Flashcards in Diet & human cancer- a difficult subject to study- L15 Deck (24)
Why is is difficult to prove that a strong geographic patter in a diet is linked to cancer?
- populations food intake is not accurate= all guesses from FFQ and food diaries
- very difficult to control confounding factors between dietary variable
- also very difficult to control for confounding factors between dietary and other economically and culturally determined variables - many confounding factors may not be known and therefore they cant be controlled for
What are the limitations of observational studies ?
- need to recall habitual diet
- period in which recall relates maybe not be the most aetiologically relevant period
- important confounders may not be known, measured or measured accurately
- each dietary variable is a potential causes and a potential confounder
Why are animal and mechanistic studies useful ?
they are useful for generating a hypothesis and can add considerably to the plausibility of an association
- on their own they are unable to answer the question of cause in humans
- they can also be quite misleading because animals dont tend to live as long as us
What are the criteria for establishing a cause and effect in diet and cancer?
1) repeatable epidemiology studies, minimising bias and identifying confounding factors
2) time course of events should be logical- e.g. the increase in lung cancer incidence must correlate with increased smoking for it to be associated as a cause
3) support from animal studies, human cell cultures
4) dose-dependent relationship between factor and cancer risk- without this then there is NO proof
5) a mechanism has to be established
Where can environmental factors induce their effects to lead to cancer?
induce mutations in...
- factors controlling proliferation
- factors controlling differentiation
- factors controlling apoptosis
- factors controlling DNA repair
- in DNA
What are the 2 most important ways to reduce cancer risk?
1) avoidance of cancer causing agents. eg. smoking
2) habitual consumption of diets high in food that protect against cancer- carrying out a positive effect, less people are as good at this aspect
What dietary factors reduces risk of all causes of mortality ?
consumption of fruit and veg
- improves protection
- pattern is consistent from many studies
What studies have shown reduced cancer risk with consumption of fruit and veg ?
epidemiological, meta-analysis case-control, meta-analysis cohort studies
cohort studies estimated reduced risk of breast, lung, bladder, stomach, colon and rectum with an increase in fruit and veg intake of 100g/day
What was seen in pancreatic cancer risk and consumption of fruit and veg ?
with veg consumption there is a almost perfect linear correlation between reduced risk and increased veg consumption
with fruit consumption the protective effect is less defined and therefore it has been suggested that veg may have a slightly more dominant protective effect
Why might fruit and veg consumption reduce risk of cancer?
- full of antioxidants ?
- contain many nutrients that keep body healthy ?
- eating lots of fruit and veg is associated with a healthy lifestyle so less smoking and obesity for example ?
- action of eating lots of fruit and veg is genetically linked to reduced cancer risk- genetically pre-disposed and therefore they have a lower risk
-eating lots of fruit and veg means other things aren't being eaten ?
- veg contains other, non-nutrient factors that protect against cancer?
What did Dr. Walter Willett from harvard uni state about antioxidants ?
- antioxidants are essential for good health
- they stand ready to neutralise free radicals
- substances that protect tissues, cells and important compounds against destructive power of oxygen and its relative
- free radicals are dangerous and over time their damage can build up and contribute to cancer, heart disease and arthritis etc
- stated that they could help fight off heart disease, cancer and age related eye problems
What is the antioxidant hypothesis?
"the antioxidant hypothesis proposes that vitamin C, vitamin E, carotenoids and other antioxidant nutrients afford protection against chronic diseases by binding to free radicals and decreasing oxidative damage
What is the direct antioxidant action ?
protection of cellular DNA, proteins and lipids against toxins and free ROS
therefore long term health benefits is a reduction in risk from toxins
How can the "antioxidant hypothesis" be tested?
could give 1 group supplements and then compare them to a control group
- it would need to be done over a very long time
What was demonstrated with beta-carotene supplements and risk of cardiovascular disease?
18 cohort studies and 8 randomised controlled trials showed there was no significant reduction in risk for high vs low intake
in 2 randomised controlled trials, it increased risk of lung cancer
What was demonstrated with vitamin E supplements and risk of cardiovascular disease?
4 of 8 studies showing reduction in risk for high vs low intake
1 studies demonstrated increased risk for high intake
none of the 4 randomised trials found any benefit from supplementation
What was demonstrated with vitamin C supplements and risk of cardiovascular disease?
2 of 11 studies showed significant reduction in risk for high vs low intake
1 study showed an increased risk associated with high intake
none of the 2 randomised controlled trials showed any beneficial effects
What have studies shown about antioxidant supplementation and cancer risk?
they have failed to show any consistent benefit and some trials even suggest harm
therefore overall the antioxidant hypothesis cant be true, modifications need to be made to it
What does the dogma dictate about ROS?
they are lethal to microbes and to DNA, proteins and lipids
ROS are linked to cancer and heart disease BUT
in a study where the large conductance calcium activated potassium channels in phagocytic cells were inhibited, the production of ROS still occurred but they lacked the ability to induce microbial killed and digestion activities were abolished. Maybe there are other factors involved
What are the conclusions about antioxidants and cancer risk ?
- antioxidant supplementation provides no protection against cancer
- supplementation may actually cause harm
- ROS may not actually be as toxic as originally thought
- "antioxidant hypothesis" requires amendments
What does the WCRF state about dietary supplementation ?
suggests that you should aim to meet nutritional needs through diet alone
PUBLIC HEALTH GOAL= maximise proportion of the population achieving nutritional adequacy without dietary supplements
dont use supplements to protect against cancer
What has been indicated for carotenoids and cancer risk ?
carotenoids within food may be protective against lung cancer
Pro-vitamin A carotenoids shown to be protective against oesophageal cancer
Why are fruits and veg protective ?
- contain nutrients= nutrients are important for normal functioning of cells
- fibre= good for intestine, can bind carcinogens preventing/reducing their absorption within the intestine
- contain protective factors