Do you sometimes snack on a handful of nuts – say a few times per week? Unless you are a vegetarian, chances are that it’s not that much; half a handful every now and then is the estimate. That’s a pity, because by now you have probably heard that eating more nuts may protect your heart.
The American Heart Association recommends that a heart-healthy eating pattern should include 4 to 5 portions (~30 g, 1 handful) of nuts and seeds per week.
What about cancer prevention?
The World Cancer Research Fund / American Institute for Cancer Research report and its continuous updates recently concluded that there is not enough evidence to suggest that nuts by themselves may lower the risk of developing a cancer. Yet emerging studies suggest that higher consumption of nuts may have a role in cancer prevention. Researchers from the Brigham and Women’s Hospital in Boston, USA, investigated the role of nut consumption in prostate cancer development and survivorship after prostate cancer diagnosis and published the findings in the latest issue of the British Journal of Cancer. Among approximately 47,000 men followed over 26 years, 6,810 developed prostate cancer. Eating nuts frequently did not protect the men from developing prostate cancer or dying specifically from prostate cancer. However, those diagnosed with prostate cancer who consumed nuts five or more times per week reduced their risk of dying from other causes (than cancer) by more than 30 percent compared to the men who ate nuts once or less a month. According to Dr Ying Bao, the corresponding author of the publication, of the 4,346 men diagnosed with non-metastatic prostate cancer (one that has not spread from prostate to other places in the body) during the 26 years of follow-up, only about 10 percent died from prostate cancer. Roughly one third of the cancer patients died from cardiovascular disease and the rest from other causes. Although not specifically reported, survival benefits related to frequent nut consumption may be particularly linked to a reduced cardiovascular disease incidence and mortality among prostate cancer patients.
One of the strengths of this study, besides its prospective design and long follow-up, is the repeated assessment of nut consumption, which reduces misclassification of study participants according to their habitual consumption levels; here they were looking at tree nuts, including almonds, Brazil nuts, cashews, pecans, pistachios and walnuts. Diet is notoriously difficult to assess, which is particularly true for nuts. Reasons for these difficulties include the large variety of nuts available, which according to consumers’ perception often also include peanuts (botanically a legume) and/or dried fruits. Nuts are typically not consumed on a daily basis and not in large amounts; they are also often ingredients in breakfast cereals, sweets and baked goods or come as a salted mixed snack or as bread spreads. These difficulties in assessing nut consumption are probably one of the main reasons why there are not that many prospective studies on nut consumption and risk of cancer. A systematic review and meta-analysis on nut consumption and risk of cancer, published last year, only identified 11 prospective studies for overall cancer, with a suggestive cancer preventive role. So far, the most promising evidence based on three prospective studies relates to a 20% reduced risk of developing colon cancer. This meta-analysis also includes the results of the only study from the European Prospective Investigation into Cancer and Nutrition (EPIC) study on nut consumption and risk of cancer. Clearly, additional evidence is needed to more confidently assess the relationship between nut consumption and the prevention of individual types of cancer.
Which mechanisms could link nut consumption to a reduced risk of cancer?
Nuts are loaded with nutrients such as unsaturated fats, protein, vitamins (vitamin E, folate and niacin) minerals (magnesium, calcium and potassium) and phytochemicals – all of which may offer anti-carcinogenic, anti-inflammatory and antioxidant properties. Increasing evidence suggests that insulin resistance, a condition in which the cells of the body become resistant to the hormone insulin, is involved in the development of certain cancers, for example, cancers of the colon.
Other emerging mechanisms may play a role as well: a recent study conducted in mice found that adding ground walnuts to the diet of mice appeared to alter their gut bacteria and suppress colon tumours, especially in male mice. Previous research has shown that certain gut bacteria metabolize phytochemicals and fibre into compounds that have tumour-suppressing properties.
Concerned about weight gain when indulging in nuts?
Nuts are perceived by the general public as “fattening” because of their high-fat content. Indeed, nuts can provide 160–200 kcal per portion (30 g) and are thus a high energy dense food; in their energy-density similar to foods such as crackers, chocolate candies, and cookies etc. Whether or not frequent nut consumption may promote weight gain is not yet entirely conclusive. Weight gain may not occur if nuts are incorporated into a healthy diet in which they are substituted for other foods such as red meat or processed meat or refined carbohydrates, as opposed to being added to an existing diet. Several mechanisms have been proposed to explain a less pronounced effect of nut intake on weight gain, including increased satiety/supressed hunger due to the high dietary fibre and plant protein content of nuts. Although nuts are high in fat, the fat is mostly unsaturated, which together with the high protein content can lead to an increase in resting energy expenditure and diet-induced thermogenesis, both of which can reduce body weight and weight gain.
Currently ongoing analyses within the EPIC study on nut consumption and prospective weight change will contribute soon further evidence whether or not nuts are a non-fattening healthy snack. That’s also essential to clarify because staying lean is also important for cancer prevention.
Higher nut consumption may play a role in reducing risk of individual cancer types, but to be more confident about such a role, more aetiological research on nut consumption and risk of cancer is needed; ideally with improved exposure assessment, which would also call for identifying novel biomarkers of nut consumption.
He is a member of the working groups on colorectal cancer and liver cancer within the EPIC study.
His research focuses on the role of nutritional and metabolic factors in the aetiology of cancer, in particular colorectal cancers, and intermediate outcomes such as weight gain and obesity.
Research Interests: Cancer aetiology; dietary exposure assessment; dietary patterns; obesity; biostatistics.
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- Can higher nut consumption lower your cancer risk? - 19th July 2016
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