The Basic Equation
As risk assessment issues go, dietary exposure assessment is
relatively simple:
Dose = Food Consumption
* Concentration
For safety assessments or risk assessments, the
concentration is usually known. So, determining
food intake is the only issue. That is
typically done using information from dietary surveys. The main survey in the United States is:
What We Eat in America
(WWEIA), NHANES is a national food survey conducted as a partnership between
the U.S. Department of Health and Human Services (DHHS) and the U.S. Department
of Agriculture (USDA). WWEIA represents the integration of two nationwide
surveys - USDA's Continuing Survey of Food Intakes by Individuals (CSFII) and
HHS' NHANES. Under the integrated framework, DHHS is responsible for the sample
design and data collection. USDA is responsible for the survey's dietary data
collection methodology, development and maintenance of the food and nutrient
databases used to code and process the data, and data review and processing.
Extracting the data from the database requires a computer
program; there are several available, including one developed by the EPA for
estimating pesticide that is available to the public (EPA, 2012). The resulting output is a statistical
distribution that describes the range of intakes in the U.S. population,
typically expressed as grams per person or grams per kilogram body weight.
For a safety assessment, one exposure value is chosen to
represent the entire population; usually by selecting an upper percentile. Like the generation of toxicology standards
(i.e. ADIs and RfDs), this is a regulatory art that embodies agency
policy. A “Level
of Concern” for a particular chemical in a particular food may be
calculated as follows:
Level of Concern =
ADI / Food Consumption
Like a speed limit, an LOC is a limit intended to reduce
risk. It doesn’t really define a point
at which there is a risk (i.e. a threshold), nor does it say anything about how big the risk is. If you want to know that, then you will need
a risk assessment.
Acute Dietary Exposure Assessment
WWEIA is a two day survey. If you want a two day exposure
assessment, that is just perfect. If you
want a one day exposure assessment that is no problem either; you can easily
generate a distribution of one day exposures. For really short exposures, per
serving consumption estimates are easily attainable as well. While it is not at all common (yeah,
regulation can be good), there are many chemicals that occur in food that can
have immediate effects. The short list
begins with anticholinergic pesticides that are still used precisely because
the only serious human health or environmental concern is from short term
exposure. There are also many naturally occurring compounds in food that can
have immediate health consequences; cyanogenic glycosides are especially
notable.
Since acute toxicity can occur after a single consumption
event, characterizing the range of contaminant concentrations is very
important. This can be done with either
an empirical distribution or by fitting a statistical distribution. The former is better when there are many
values (>100) and most of the values are above the limit of quantitation
(LOQ). Otherwise a fitted distribution
is useful for estimating values both below the LOQ and at higher percentiles.
Chronic Dietary Exposure Assessment
Effects occurring after long term exposure are surely less
important; if you are going to be poisoned at all, later is better. But still, when delving into the realm of
what might happen later, WWEIA is less useful.
Since more often than not, food safety issues involve medium to long
term effects, that is a problem.
For estimating lifetime food consumption, the one piece of
useful information that can be easily obtained from the WWEIA is average per
capita food intake. Since the survey is
designed to characterize the consumption habits of the entire population at all
ages, the average intake represents average lifetime exposure for the entire
population. For risk assessments where the dose-response relationship is
linear, or even approximately linear over the range of dietary exposures, then
the average per capita food intake (grams per kg body weight per day) can be
used to estimate population risk (e.g. disease frequency).
There are other surveys that can be used, at least in part,
to estimate long term food intake. In
particular:
NCI developed a new instrument
called the NHANES Food
Frequency Questionnaire (formerly called Food Propensity Questionnaire) and
supported its application in the National Health and Nutrition Examination
Survey (NHANES). This instrument collects much of the same information as a
food frequency questionnaire (FFQ), but without asking about portion size. The
resulting data can be employed in the NCI usual dietary intakes model as
covariates. By not asking about portion size, the NHANES FFQ requires less
respondent burden than other FFQs.
The NHANES FFQ does not over as broad of a range of food
categories as WWEIA, but it does cover major staples consumed over a one year
period. Since portion size is not
included, the NHANES FFQ must use portion sizes from WWEIA or some other
source. While the portion sizes will not
be individually matched, as long as variation in portion size is small relative
to the variation that is not serious problem; meaning a reasonable statistical
characterization of intakes over the span of a year is possible. One year is still not the same as “lifetime”,
but that is about as good as it gets.
NHANES also includes a detailed 30 day FFQ for fish
consumption that characterizes the frequency of consumption of many different
species of fish, which is useful for exposure assessments for methylmercury and
other contaminants in seafood.
Although estimating food intake is more difficult,
characterizing contamination in a food is easier. Because consuming many different food items
over many years will tend to draw from the entire distribution of
concentrations that will occur in food, the arithmetic average of the
distribution is a good approximation of exposure over a long period of time.
References
U.S. Department of Agriculture (2015). Agricultural Research Service, Beltsville
Human Nutrition Research Center, Food Surveys Research Group (Beltsville, MD)
and U.S. Department of Health and Human Services, Centers for Disease Control
and Prevention, National Center for Health Statistics (Hyattsville, MD). What We Eat in America.
USEPA (2012). http://www.epa.gov/pesticides/science/deem/. DEEM-FCID/Calendex Version 4.02/10.00 Release
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Post Note
Thesis Post #27. Part of Risk Assessment paradigm thread.
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