Fat intake, obesity and cancer
There is certainly evidence that saturated fat and calorie excess adds to the risk of developing cancer increase risks of side effects during therapy, reduce the cure rate and increase the risks of fatal diseases other than cancer
The evidence A large analysis of USA health professionals who had a high energy and fat intake subsequently were associated with an increased risk of advanced prostate cancer especially for cases with a young age at diagnosis or had a positive family history of cancer. At a recent conference in the National Institute of Health, a Cambridge University scientist, presented data to show that women who eat more than 90g of fat a day had twice the risk of developing breast cancer than those eating 40g per day. Overweight women also had an increased risk of breast cancer irrespective of their daily saturated fat intake.
After cancer has developed, the NSABP and other studies have demonstrated a strong association between obesity (adiposity, BMI>30kg/m2) and outcome after treatment for bowel, breast and prostate cancer, with the improvement in survival being a result of both decreased cancer and non-cancer related deaths such as heart attacks and strokes.
Other evidence for the risks of fats comes from studies using statins, which are now commonly used across the world to reduce blood fat levels by chemically reducing fat absorption from the gut. Five randomised trials have demonstrated fewer colon, breast, and melanoma in long term users of statins compared to controls. The data for prostate cancer however, is non conclusive as two large clinical cohort studies did not demonstrated a reduce risk with statin intake. A further cohort study of 16,976 subjects showed a 63% reduction of prostate cancer although this did not stand up to robust statistically analysis. Furthermore, it is also still not completely clear that statins may have a direct anti cancer affect independent of their ability to reduce blood fat levels.
The underlying mechanisms of why high fat intake and obesity affects the risk of cancer include:-
Oestrogen levels. Adiposity influences the production of and availability of the body’s sex hormones including oestrogen, androgens and progesterone. In post menopausal women oestrogen is made in the peripheral body fat whilst in pre-menopausal women it is produced primarily in the ovary. This may explain a higher risk of breast and endometrial cancer for overweight post menopausal women but not pre-menopausal women. Fortunately, oestrogen levels have been shown to reduce following weight reduction programmes.
Insulin and Insulin-like growth factor receptor (IGF-1). The increased risks of cancer or its rate of progression is not just hormone related. We know this because overweight women have a worse prognosis after cancer whether their cancers where sensitive to oestrogen (ER+ve) or not (ER-ve). One mechanism for a non hormone related increased risk of cancer progression is via IGF-1 which is higher in overweight people. Higher levels have also been associated with breast, ovary and prostate cancer. This protein, also higher in sedentary individuals, has been shown to promote cancer cell division (encourage proliferation), inhibit apoptosis (cells don’t die when they should) and encourage them to spread.
Leptin. This is a multifunctional neuro-endocrine hormone generated primarily from fat cells. There is a direct correlation with the amount of body fat levels and circulating blood levels of leptin. There is also a correlation between leptin and serum insulin, IGF-1 and progesterone levels. Leptin has been shown in several laboratory experiments to promote proliferation, reduce apoptosis and reduce the stickiness of cancer cells encouraging them to spread and metastasise. Higher leptin levels are associated with higher expression of Cycloxidase 2 which also, as described below, encourages cancers to grow faster and spread.
Progesterone. Another important hormone which would affect women is progesterone. Compared to women with ‘normal’ weight pre-menopausal obese women in particular have reduced serum progesterone. There is a significant body of evidence that progesterone plays a protective role in cancer progression particularly ovary. Progesterone increases in pregnancy which also adds some protection against breast and ovarian cancer. In post menopausal women who are not overweight (BMI < 35kg/m2) the evidence is less clear. The risk of breast cancer, in a one large study from Sweden, was higher in women taking HRT containing progestin than those containing oestrogen alone. On the other hand, another study of postmenopausal women with breast cancer from Boston USA, women with higher blood levels of oestrogen and androgens had a worse prognosis but no such correlation was found with progesterone. It well be therefore, that the protective affect of progesterone is greater in pre-menopausal women.
Types of fats
Healthy fats - Replacing saturated fats with polyunsaturates, and to a lesser extent, monounsaturates, have been shown to reduce serum blood cholesterol and other unhealthy fat levels. Evidence from two large prospective studies and a smaller case-control study suggests a protective effect of oily fish intake on cancer incidence and risk of dying from it. A unique nutritional component of fish is the long-chain marine omega-3 fatty acids. Cancer cells grown in the laboratory in petri dishes and in mice, as well as small human studies have suggested that marine omega-3 fatty acids or even more importantly the ratio of marine omega-3 to omega-6 fatty acid can modulate cancer progression. The entire reason for this not fully understood but one mechanism appeared to be via the cyclooxygenase-2 (COX-2) pathway, described below.
Another healthy fat is olive oil which provided it is not over processed, is predominantly the monounsaturated fatty acid oleic acid having strong antioxidant properties. Olive oil also exhibits a number of other advantageous biological functions, including an ability to reduce blood saturated fat levels. Environmental studies have shown that higher olive oil use correlates to a lower incidence of atherosclerosis, diabetes, inflammatory and autoimmune diseases, skin wrinkling and skin aging. For centuries, Greeks and Egyptians have also used olive oil topically for the treatment of what they termed “erythema” or redness and wealthy Romans would rub it on their skin before a steam bath.
The potent antioxidant effect of olive oil has been confirmed in cancer cells grown in the laboratory. They have demonstrated reduced DNA damage when exposed to carcinogens in the presence of oleic acid. In mice, massaging olive oil for 5 minutes every evening after a days exposure to UV light had a major effect on reducing skin damage and number of skin cancers. It was then found that the mechanism for this protection lies in the finding that the formation of 8-hydroxy-deoxyguanosine (8OhdG) in the mice skin was reduced by the application of olive oil. This is a marker of DNA damage so clearly the olive oil is protecting the DNA from the effect of the UV light.
More recently, oleic acid, in laboratory experiments, has been shown to suppress a protein on breast cancer cells called Her-2. Over expression of this protein is present in 25% of patients with breast cancer and is the main target for the biological agent herceptin. In general the higher the concentration (over expression) of the HER-2 protein the more aggressive the cancer. Olive oil can enhance the effects of herceptin by promoting apoptosis, reducing proliferation etc. It may have an affect on its own even without herceptin. This natural herceptin is being investigated in further clinical studies.
Harmful fats The saturated fats found in meat the harmful in excess last chapter explained why we should be aiming to cut down on fat, and particularly trying to replace. A particularly harmful group of fats are the trans-fats or hydrogenated fats. Hydrogenation is one of the processes that can be used to turn liquid oil into solid fat. The final product of this process is called hydrogenated vegetable oil. It's used in some biscuits, cakes, pastry, margarine, processed foods and particularly the fast food industry. The trans fats found in food have no known nutritional benefits and emerging evidence suggests that their may be worse than saturated fats and as a result, Denmark, Canada and the USA have started labelling foods with their trans-fat content.
Tips to reduce saturated and trans-fat intake:
Tips to increase unsaturated fats and omega-3:
Links and further resources:
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