 |
Vitamin C and Cardiovascular Diseases (Heart Disease and
Stroke)
Vitamin C and Cardiovascular Diseases
(heart disease and stroke):
Seven out of 12 prospective studies, which examined large
numbers of people (700 to 87,000) over a number of years
(3 to 20), found a significant relationship between higher
levels of vitamin C intake and a lower risk of heart disease
and stroke. The remaining studies, which did not find a
relationship between vitamin C intake and cardiovascular
diseases, compared individuals who were already consuming
close to 100 mg daily with those consuming higher amounts.
A careful experimental study at the NIH demonstrated that
some human tissues (leukocytes) tend to become saturated
with vitamin C at a dose of 100 mg/day. Thus, it is possible
that once tissue saturation has been achieved, additional
protective effects of vitamin C against cardiovascular diseases
are small and therefore difficult to detect in prospective
studies. Consistent with this possibility, at least 6 prospective
studies have found low blood levels of vitamin C at baseline
to be associated with a subsequent increase in the risk
of heart disease or stroke. In a prospective study that
followed more than 2,000 residents of a rural Japanese community
for 20 years, the risk of stroke in those whose blood levels
of vitamin C were in the highest quartile (1/4) was only
59% of those whose blood levels were in the lowest quartile.
Additionally, the risk of stroke in those who consumed vegetables
6 to 7 days of the week was only 58 % of the risk in those
who consumed vegetables 0 to 2 days of the week. The participants'
blood levels of vitamin C were highly correlated with their
fruit and vegetable intake. Therefore, as in many studies
of vitamin C intake and cardiovascular disease risk, it
is difficult to separate the effects of vitamin C on stroke
risk from the effects of other components of fruits and
vegetables, emphasizing the benefits of a diet rich in fruits
and vegetables.
Vitamin
C and Cancer
- Preventing
A large number of studies have shown
that increased consumption of fresh fruits and vegetables
is associated with a reduced risk for most types of cancer.
Such studies are the basis for dietary guidelines endorsed
by the U.S. Department of Agriculture and the National Cancer
Institute, which recommend at least 5 servings of fruits
and vegetables per day. A number of case-control studies
have investigated the role of vitamin C in cancer prevention.
Most have shown that higher intakes of vitamin C are associated
with decreased incidence of cancers of the mouth, throat
and vocal chords, esophagus, stomach, colon-rectum, and
lung. Because the possibility of bias is greater in case
control studies, prospective studies are generally given
more weight in the evaluation of the effect of nutrient
intake on disease. In general, prospective studies in which
the lowest intake group consumed more than 86 mg of vitamin
C daily have not found differences in cancer risk, while
studies finding significant cancer risk reductions found
them in people consuming at least 80 to 110 mg of vitamin
C daily.
A prospective study of 870 men over a period of 25 years
found that those who consumed more than 83 mg of vitamin
C daily had a striking 64% reduction in lung cancer compared
with those who consumed less than 63 mg per day. Although
most large prospective studies found no association between
breast cancer and vitamin C intake, two recent studies found
dietary vitamin C intake to be inversely associated with
breast cancer risk in certain subgroups. In the Nurses'
Health Study, premenopausal women with a family history
of breast cancer who consumed an average of 205 mg/day of
vitamin C from foods had a 63% lower risk of breast cancer
than those who consumed an average of 70 mg/day. In the
Swedish Mammography Cohort, women who were overweight and
consumed an average of 110 mg/day of vitamin C had a 39%
lower risk of breast cancer compared to overweight women
who consumed an average of 31 mg/day. A number of observational
studies have found increased dietary vitamin C intake to
be associated with decreased risk of stomach cancer, and
laboratory experiments indicate that vitamin C inhibits
the formation of carcinogenic compounds in the stomach.
Infection with the bacteria, helicobacter pylori (H. pylori)
is known to increase the risk of stomach cancer and also
appears to lower the vitamin C content of stomach secretions.
Although two intervention studies did not find a decrease
in the occurrence of stomach cancer with vitamin C supplementation,
more recent research suggests that vitamin C supplementation
may be a useful addition to standard H. pylori eradication
therapy in reducing the risk of gastric cancer.
- Treating
Studies in the 1970's and 1980's conducted
by Linus Pauling and colleagues suggested that very large
doses of vitamin C (10 grams/day intravenously for 10 days
followed by at least 10 grams/day orally indefinitely) were
helpful in increasing the survival time and improving the
quality of life of terminal cancer patients. However, two
randomized placebo-controlled studies conducted at the Mayo
clinic found no differences in outcome between terminal
cancer patients receiving 10 grams of vitamin C/day orally
or placebo There were significant methodological differences
between the Mayo Clinic and Pauling's studies, and recently,
two researchers from the NIH suggested that the route of
administration (intravenous versus oral) may have been the
key to the discrepant results. Intravenous (IV) administration
can result in much higher blood levels of vitamin C than
oral administration, and levels that are toxic to certain
types of cancer cells in culture can be achieved with intravenous
but not oral administration of vitamin C. Thus, it appears
reasonable to reevaluate the use of high-dose vitamin C
as cancer therapy.
Currently, there is no clinical evidence suggesting that
vitamin C would adversely affect the survival of cancer
patients. However, vitamin C should not be used in place
of therapy that has been demonstrated effective in the treatment
of a particular type of cancer, for example, chemotherapy
or radiation therapy. If an individual with cancer chooses
to take vitamin supplements, it is important that the clinician
coordinating his or her treatment is aware of the type and
dose of each supplement. While research is underway to determine
whether combinations of antioxidant vitamins might be beneficial
as an adjunct to conventional cancer therapy, definitive
conclusions are not yet possible.
In a presentation at a meeting of the American Cancer Society,
a scientist suggested that supplemental vitamin C might
enhance the growth of cancer cells or protect them from
cell-killing free radicals produced by radiation and some
forms of chemotherapy. An article published in the Spring/Summer
2000 issue of the Linus Pauling Institute newsletter, Is
vitamin C harmful for cancer patients?, provides additional
insight on this topic.
Vitamin
C and Cataracts
Cataracts are a leading cause of blindness
in the U.S. Cataracts occur more frequently and become more
severe as people age. Decreased vitamin C levels in the
lens of the eye have been associated with increased severity
of cataracts in humans. Some, but not all, studies have
observed increased dietary vitamin C intake and increased
blood levels of vitamin Cto be associated with decreased
risk of cataracts. Those studies that have found a relationship
suggest that vitamin C intake may have to be higher than
300 mg/day for a number of years before a protective effect
can be detected. Recently, a 7-year controlled intervention
trial of a daily antioxidant supplement containing 500 mg
of vitamin C, 400 IU of vitamin E, and 15 mg of b-carotene
in 4,629 men and women found no difference between the antioxidant
combination and a placebo on the development and progression
of age-related cataracts. Therefore, the relationship between
vitamin C intake and the development of cataracts requires
further clarification before specific recommendations can
be made.
Vitamin
C and Lead toxicity
Although the use of lead paint and leaded
gasoline has been discontinued in the U.S., lead toxicity
continues to be a significant health problem, especially
in children living in urban areas. Abnormal growth and development
has been observed in infants of women exposed to lead during
pregnancy, while children who are chronically exposed to
lead are more likely to develop learning disabilities, behavioral
problems, and to have low IQs. In adults, lead toxicity
may result in kidney damage and high blood pressure. In
a study of 747 older men, blood lead levels were significantly
higher in those who reported total dietary vitamin C intakes
averaging less than 109 mg/day compared to men who reported
higher vitamin C intakes. A much larger study of 19,578
people, including 4,214 children from 6 to 16 years of age,
found higher serum vitamin C levels to be associated with
significantly lower blood lead level. An intervention trial
that examined the effects of vitamin C supplementation on
blood lead levels in 75 adult male smokers found that 1,000
mg/day of vitamin C resulted in significantly lower blood
lead levels over a 4-week treatment period compared to placebo.
A lower dose of 200 mg/day did not significantly affect
blood lead levels, despite the finding that serum vitamin
C levels were not different than those of the group that
took 1,000 mg/day. The mechanism for the relationship between
vitamin C intake and blood lead levels is not known, although
it has been postulated that vitamin C may inhibit intestinal
absorption or enhance urinary excretion of lead.
Vitamin
C and Cardiovascular Diseases
- Vasodilation
The ability of blood vessels to relax
or dilate is compromised in individuals with atherosclerosis.
The damage to the heart muscle caused by a heart attack
and damage to the brain caused by a stroke is related, in
part, to the inability of blood vessels to dilate enough
to allow blood flow to the affected areas. The pain of angina
pectoris is also related to insufficient dilation of the
coronary arteries. Treatment with vitamin C has consistently
resulted in improved dilation of blood vessels in individuals
with atherosclerosis as well as those with angina pectoris,
congestive heart failure, high cholesterol, and high blood
pressure. Improved blood vessel dilation has been demonstrated
at a dose of 500 mg of vitamin C daily.
-Hypertension (high blood pressure)
Individuals with high blood pressure
are at increased risk of developing cardiovascular diseases.
Several studies have demonstrated a blood pressure lowering
effect of vitamin C supplementation. One recent study of
individuals with high blood pressure found that a daily
supplement of 500 mg of vitamin C resulted in an average
drop in systolic blood pressure of 9% after 4 weeks. It
should be noted that those participants who were taking
anti-hypertensive medication continued taking it throughout
the 4-week study. Because the findings regarding vitamin
C and high blood pressure have not yet been replicated in
larger studies it is important for individuals with significantly
high blood pressure to continue current therapy (medication,
lifestyle changes, etc.) in consultation with their health
care provider.
Vitamin
C and Diabetes Mellitus
Numerous observational studies have found
that people with diabetes have lower plasma levels of vitamin
C (approximately 30% lower) than do people without diabetes.
However, a number of methodological flaws have been attributed
to such studies, and it is not clear whether diabetes is
the cause of lower plasma levels of vitamin C. Recently
a large population-based study found no difference in blood
levels of vitamin C between over 200 individuals with newly
diagnosed diabetes and 1,800 individuals without diabetes
once dietary intake of vitamin C and cigarette smoking were
taken into consideration. Though few studies have demonstrated
improved blood glucose levels upon supplementation of diabetic
individuals with vitamin C, doses of between 100 and 600
mg of vitamin C daily have been found to normalize cellular
sorbitol levels, which may have implications for decreasing
some of the long-term complications of diabetes. Cardiovascular
diseases are the leading cause of death in individuals with
diabetes. Vitamin C has also been found to improve blood
vessel dilation, which is often impaired in diabetic individuals.
The link between cardiovascular diseases and diabetes may
be related to increased oxidative stress, giving credibility
to the idea that the antioxidant properties of vitamin C
may benefit individuals with diabetes. While the role of
vitamin C in the management of diabetes is by no means clear,
maintaining an adequate intake of vitamin C may help prevent
some of the complications of diabetes.
Vitamin
C and Common Cold
The work of Linus Pauling stimulated
public interest in the use of large doses (greater than
1 gram/day) of vitamin C to prevent infection with the viruses
responsible for the common cold. Reviews of the research
conducted on this issue over the past 20 years conclude
that, in general, large doses of vitamin C do not have a
significant effect on the incidence of the common cold.
However, a few studies have indicated that certain susceptible
groups (e.g., individuals with low dietary intake and marathoners)
may be less susceptible to the common cold when taking supplemental
vitamin C. Additionally, large doses of vitamin C have been
found to decrease the duration and severity of colds, an
effect that may be related to the antihistamine effects
found to occur with large doses (2 grams) of vitamin C.
|
