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Routes by which lead is
absorbed by humans and its
fate in the body
Inorganic lead can, to some extent, be absorbed through the
skin, but primarily enters the bloodstream following ingestion of
contaminateddust,paintfragments,foodandwaterorinhalation
of dust. The primary route of exposure to lead in Europe is in the
diet (EFSA 2010). The amount and rate of absorption of ingested
lead depends on the individual (age, nutritional status etc.)
and the physical and chemical characteristics of the material
ingested. Children absorb proportionately more ingested lead
than adults. Once absorbed, lead is transported around the
body in the bloodstream. It is excreted primarily in faeces and
urine, but is also incorporated into hair and lost when hair is
shed. Lead is also transferred from the blood to soft tissues such
as the liver and kidneys and to bone where it accumulates. The
half-life of lead in blood is about 30 days, but in bone it is several
decades, although a labile compartment exists (USASTDR 2007).
Hence, lead is accumulated in the body over the lifetime of an
individual, primarily in bone, and lost only slowly. About 94% of
the total lead body burden in adults is in the bone, compared
with about 73% in children. Lead may be mobilised from bone
in times of physiological stress, resulting in elevated blood lead
concentrations (USATSDR 2007).
Quantity of gamebird
meat consumed annually
and minimum number of
consumers in the UK
We used data from the UK National Diet and Nutrition
Survey (NDNS) programme to estimate the mean quantity of
gamebird meat eaten per year by people in the UK (NatCen
Social Research 2014). NDNS provides detailed quantitative
information on food intake and diet composition based on
surveys of a representative sample of UK citizens. We used
data from the core survey based on 4-day diet diary results
collected in the four survey years 2008/09-2011/12 (NatCen
Social Research 2014). We used data from the 4,071 subjects
for whom the diet was reported on all four diary survey days.
For each subject, we extracted the variable GameBirdsg, which
is the mean quantity in grams of gamebird meat consumed
per day. This is the only measure of game meat consumption
included in the NDNS. This variable was non-zero for 87
subjects. We coded the age of each subject as the midpoint of
the age class. For example, the midpoint of the age class coded
as 15 years was 15.5. The exception to this was the age class 1
year. The survey only covers children older than 1.5 years, so
this class midpoint was coded as 1.75 years.
To relate the proportion of subjects for which consumption of
gamebirdmeat was reported in the 4-day diary period to subject
age and sex, we fitted three asymptotic non-linear models:
Pg = exp(A) Model 1,
Pg = exp(A – B exp(-C Age)) Model 2,
Pg = exp(As – B exp(-C Age)) Model 3,
where Pg is the proportion of subjects for whom gamebird
consumption is reported, A is a constant representing the
logarithm of the asymptotic proportion of subjects who eat
gamebird meat, B and C are constants and Age is the age class
midpoint in years. The parameter A was assumed not to differ
between males and females in Model 2, but to take different
values for the two sexes in Model 3. We calculated the binomial
probability of observing the recorded numbers of subjects of
each age and sex who did and did not consume gamebird meat
under eachof the threemodels. For eachmodel, weused a quasi-
Newton algorithm to obtain the parameter values at which the
log-likelihood of the data was maximised. We used bootstrap
resampling, with replacement, of the 4,071 subjects to obtain
confidence intervals of parameter estimates and derived values.
We performed 1,000 bootstrap replicates and took the bounds
defined by the central 950 bootstrap estimates to represent the
95% confidence limits.
Model 2, which assumes that the proportion of people who
consumed gamebird meat changed with age, but did not
differ by sex, had the lowest value of the Akaike Information
Criterion (AIC) (Model 1 AIC = 843.29, Model 2 AIC = 832.64,
Model 3 AIC = 833.95). Likelihood-ratio tests indicated a
highly statistically significant effect of age on the proportion
consuming gamebird meat (Model 2 vs Model 1, χ2
(2) = 14.65, P
= 0.0007), but no indication of a significant effect of sex (Model
3 vs Mode l 2, χ2
(1) = 0.69, P = 0.405). We therefore selected
Model 2 as providing an adequate description of the data. The
proportion of subjects consuming gamebird meat increased
most rapidly with advancing age over about the first 20 years,
being less than 1% for the youngest infants and about 3% for
adults (see Figure 1).
Rhys E. Green & Deborah J. Pain