Patients often come seeking our advice on what foods are good for them to eat and expecting clear-cut answers. These aren’t the sorts of questions that answers come easily for. Take wine as an example. Sure it lowers risk of cardiovascular disease but it raises risk of upper GI cancers. Then again it lowers risk of anal cancer, oh but it increases risk of breast cancer recurrence. The answer to whether it is good to drink wine is, “It depends.” This isn’t what the patient wants to hear; they want us to pronounce a food to either good or bad. They seem to mistake us for a religious leader who has all the answers. PubMed does not have all the answers, nor do we.
I remind patients that I practice medicine not religion.
A new study that provides a rationale for eating potatoes with meat potatoes stirred up these thoughts on food.
I typically express a low opinion of potatoes and discourage patients from eating them. I have several reasons for disparaging them. Foremost is their amazingly high glycemic index.
On Harvard University’s Glycemic Index a baked russet potato scores at 111 (on a scale on which pure glucose is 100) [i]. Peanut M&M’s by contrast score at 33.
Whether it’s this high glycemic index at fault or not isn’t clear, but it appears eating potatoes may be rather bad for cancer patients, well at least for lung cancer patients in Denmark. Consuming large quantities of fruits and vegetables is considered a useful strategy in treating cancer. This appears to make a difference in lung cancer. A 2006 study reviewed data from a Danish cohort of 57,053 subjects. Individuals with lung cancer who had the highest intakes of vegetables had a hazard ratio (HR) of dying of 0.84, and those with high fruit intakes had a HR of 0.81. High potato intakes increased the hazard ratio to 1.51. [ii] Eating potatoes may be far worse for cancer patients than the benefit of eating fruits and vegetables.
There’s another reason to think twice about eating potatoes. They contain substantial amounts of two glycoalkaloids, namely solanine and chaconine that can wreck havoc on a person’s bowels.
These chemicals disrupt gut epithelial barrier integrity and aggravate or maybe even cause inflammatory bowel disease (IBD). Frying potatoes concentrates glycoalkaloids so it is no surprise to read that IBD incidence is highest in countries where fried potato consumption is highest.
So that is enough bad news about potatoes. Here’s some better news: Most bakers already appreciate potatoes when making bread as potatoes are exceedingly high in potassium and adding them to yeasted breads stimulates the yeast to grow faster producing lighter airier breads and doughnuts. I’m sure none of you really care about this bit of good news. But interestingly, while bread yeast loves potatoes, Candida, that ‘other yeast,’ hates potatoes.
A 2012 article published in the International Journal of Molecular Science by JK Lee et al describes how an isolated and purified extract from potatoes has a pronounced antifungal effect in particular against Candida albicans. This compound was effective enough that we may someday see it turned into an anti-fungal drug. [iv]
Here’s the most recent good news. In August 2014 Karen Humphreys and colleagues reported that cold potato starch could offset the increased colon cancer risk caused by a high red meat diet. In their small trial, 23 volunteers ate a diet heavily loaded with red meat for a month. Tissue samples from the volunteers intestines showed that this diet increased the number of micro RNA molecules, a bad thing when it comes to colorectal cancer as elevated micro RNAs seem to aid and abet cancer cells and are associated with poor outcomes for people with this type of cancer.
The following month, the same volunteers kept up their red meat diet but added what is called a ‘resistant starch’ in the form of a drink. ‘Resistant starches’ are found in cold potatoes. Adding the starch brought levels of micro RNA back down to the level they had been before starting the high meat diet. [v]
Thus eating cold potato salad with one’s portion of meat may in the end be protecting people from colon cancer. Who would have guessed?
Can we find any evidence that eating potatoes lowers risk of colorectal cancer?
That’s tricky as in diet surveys meat and potatoes are often lumped together as a dietary pattern in contrast to eating a diet high in fruits and vegetables. For example a 2008 study by Flood et al assessed dietary patterns among 293,615 men and 198,767 women in the National Institutes of Health-AARP Diet and Health Study seeking associations between diet and colorectal cancer. They divided people into three different dietary patterns: a diet high in fruits and vegetables, a diet high in red meat and potatoes and a diet high in ‘diet foods.’ In the five years these people were followed, 2151 of the men and 959 of the women were diagnosed with colorectal cancer. Men who ate a lot of fruits and vegetable were at a lower relative risk for this cancer (RR) (0.81). Men and women who were dieting also had lower RR (0.82 and 0.87) respectively. Men in the meat and potato group had a slight increase in risk (RR: 1.17) but women were at about 50% above average in risk (RR: 1.48). [vi]
Would leaving the potatoes out of the diet lower or increase these risks?
According to the National Potato Association website, about 44 billion pounds of potatoes are grown in the US each year. Only about a quarter of them are eaten (28%) as fresh potatoes. Nearly half (47%) are consumed as either French fries or potato chips [vii].
[We might note somewhere that Thomas Jefferson is credited with ‘inventing’ French fried potatoes [viii], He is also credited with bringing a recipe for batter fried fish home from London, which he cooked for guests at Monticello [ix]. Thus we might call Jefferson the ‘Fish and Chips President.’]
Ireland leads the world in annual potato consumption at 111 kg/year (2009 data) Kyrgyzstan comes in second at 104 kg/yr followed by Latvia and Lithuania with 104 and 98 kg/yr. Vietnam, Indonesia and Malaysia may have the lowest consumption, with 4.5, 5 and 9 kg/yr respectively.[x]
Korea, a country that consumes only about 10 kg/yr of potatoes, has the highest rate of colorectal cancer in the world (45/100,000 people/yr). Maybe there is something to this idea? Slovakia and Hungary take second and third place for colorectal cancer incidence (about 42/100000) [xi] but are big potato eating countries with 58 and 60-kg/person consumption respectively. So without any more statistical analysis, and seeing no easily discernable pattern, I’m ready to drop my theory of potatoes protecting people from cancer. Of course hardly anyone in Korea eats red meat the way we do in the US.
Given my patient population, most of whom have cancer, I’m not about to start advocating that patients eat more potatoes, but if they are going to make an exception, I’m going to suggest cold potato salad. Even with potatoes, whether they are healthy or bad for you is not a black and white issue.
I sometimes get on a soapbox when patients start on about super-foods and tell them that there is no such thing. There are whole foods and there are processed foods, goes my spiel. We want to eat the former and avoid the later, is the gist of what I say and that if anything we should put our attention on eating as diverse a variety of foods that we can. I dig out the numbers for a breast cancer study looking at the diets of BRCA positive patients.
This 2009 study by Ghadirian et al reported on diet diversity and its effect on BRCA+ carriers’ risk for breast cancer. This was “… a case-only study … carried out in a French-Canadian population including 738 patients with incident primary BC comprising 38 BRCA mutation carriers. Diet diversity was assessed… [for] case-only odds ratio (COR) and 95% confidence interval (CI) while adjusting for age, body mass index, smoking, hormonal replacement therapy, and total energy intake. …. results reveal a strong and significant interaction between BRCA mutations and vegetable and fruit diversity (COR = 0.27;…) when comparing the upper to the lower quartiles. ….. The results of this study suggest that the combination of BRCA mutations and vegetable and fruit diversity may be associated with a reduced risk of BC.”
In simple terms what they are saying is that women who were BRCA + and who ate the greatest variety of vegetables and fruit had a 73% reduction in cancer incidence compared to whose who ate a narrow range of choices [xii].
Given the high risk BRCA carriers have for breast cancer, these finding are remarkable. Advocating for increased dietary diversity is a reasonable goal not only for this patient group but also probably for a wider range of patients at risk for numerous other cancer types.
Here is another similar study that I’ve come across. In May 2013, Isa et al reported on the effect dietary diversity had on risk of bladder cancer in China. As we might expect high intake of red meat increased risk for bladder cancer (OR=1.8). Some of their findings come as a surprise. High intake of leafy vegetables nearly tripled risk (OR=2.9), bulb vegetables and preserved vegetables more than doubled risk (OR=2.3 for each). An interesting list of foods were associated with lower risk: white fish consumption and vine fruits cut risk in half (OR=0.5), citrus fruits and potatoes even more (OR=0.4) and flower vegetables even more so (OR=0.3). Those subjects with the highest total fruit diversity had the lowest risk of developing cancer, 90% less than those subjects with the lowest diversity in fruit consumption. (OR=0.1). [xiii]
Rather than suggesting super-foods or prohibiting particular foods, it may make more sense for us to advocate for diversity in foods eaten.
[ii] Skuladottir H, et al. High Intake of Vegetables and Fruit May Improve the Prognosis of Persons with Lung Cancer. Lung Cancer, 2006; Mar;51(3):267-73.
[iii] Patel B, Schutte R, Sporns P, Doyle J, Jewel L, Fedorak RN. Potato glycoalkaloids adversely affect intestinal permeability and aggravate inflammatory bowel disease. Inflamm Bowel Dis. 2002 Sep;8(5):340-6.
[iv] Lee JK, Gopal R, Seo CH, Cheong H, Park Y. Isolation and Purification of a Novel Deca-Antifungal Peptide from Potato (Solanum tuberosum L. cv. Jopung) Against Candida albicans. Int J Mol Sci. 2012;13(4):4021-32.
Humphreys KJ, Conlon MA, Young GP, Topping DL, Hu Y, Winter JM, Bird AR, et al. Dietary Manipulation of Oncogenic MicroRNA Expression in Human Rectal Mucosa: A Randomized Trial. Cancer Prev Res (Phila). 2014 Aug;7(8):786-95.
[vi] Flood A, Rastogi T, Wirfält E, Mitrou PN, Reedy J, Subar AF, Kipnis V, et al. Dietary patterns as identified by factor analysis and colorectal cancer among middle-aged Americans. Am J Clin Nutr. 2008 Jul;88(1):176-84.
[xii] Ghadirian P, Narod S, Fafard E, Costa M, Robidoux A, Nkondjock A. Breast cancer risk in relation to the joint effect of BRCA mutations and diet diversity. Breast Cancer Res Treat. 2009 Sep;117(2):417-22.
[xiii] Isa F, Xie LP, Hu Z, Zhong Z, Hemelt M, Reulen RC, Wong YC, et al. Dietary consumption and diet diversity and risk of developing bladder cancer: results from the South and East China case-control study. Cancer Causes Control. 2013 May;24(5):885-95.