Nutritional Medicine: Sweet Potatoes
By Sarah Brewer
How often do you eat sweet potatoes? By enjoying them regularly, perhaps once or twice a week, you could reduce your risk of developing breast or prostate cancer.
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The sweet potato (Ipomoea batatas) was brought from Central America to Europe by Columbus and, gram for gram, they are one of the most nutrient dense foods available.
In Japan, both the tuberous roots and leaves are a dietary staple – especially in Okinawa where, together with fish, seaweed and turmeric, their consumption is associated with extraordinarily long lifespans.
Nutrition in Disguise
Sweet potatoes come in a range of colors of which those with an orange flesh are exceptionally rich in betacarotene, an antioxidant carotenoid which your cells can convert into vitamin A (retinol) when needed.
Just 100g of orange-fleshed sweet potato provides over 8500mcg of betacarotene, which is at least as good as new carrots. If you ever see the red or purple-fleshed versions for sale, grab them quick, too, as they are unusually rich in anthocyanins; one study found the antioxidant activity of purple/red sweet potatoes was more than three-fold higher than blueberries.
Sweet potatoes have a lower starch content than normal white potatoes and therefore have less impact on blood glucose levels.
Also, this is assessed by their glycemic index and glycemic load (boiled sweet potato GI 44 / GL 11 versus boiled Maris Piper white potato GI 85, GL 25). As a result, sweet potatoes are a valuable medicinal food for treating diabetes in Kagawa, Japan, while the leaves are used as an antidiabetic remedy in Ghana.
Sweet potatoes are one of the richest dietary sources of oestrogen-like plant hormones classed as lignans.
Like their close cousins, the isoflavones, these are present in an inactive form. When you eat them, certain bowel bacteria convert them to mammalian enterolignans (eg enterolactone and enterodiol) which have a weak, oestrogen-like action. By interacting with human oestrogen receptors, these enterolignans block access to the stronger human oestrogens which are associated with hormone-dependent cancers of the prostate and breast.
Enteroligans also have anti-inflammatory and anti-cancer actions by reducing the proliferation of abnormal cells, and triggering their programmed cell death (apoptosis).
The Healthy Choice
Enterolignans are also noteworthy in having a unique ability among plant oestrogens to inhibit the enzyme, 5-alpha reductase, which converts testosterone to the stronger dihydrotestosterone (DHT).
The formation of DHT in scalp hair follicles is linked with male-pattern baldness, while in the prostate gland increased DHT levels are associated with both benign prostate enlargement (BPH) and prostate cancer. DHT concentrations are five times higher in an enlarged prostate gland than in those of normal size, for example, and if the conversion of testosterone to DHT is prevented, by blocking the action of 5-alpha reductase, BPH does not occur and can even be reversed once it has developed.
Protection Against Prostate Cancer
Enterolactone and enterodiol are able to suppress the growth of prostate cancer cell cultures through both hormone-dependent and independent mechanisms.
Recent studies also suggest that dietary lignans may block cancer cell proliferation in men with a diagnosis of localised prostate cancer.
One Scottish study, involving over 900 men, aged 50 – 74 years, found that those with the highest blood concentrations of enterolactone were 60% less likely to develop prostate cancer compared with those who had the lowest levels, suggesting that dietary lignans protect against prostate cancer in this population.
Some other studies found no significant association between blood lignan concentrations and overall risk of prostate cancer
Make it a Habit
However, although this could be because dietary intakes were too low to show a protective effect, or because of the individual genetic profiles of the men involved.
It’s now known that enterolactone exerts the greatest protection in men with a particular gene profile that is known to increase the risk of prostate cancer. In this group of men, those with a low enterolactone concentration are 28% more likely to have prostate cancer than those with the highest blood enterolactone concentrations.
Protection Against Breast Cancer
A large meta-analysis, which pooled data from 21 studies (11 prospective cohort studies and 10 case-control studies) found that lignan intakes were associated with an 8% reduction in breast cancer risk; although this was not statistically significant when all age groups were taken into account.
In postmenopausal women, however, a high lignan intake was associated with a significant 14% reduction in the risk of breast cancer compared to those with the lowest intakes.
In women who already have a diagnosis of breast cancer, a higher intake of dietary lignans is associated with a lower mortality.
A study involving over 2650 postmenopausal women with breast cancer, who were followed for over 6 years on average, found that those with the highest blood levels of enterolactone and enterodiol were 40% less likely to die from any medical cause during follow-up than those with the lowest levels. The researchers suggested that postmenopausal women with breast cancer who have high serum enterolactone levels might have better survival.
Reach for Sweet
Good for Health
A meta-analysis of five studies exploring lignan intakes and breast cancer prognosis found that those with the highest intakes had a 43% lower risk of all-cause mortality and a 46% lower risk of breast-cancer-specific mortality compared with those whose intakes were low.
A significant association remained after accounting for weight, smoking status and physical activity so these findings were not just down to having adopted a healthy lifestyle.
So next time you plan to eat potatoes, consider opting for sweet potatoes instead.
They are eaten in a similar way – baked, mashed or added to soups and stews for example – but can also be used raw as in this delicious Sweet Potato, Fig & Feta Salad. I promise you’ll love them for their flavor as much as for their health benefits.