thus, sunscreen use may not actually diminish vitamin D synthesis in real world use, although further study is needed to verify its actual impact (Diehl and Chiu, 2010; Springbett et al., 2010).

Other variables affecting synthesis A number of other variables can impede sun exposure and thus inhibit cutaneous vitamin D synthesis. Clothing is an effective barrier to sun exposure and the UVB waveband, but the effectiveness of sun blocking depends on the thickness or weave of the fabric (Diehl and Chiu, 2010). Likewise, ethnic practices, such as extensive skin coverage with clothing, urban environments that reduce or block sunlight, air pollution, and cloud cover that reduces solar penetration can variously reduce sun exposure. In contrast, high altitude reduces the atmospheric protection against UVB waveband and can increase risk for sun damage as well as increase vitamin D synthesis (Misra et al., 2008). There may be a role for measures of physical activity in affecting vitamin D synthesis, although many covariates may be relevant, and some have suggested that genetics can account for some of the differences in synthesis of serum 25OHD.

Adiposity Interpreting data on serum 25OHD concentrations in obese and overweight persons is particularly challenging. Data from NHANES showed lower circulating levels of 25OHD among young adult obese non-Hispanic white women compared with their leaner counterparts; the relationship appeared to be weaker among non-Hispanic blacks. Differences in physical activity levels partially explain these differences (Looker, 2005, 2007). However, overweight and obese persons in NHANES also reported lower use of dietary supplements than did leaner persons of the same age or gender group (Radimer et al., 2004; Picciano et al., 2007), suggesting that lower dietary exposures could also contribute to the lower serum 25OHD levels in obese and overweight people.

Sequestration of vitamin D into fat likely also plays a significant role in reducing the amount that can be presented to the liver for 25-hydroxylation. As noted previously, vitamin D is absorbed with fat as part of chylomicrons and is taken up first by peripheral tissues that express lipoprotein lipase, especially adipose tissue and skeletal muscle. This pathway predicts that increased adiposity should lead to lower serum 25OHD levels and, conversely, that weight loss should reduce peripheral sequestration and enable higher 25OHD levels. Consistent with this, not only does increasing adiposity correlate with lower 25OHD levels, but also a few studies of modest weight loss have found the circulating 25OHD levels to increase despite no increased intake of vitamin D from diet or sunlight exposure (Riedt et al., 2005; Reinehr et al., 2007; Zittermann et al., 2009; Tzotzas et al., 2010). The measured increase in serum 25OHD levels in overweight and obese individuals was about 1.5 nmol/L for a 100 IU/day vitamin D intake over 12 months (Sneve et al., 2008; Zittermann et al., 2009). Others found that obese subjects show a lower rise in serum 25OHD levels in response to both oral vitamin D intake and UVB exposure (Wortsman et al., 2000) or in a retrospective analysis in response to 700 IU/day vitamin D3 supplementation (Blum et al., 2008). It is interesting that in severely obese individuals after malabsorptive gastric bypass surgery, vitamin D supplementation resulted in a marked rise in serum 25OHD level of approximately 3 nmol/100 IU intake when the dose was 800 to 2,000 IU/day, but only a 1 nmol/L rise when intake was increased to 5,000 IU/day (Goldner et al., 2009).

African American ancestry Serum 25OHD levels are lower in African Americans compared with light-skinned population groups (Looker et al., 2008), yet the risk for fracture is lower for African Americans than for other ethnic groups (Aloia, 2008). It should be noted, however, that there is a wide range of variability among individuals of any race or ethnicity (Aloia, 2008). Serum 25OHD levels in African Americans and whites have been shown to be similarly responsive to vitamin D supplementation at 40°N latitude (equivalent to Philadelphia or Indianapolis), increasing by 1 to 2 nmol/L per 100 IU/day at a dose of 3,440 IU/day (Aloia et al., 2008), although at doses below 2,000 IU/day, serum 25OHD levels do not increase above 50 nmol/L in African American girls (Talwar et al., 2007). The significance of maintaining a higher serum 25OHD level in African Americans is not understood at this time because of a lack of evidence on extra-skeletal effects of vitamin D.

Size and frequency of dose Dosing of vitamin D daily, weekly, or monthly has been tested, and there are reports of annual dosing as well. The results of a study by Chel et al. (2008) suggested that daily (600 IU/day) and weekly doses of vitamin D will increase serum 25OHD levels more than monthly doses, but Ish-Shalom et al. (2008) using a dose of 1,500 IU/day found no difference due to timing of the doses. Ish-Shalom et al. (2008) suggested that the attenuated response to monthly dose in the Chel et al. (2008) study may have been due to poor compliance with a powdered monthly supplement compared with pills used for their daily and weekly doses. An alternative explanation is that only a lower (Chel et al., 2008) and not a higher (Ish-Shalom et al., 2008) dose is influenced by timing of the vitamin D supplement. Thus far, studies suggest that weekly and daily dosing give similar serum 25OHD responses.