Summarized by Robert
W. Griffith, MD
January 21, 2005
Introduction
Lead is a toxic substance that can enter the body in a number of ways - inhalation
of lead dust (paint) or fumes (gasoline), or through the gastrointestinal tract
(contaminated hands, food, or water). Once in the body, lead is distributed by
the blood to all parts, but more than 90% of the total body lead is accumulated
in the bones. From here it can be released into the blood again, re-exposing
organs long after the original exposure.
Numerous organs and tissues are affected by lead, to varying degrees. Common
lead toxicities are: neuropathy (nerve damage), seizures, fatigue, nausea, vomiting,
constipation, miscarriage, infertility, anemia, and kidney damage. Now a new
organ must be added to the list: the cornea. A recent publication has reported
a link between bone lead levels and the occurrence of cataracts in older men.
It's published in the Journal of the American Medical Association, and summarized
here.
What was done
Fortunately the bone lead levels can be measured by a type of radiography: K
x-ray fluorescence. Levels in the tibia (lower leg bone) and patella (knee-cap)
were measured in 642 men over 60 who were taking part in the Normative Aging
Study (NAS) running in Boston, Massachusetts. All participants in the NAS had
full physical exams every 3 to 5 years that included an extensive eye exam.
The subjects were classified into 5 groups (called quintiles) according to
their tibia lead levels. This allowed analysis for a link between high lead levels
and the presence of cataracts.
What was found
The average age of the subjects was 69. The concentration of lead in the tibia
ranged from 0 to 126 microgram/gram of bone, and in the patella from 0 to 165
microgram/gram. Blood levels ranged from 0 to 35 microgram/dL. Higher bone lead
levels were found in those who were older, smoked more, and had diabetes.
There were 122 cases of cataract among the 622 men. After adjusting for the
possible effect of age, the men in the highest lead level quintile were 2.7 times
as likely to have a cataract as those in the lowest quintile.
After further adjustments for possible effects of smoking, diabetes, and vitamin
C or carotenoid intake (which can influence blood lead levels), the highest quintile
group were 3.2 times as likely to have a cataract as those in the lowest quintile.
Finding for the patella lead levels were similar. Blood lead levels were not
significantly associated with cataract; this was not surprising, as they are
more likely to reflect short-term lead exposure.
What this means
Over 80% of home built in the USA before 1980 are contaminated with lead-based
paint or leaded water pipes. It's not surprising that most US adults have accumulated
a substantial amount of lead in their bones. This present study suggests that
cumulative lead exposure is a risk factor for cataract, a condition responsible
for 40% of the cases of blindness worldwide.
The investigators propose several ways that lead can promote the development
of cataract. First, the metal can upset the electric potential in the lens that's
necessary to maintain lens clarity. Alternatively, lead may interrupt the metabolism
in the lens of two substances, glutathione and malondialdehyde, allowing one
or both to accumulate there. Finally, lead can interfere with calcium levels
in tissues, and this may occur in the lens.
As we age, the likelihood of needing cataract surgery increases. About 1.5
million cataract surgeries are done each year in the USA, and their cost is the
largest single item in the Medicare budget. Approximately 10 million cataract
surgeries are done in the entire world each year. The cataract burden is greater
in many developing countries, and that's where lead exposure continues to be
high. It would be a wonderful thing if ongoing global reduction of lead exposure
resulted in decreased cataracts around the world.
Source
Accumulated lead exposure and risk of age-related cataract in men. DA. Schaumberg,
F. Mendes, M. Balaram, et al., JAMA, 2004, vol. 292, pp. 2750--2754 |
