Plant size, latitude, and phylogeny explain within-population variability in herbivory.

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth.

Prospective study of toenail selenium levels and cancer among women.

BACKGROUND: Inverse associations between selenium status and cancer risk have been observed in animal studies, ecologic studies, and some case-control and prospective studies. Whereas results of some prospective studies have suggested an overall inverse relationship between selenium levels and cancer, other prospective studies have failed to confirm this finding. Prospective data on women are particularly limited because fewer women than men have been studied prospectively. PURPOSE: The aim of this study was to prospectively examine the relationship between selenium levels in toenails (previously shown to reflect selenium intake) and incidence of cancer among women. METHODS: The Nurses' Health Study cohort began in 1976 with 121,700 female nurses aged 30-55 years living in 11 U.S. states. In 1982, we requested toenail clippings from the members of the cohort, and 62,641 participants with no history of cancer returned these clippings. During 41 months of follow-up, 503 cases of cancer other than breast cancer (results previously reported) or nonmelanoma skin cancer were analyzed. For each case patient, a control subject was chosen from women who remained free of diagnosed cancer, matched by age and by date of nail return. RESULTS: No inverse association was observed between selenium levels in toenails and cancer risk. The age- and smoking-adjusted relative risk (RR), comparing the highest with the lowest quintile of toenail selenium level, was 1.44 (95% confidence interval [CI] = 0.97-2.13), and the trend across quintiles was marginally significant (two-sided P = .06). Comparing the highest with the lowest decile, the RR (age- and smoking-adjusted) was 1.77 (95% CI = 1.04-3.02). When these data were combined with the data from 434 breast cancer case patients and their matched control subjects identified in parallel from this same cohort, the RR comparing the highest with the lowest quintile was 1.24 (95% CI = 0.93-1.65). Toenail selenium level was not inversely associated with cancer at any major site, including uterine cancer, colorectal cancer, melanoma, ovarian cancer, or lung cancer (after adjusting for smoking); in fact, nonsignificant positive associations were observed at several sites. CONCLUSIONS: Toenail selenium levels were not inversely associated with cancer risk in this study. IMPLICATIONS: These data, in conjunction with previous findings of no association between toenail selenium status and breast cancer risk, strongly suggest that higher selenium intake within the range consumed by most U.S. women (as reflected by toenail selenium levels) is not protective against overall cancer incidence in women.

A 1-y trial of the effect of high-selenium bread on selenium concentrations in blood and toenails.

The concentration of selenium in toenail clippings and blood reflects dietary intake better than does intake calculated from dietary data because of the highly variable selenium concentration in different samples of the same food. However, the time course of selenium intake in relation to subsequent concentrations in toenail clippings is unclear. Therefore, 12 males were fed high-dose (4.91 mumol Se/d), medium-dose (2.61 mumol Se/d), or control (0.41 mumol Se/d) whole-wheat-bread for 1 y and the concentration of selenium was measured in toenail clippings collected every 12 wk for 2 y. Toenail selenium concentration was unaffected by dietary intake in the previous 3 mo and appeared to provide a time-integrated measure of intake over a period of 26-52 wk. Use of selenium concentration in toenail clippings may be an alternative to blood when a measure of long-term average intake is desired. The absence of a short-term effect of diet on toenail selenium concentration also makes this a useful marker of intake in retrospective studies.

Toenail samples as an indicator of drinking water arsenic exposure.

We conducted a pilot study to assess the utility of toenail arsenic concentrations as an indicator of ingestion of arsenic-containing water. We enrolled 21 individuals whose household drinking water supply was provided by a private well, including 10 individuals who lived in areas of New Hampshire where elevated water levels of arsenic had been reported previously. Participants were interviewed regarding use of their private (unregulated) wells for drinking and cooking, and each provided a sample of water and toenail clippings. All specimens were analyzed using instrumental neutron activation analysis with a sensitivity of approximately 0.001 parts per million (ppm). Trace concentrations of arsenic were detected in 15 of the 21 well water samples and in all toenail clipping samples. Among the 10 individuals who lived in areas with reportedly high arsenic levels in the water supply, the geometric mean toenail concentration was 0.39 ppm (SE, 0.12 ppm); among the other 11 persons, the geometric mean was 0.14 ppm (SE, 0.02 ppm; P = 0.005 for the difference between the two means). The overall Spearman correlation between toenail and well water arsenic was 0.67 (P = 0.009), and among those with detectable well water levels of arsenic, the Spearman correlation was 0.83 (P = 0.0001). Based on the regression analysis of those who had detectable water levels of arsenic, a 10-fold increase in well water concentrations of arsenic was reflected by about a 2-fold increase in toenail concentrations. These results indicate that concentrations of arsenic in toenails reflect use of arsenic-containing drinking water.

Toenail trace element levels as biomarkers: reproducibility over a 6-year period.

We assessed the reproducibility over a 6-year period of 16 trace elements measured in toenails by comparing levels in paired specimens collected in 1982-1983 and 1988 from 127 women in the United States. The Spearman correlation coefficients for the reproducibility of toenail levels of selenium and arsenic (both known to reflect intake of these elements) were 0.48 and 0.54. Correlations for other elements ranged from 0.26 (copper) to 0.58 (zinc). In utilizing biomarkers to assess exposure in epidemiological studies of cancer and other chronic disease, random within-person variability in exposure leads to attenuation of measures of association between exposure and disease. We demonstrate the effect of such variability on odds ratios from a hypothetical case-control study. For a true odds ratio of 3.0 (for a comparison of the highest quintile versus the remaining 4 quintiles of exposure) the odds ratios which would be observed in the presence of the degree of within-person variability demonstrated in this study were 2.15 for toenail arsenic and 1.67 for toenail copper levels. Toenail concentrations of certain trace elements are useful biomarkers of exposure in which a single sample is assumed to represent long-term exposure. However, substantial attenuation in measures of association may occur.

Toenail trace element levels and breast cancer: a prospective study.

The associations between toenail levels of five trace elements and breast cancer risk were studied among a cohort of 62,641 US women who provided toenail clippings and were free from diagnosed breast cancer in 1982. Among 433 cases of breast cancer identified during 4 years of follow-up and their matched controls, the odds ratios comparing the highest with the lowest quintiles and adjusted for established breast cancer risk factors were as follows: for arsenic, 1.12 (95% confidence interval (CI) 0.66-1.91); for copper, 0.91 (95% CI 0.59-1.42); for chromium, 0.96 (95% CI 0.61-1.52); for iron, 0.89 (95% CI 0.56-1.40); and for zinc, 1.09 (95% CI 0.70-1.70). Among postmenopausal women, a marginally significant positive association was observed between toenail chromium levels and breast cancer risk (odds ratio = 1.71, 95% CI 0.87-3.35) (p for trend = 0.07). However, the association between chromium and breast cancer risk was inverse among premenopausal women. Although data on the validity of toenail levels of certain of these elements are limited, these results do not provide evidence for an important effect of arsenic, copper, chromium, iron, or zinc on breast cancer risk.