Pre-pregnancy potato consumption and risk of gestational diabetes mellitus: prospective cohort study
BMJ 2016; 352 doi: https://doi.org/10.1136/bmj.h6898 (Published 12 January 2016) Cite this as: BMJ 2016;352:h6898
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We read with interest the research article by Bao et al (1) on pre-pregnancy potato consumption and risk of gestational diabetes mellitus (GDM). The authors have opined that consumption of higher levels of potato before pregnancy is associated with greater risk of GDM. They have also suggested that substituting potatoes with other vegetables, legumes or whole grain-rich foods might lower this risk. Additionally, we would like to suggest that sweet potato [Ipomoea batatas (L.) Lam] can be used instead of potato. It is the second most important root tuber and overall seventh most important food crop of the world, categorized as “poor man’s food”. (2) It also possesses a high nutritional value – about fifty percent more than potato, and therefore can be consumed in moderation unlike potato to achieve satiety. (3) There is a myth among general public as well as some physicians that sweet potatoes, being sweet in taste and having the word “sweet” in name itself, have high sugar content and thus, must be avoided by diabetic patients. However, the fact is they have a lower glycaemic index (GI) compared to potatoes in general. When boiled, GI of sweet potato is 44 whereas that of boiled potatoes has been shown to vary from 58-89 (depending on the variety of potato used). (4) Fiber content of the sweet potato is almost double than that found in common potato varieties (5) and as constipation is a common complaint in pregnant women (due to the pressure of the enlarged uterus on the intestine), sweet potatoes may find preference over potato in pregnant diabetic patients. Pregnant women are often supplemented with calcium for the fetal and maternal well-being. Sweet potato contains almost four times more calcium than found in potato. (5) Additionally, sweet potato contains polyphenolics, vitamins, and anthocyanins which have shown to produce anti-hypertensive effect. (6) It is a well known fact that pregnant women, especially suffering from GDM, are more prone to develop hypertension.
Studies by Jaarsveld et al (2005) and Abubakar et al (2010) have shown that tubers of sweet potato possess anti-diabetic, anti-oxidant, and anti-proliferative properties attributed to the presence of valuable nutritional and mineral components. (7) (8) It has been suggested that sweet potatoes have the ability to raise blood levels of adiponectin, a protein hormone created by adipocytes, which helps to regulate insulin metabolism and can in turn, regulate blood glucose levels. (9)
In view of these, we strongly feel that sweet potato can be used as an alternative to potato not only in pregnant-diabetic women but also in population suffering from diabetes. Unfortunately, it appears that sweet potatoes are being under utilized in the management of diabetes. We feel that extensive scale studies employing sweet potatoes in GDM will be extremely worthwhile.
References
1) Bao W, Tobias DK, Hu FB, Chavarro JE, Zhang C. Pre-pregnancy potato consumption and risk of gestational diabetes mellitus: prospective cohort study. BMJ 2016; 352:h6898.
2) Mitra S. Nutritional Status of Orange-Fleshed Sweet Potatoes in Alleviating Vitamin A Malnutrition through a Food-Based Approach. J Nutr Food Sci 2012; 2:160.
3) Krochmal-Marczak B, Sawicka B, Supski J, Cebulak T, Paradowska K. Nutrition value of the sweet potato (Ipomoea batatas (L.) Lam) cultivated in south – eastern Polish conditions. Int J Agron. & Agri R 2014; 4: 169-78.
4) The University of Sydney. The International Glycemic Index (GI) Database. Accessed February 06, 2016.
5) National Institute of Nutrition. Nutritive value of Indian foods, 2011. 2nd ed India. Indian Council of Medical Research (ICMR), 2011.
6) Jawi IM, Yasa IWPS, Subawa AAN, Suprapta DN. Comparison of Potential Antihypertensive and Antioxidant Between Aqueous Extract of Purple Sweet Potato Tuber and Captopril in Hypertensive Patients. Journal of Biology, Agriculture and Healthcare 2015; 5: 128-33.
7) van Jaarsveld PJ, Faber M, Tanumihardjo SA, Nestel P, Lombard CJ, Benadé AJ. Beta-carotene-rich orange-fleshed sweet potato improves the vitamin A status of primary school children assessed with the modified-relative-dose-response test. Am J Clin Nutr 2005 ;81:1080-7.
8) Abubakar HN, Olayiwola IO, Sanni SA, Idowu MA. Chemical composition of sweet potato (Ipomea batatas Lam) dishes as consumed in Kwara state, Nigeria. International Food Research Journal 2010; 17: 411-16.
9) Ludvik BH, Mahdjoobian K, Waldhaeusl W, Hofer A, Prager R, Kautzky-Willer A, et al .The effect of Ipomoea batatas (Caiapo) on glucose metabolism and serum cholesterol in patients with type 2 diabetes: a randomized study. Diabetes Care 2002; 25, 239-40.
Competing interests: No competing interests
This is an interesting article by Bao et al that utilises data from a questionnaire that has been sent every 2 years to nurses in USA which had a very high completion rate of over 90%.1 To be eligible for this questionnaire initially when it was commenced in 1989 minimum age was 24, the 1991 questionnaire was the first to have dietary questions meaning that the minimum age for inclusion was 26 years and above which weakens the generalisability of this study to not all women of child bearing age. The article contains a documented policy for handling incomplete questionnaires as well as unrealistic responses although did not quantify how many women were excluded due to this reason. It also seems sensible to exclude women who have had previous gestational diabetes not only for the fact as outlined that they may change their lifestyle but clearly these women with a personal history are more susceptible to developing gestational diabetes in future pregnancies than the general population.
It is very impressive that previous work that has validated the self-reporting of gestational diabetes mellitus in this cohort revealing a 94% accuracy.2 Even medical records are inconsistently and at times incorrectly coded so this self-reporting of gestational diabetes particularly in the context of health care professionals is in my opinion robust.3 It has also been noted that the Diabetes Data Group criteria for diagnosing gestational diabetes was widely used during the time period of the study, this included an oral glucose tolerance test and although guidelines and diagnostic criteria are regularly updated an oral glucose tolerance test is still included in the diagnostic pathway.4 5
Clearly the largest potential confounder of the results is the susceptible to recall bias in terms of estimating dietary intake – which has been acknowledged as a limitation by the authors and it is worthwhile noting that data from the study is 15 years old at the time of publication. Although this cohort has been validated for weight and physical activity the authors do not highlight that it has been validated for self-reported dietary consumption.1 6 When the survey was validating for physical activity it was performed by seeking a correlation of questionnaires answers with self-reported activities recalled over the past week and seven day activity diaries rather than measuring objectively or observing any activities.7
The statistical analysis incorporating different modelling approaches was impressive and major variables were adjusted for; it was particularly good to see that the body mass index was entered into the model separately as it could be an intermediate on the pathway between potato consumption and GDM risk. A series of different sensitivity analyses were performed which is adds to the robustness of the study, an additional analysis could have been performed using multiple imputation to account for missing data although only 4.5% had a missing covariant means this is unlikely to affect the conclusion of statistical analysis.8 Performing the analysis excluding participants with missing data seems sensible. A further analysis could have also been performed excluding an adjustment for age. This is because gestational diabetes tends to be more prevalent with an increasing age and it is of interest that the participants with higher potato consumption were in fact younger than the others - this means that when performing statistical analysis adjusting for age it may strengthen the apparent association.
Table 1 reveals that people with higher consumption of potatoes also have less protein, less magnesium, less vitamin C and E than people who consumed less potatoes and higher levels of fat, these have been deduced from the self-reported dietary data.9 An author could potentially draw a likely incorrect conclusion from these associations.
Although higher potato consumption was found to be statistically significantly associated with gestational diabetes in this cohort and statistical adjustments were used to account for the major variants due to the complex interplay between all of these factors is extremely difficult to draw any conclusions for this association. It was noted that participants with higher potato consumption at baseline was associated with higher BMI, current smokers, people who performed less physical activity, consumed greater total energy and had a lower quality of overall diet score. A recent Swedish study that was also questionnaire based revealed significant differences between the self-reported lifestyle and health status of women who had gestational diabetes and those who did not.10 When taking into account the element of recall bias and the interaction of these complex factors of lifestyle differences I believe it is very difficult to draw any clinical associations about the statistically significant association between potato consumption and the risk of gestational diabetes mellitus.
1. Bao W, Tobias DK, Hu FB, et al. Pre-pregnancy potato consumption and risk of gestational diabetes mellitus: prospective cohort study. BMJ 2016;352:h6898.
2. Solomon CG, Willett WC, Carey VJ, et al. A prospective study of pregravid determinants of gestational diabetes mellitus. JAMA 1997;278(13):1078-83.
3. Robertson AR, Fernando B, Morrison Z, et al. Structuring and coding in health care records: a qualitative analysis using diabetes as a case study. J Innov Health Inform 2015;22(2):275-83.
4. Classification and diagnosis of diabetes mellitus and other categories of glucose intolerance. National Diabetes Data Group. Diabetes 1979;28(12):1039-57.
5. Gestational diabetes mellitus. BMJ Best Practice.
6. Rimm EB, Stampfer MJ, Colditz GA, et al. Validity of self-reported waist and hip circumferences in men and women. Epidemiology 1990;1(6):466-73.
7. Wolf AM, Hunter DJ, Colditz GA, et al. Reproducibility and validity of a self-administered physical activity questionnaire. Int J Epidemiol 1994;23(5):991-9.
8. Yucel RM. State of the Multiple Imputation Software. J Stat Softw 2011;45(1).
9. Chiuve SE, Fung TT, Rimm EB, et al. Alternative dietary indices both strongly predict risk of chronic disease. J Nutr 2012;142(6):1009-18.
10. Persson M, Winkvist A, Mogren I. Lifestyle and health status in a sample of Swedish women four years after pregnancy: a comparison of women with a history of normal pregnancy and women with a history of gestational diabetes mellitus. BMC Pregnancy Childbirth 2015;15:57.
Competing interests: No competing interests
Letter to editor
Re: Pre-pregnancy potato consumption and risk of gestational diabetes mellitus: prospective cohort study (BMJ 2016; 352: h6898).
Mullie P1,2, Boniol M1,3, Autier P1,3
1 International Prevention Research Institute, Lyon, France
2 Vrije Universiteit Brussel, Elsene, Belgium
3 Strathclyde University Global Public Health Institute, Lyon, France
Corresponding author: Patrick Mullie, International Prevention Research Institute, 95 cours Lafayette, 69006 Lyon, France. Email: patrick.mullie@i-pri.org.
Using data from the Nurses’ Health Study II, Bao et al. (1) found a higher prevalence of gestational diabetes mellitus (GDM) with increasing potato consumption. The authors stated that the high glycaemic index (GI) of potatoes, resulting in sharp postprandial rise in blood glucose concentrations with risk of associated pancreatic β cells exhaustion, could explain this relationship.
However, the use of GI has methodological limitations. Dodd et al. (2) observed that the GI formula of a meal is overestimated compared to the measured GI of the same meal. This overestimation was between 22% and 50% for different meals, and the overestimations were unpredictable and food dependent, i.e., higher for spaghetti than for potato. This means that the reproducibility of GI is very low, because potatoes are almost always consumed during meals.
Secondly, it is not clear from Bao et al. (1) how the GI and glycaemic loads (GL) were calculated, there is no information in the method section. It is also not clear why Bao et al. choose potatoes to test this GI hypothesis. The GI of potatoes range from 60 to 120, depending of the cooking method and the sort of potatoes consumed. It would be easier to relate the GI to GDM with a more homogenous food group, as for example breakfast cereals, with GI between 100 and 120 (3).
Bao et al. found that the adjusted relative risk for GDM associated with baked, boiled, or mashed potatoes consumption before pregnancy was 1.52 (95% CI: 1.11 to 2.07), and for French fries 1.18 (95% CI: 0.91 to 1.53). This discrepancy between results is difficult to explain by the GI theory, because the GI of French fries is higher than for cooked potatoes.
A last point is that, as indicated in Table 1 of Bao et al., the mean (SD) GI and GL for the total dietary pattern across the weekly consumption servings of potatoes remained stable, ranging from 53.1 (3.7) to 55.6 (2.7) and 124.4 (22.9) to 124.1 (20.2), respectively.
As pointed out by Bao et al., women with a higher potato consumption had a higher prevalence of family history of diabetes, smoking, adiposity and were less physical active. They also had a lower score for the Alternate Healthy Eating Index 2010, indicating a less healthy dietary pattern. The mean (SD) BMI for low and high pre-pregnancy potato consumers were 22.9 (4.0) kg/m2 and 24.2 (4.8) kg/m2. The mean energetic intake was 2,049 kcal/day versus 2,249 kcal/day, respectively. Unfortunately, there was no information about added sugar consumption across the potato consumption groups.
The authors observed a clustering of unhealthy behaviours associated with high potato consumption. A consequence of a clustering for observational research is that trying to single out a specific contribution of one component is elusive because of associated statistical limitation: even after adjustment, residual confounding may remain, due to inaccurate measurements of variables and, even after hypothetical perfect measurement, risk of multicollinearity is likely to threaten the correct interpretation of multivariate models.
In conclusion, we believe that this Bao et al. reductionism approach of GDM will add to the actual nutritional mess and confusion concerning food. Moreover, this approach will create inappropriate nutritional certitudes, and this by exaggerating the food scientists’ knowledge about the relationship between specific food and health. Fetishizing foods on the basis of the nutrient composition has shift the focus to details, and forgetting the essential to stay healthy: a plant-based dietary pattern, not smoking, a healthy weight and physical activity.
References
1. Bao W, Tobias DK, Hu FB, Chavarro JE, Zhang C. Pre-pregnancy potato consumption and risk of gestational diabetes mellitus: prospective cohort study. Bmj. 2016;352:h6898.
2. Dodd H, Williams S, Brown R, Venn B. Calculating meal glycemic index by using measured and published food values compared with directly measured meal glycemic index. The American journal of clinical nutrition. 2011;94(4):992-6.
3. Foster-Powell K, Holt SA, Brand-Miller JC. International table of glycemic index and glycemic load values: 2002. The American journal of clinical nutrition. 2002;76:5-56.
Competing interests: No competing interests
I welcome the research study by Bao et al (1) that highlights the difficulties experienced with gestational diabetes (GDM). Gestational diabetes can be a traumatic diagnosis and a difficult condition to manage. Have we not come full circle with recommendations on ‘diabetes’ diets? The recommendation to eat more vegetables and to appropriately manage carbohydrates such as potatoes, bread, rice, pasta, cereals, amongst others, were recommended as best protocol before the 1980s. The following studies may be of interest:
A 1973 UK study of two hundred newly diagnosed, overweight type 2 diabetics (T2DM) concluded that control of diabetes in obese patients who respond to diet alone is due to carbohydrate restriction rather than to weight loss. This result was achieved in 80% of the participants. A highly significant statement regarding this research was - They did not start drug treatment until it was clear that diet alone had failed, which was usually after at least four months. (2)
Conversely a 1979 study concluded that in T2DM a high carbohydrate diet composed of readily available cereal foods and tuberous vegetables, resulted in lower fasting and preprandial blood glucose concentrations than a standard low carbohydrate diet. (3) Similar findings were found in T1DM. (4) By the 1980’s the UK and US diabetes association’s were recommending diets with a substantial amount of carbohydrate and fibre and little fat to improve glycaemic control and reduce the risk factors for ischemic heart disease. (5)(6) These findings were largely based on the now discredited diet heart hypothesis, which remains a controversial issue within science.
It is interesting to note the description of gestational diabetes is: “Carbohydrate intolerance of varying degrees of severity with onset or first recognition during pregnancy,” (7)
It is widely accepted that GDM affects a significant number of pregnancies worldwide and that prevalence levels have been increasing rapidly in recent years (8). Does this not reflect the increasing dominance of carbohydrates in our diet? Unlike many other carbohydrates, potatoes don’t need a sprinkle of sugar to make them palatable. Although there is a danger of lipid peroxidation when fried. (The issue of fats consumed within the diet were not addressed in this study). The research findings (1) suggest there is a need to reassess dietary recommendations particularly in those planning to become pregnant. It is also important to review carbohydrate consumption in gestational and type 2 diabetes.
The research authors acknowledge several study limitations and say because of the observational nature of their study, no definitive conclusions can be drawn about cause and effect. (1) However potatoes will gain a negative reputation. All carbohydrates are converted at different absorption rates into glucose by the action of insulin. All carbohydrates are not equal and have varying blood glucose responses. It is difficult to understand why potatoes, which are a highly nutritious food, were chosen to be highlighted in this study. The authors point out that potatoes have a high glycaemic index (GI) compared with other vegetables, so can trigger a sharp rise in blood sugar levels, and this could be one explanation for the findings (1) However if using the principles of the GI diet, potatoes when eaten with protein and/or fat lowers the GI response.
A basic feature of pregnancy is the baby receives all their nourishment from their mother, which means the mother, will be nutritionally depleted if her diet is inadequate for the needs of pregnancy and indeed breastfeeding. It is not surprising that our poor dietary transgressions are continued into pregnancy with unwelcome consequences such as insulin resistance accompanied by the diagnosis of GD.
References:
(1) Bao Wei, Tobias Deirdre K, Hu Frank B, Chavarro Jorge E, Zhang Cuilin.Pre-pregnancy potato consumption and risk of gestational diabetes mellitus: prospective cohort study BMJ 2016; 352:h6898
(2) Effect of Carbohydrate Restriction in Obese Diabetics:Relationship of Control to Weight Loss J. R. Wall, D. A. Pyke, W. G. Oakley British Medical Journal, 1973, 1, 577-578
(3) A S Truswell, Briony J Thomas, Ann M Brown British Medical Journal, 1975, 4, 7-11 Survey Of Dietary Policy And Management In British Diabetic Clinics
(4) R W Simpson, J I Mann, J Eaton, R D Carter, T D R Hockaday. British Medical Journal, 1979, 2, 523
(5) American Diabetes Association. Nutritional recommendations and principles for individuals with diabetes mellitus. Diabetes Care 1987;10: 126-32.
(6) Diabetes and Nutrition Study Group of the European Association for the Study of Diabetes. Nutritional recommendations for individuals with diabetes mellitus. Diabetes, Nutrition and Metabolism 1988;1:145-9.
(7) Metzger BE, Coustan DR. Summary and recommendations of the Fourth International Workshop-Conference on Gestational Diabetes Mellitus. Diabetes Care 1998;21(suppl 2):161-7S.
(8) The global challenge of diabetes. Lancet 2008;371:1723. [PubMed]
Competing interests: No competing interests
This may be explained by increased testosterone caused by consumption of potatoes.
Potatoes are a good source of vitamin B6. B6 is known to increase testosterone.
In “Role of first trimester total testosterone in prediction of subsequent gestational diabetes mellitus," (J Obstet Gynaecol Res. 2015 Feb;41(2):193-8), it is reported that "Compared with women without GDM, first trimester total testosterone levels were higher among women in whom GDM subsequently developed."
I suggest the findings of Bao, et al., may be explained by negative effects of increased testosterone induced by increased vitamin B6 levels found in potatoes.
Competing interests: No competing interests
Good day,
This is a very interesting study, my concern here is it has not been stated clearly if sweet potatoes can also predispose one to GDM please I need clarification on this.
Adiponectin has been shown to reduce resistance to insulin in various studies and is found to be rich in sweet potatoes. Hence some school of thought advice on benefits of sweet potatoes in the diabetics. Is this research refuting that? Thank you and kind regards
PHARM A S Gachi
Competing interests: No competing interests
Decreased insulin sensitivity or insulin resistance due to pregnancy hormones results in an insulin resistant state. This insulin resistance is not due to GLUT 4 receptor number but due to defect in the post receptor signaling. It is reported that there is a defect in tyrosine phosphoryaltion following insulin binding. The changes in maternal hormonal mileu due to effects of pregnancy hormones - the need for more dietary sources to provide more nutrients to the growing fetus - places a great stress on metabolic control.
Dietary carbohydrates with a high glycemic index like potatoes carry greater risk for women with a family history of diabetes or with a history of glucose intolerance. There is also the role of ethnicity in dealing with genetic predisposition to glucose intolerance.
Therefore it is necessary for Gynecologists to advise patients with such a history to avoid foods with a high glycemic index and to promote a balanced diet.
Competing interests: No competing interests
Re: Pre-pregnancy potato consumption and risk of gestational diabetes mellitus: prospective cohort study
We appreciate the great interest from the readers in our recent article on pre-pregnancy potato consumption and risk of gestational diabetes mellitus (GDM). (1) Below we provided responses to a couple of major comments as raised by the readers
Sweet potato and GDM risk
In the present study, pre-pregnancy consumption of potato did not include sweet potato. Sweet potato consumption in our study population is too low (i.e. ~95% consumed sweet potato less than 1 serving/week) for us to provide a reliable evaluation of its association with the risk of GDM. The role of pre-pregnancy sweet potato consumption in the development of GDM warrants further investigation. We are aware that sweet potato which is rich in complex carbohydrates, dietary fiber, beta-carotene, vitamin C, vitamin B6, anthocyanin, and minerals is considered as a nutritious subtropical and tropical vegetable in some populations. (2) Previous studies in animal models indicated hypoglycemic effects of sweet potatoes through ameliorating oxidative stress and inflammation and improving pancreatic β-cells mass. (3) In humans, sweet potato has been used in traditional medicine practices for the treatment of type 2 diabetes. However, data from randomized controlled trials or large observational studied among humans assessing the efficacy of sweet potato on glycemic control remains insufficient and inconclusive.(2)
Potato, glycemic index, and GDM
The present study was not designed to address the underlying mechanisms for the observed positive association between pre-pregnancy potato intake and GDM risk. The biological mechanisms are yet to be elucidated in future studies. Available evidence suggest that high glycemic index (GI) in potato could be one of the possible explanations of our findings. High GI diet has been associated with greater risk of type 2 diabetes in previous studies across different populations. (4) The calculations of GI and glycemic load in this cohort have been described previously.(4-6) Briefly, the GI values for single food items on the food frequency questionnaire were derived from available databases and publications.(7-9) Foods not represented in the database were sent to the University of Sydney for GI analysis. These foods included breakfast cereals, cakes, cookies, muffin mixes, pancake mixes, and candy bars. For the remaining foods not represented in the database, their GI values were imputed from similar foods in the database. For each participant, total glycemic load (GL) was calculated by first multiplying the carbohydrate content of each food by its GI value, then by multiplying this value by the frequency of consumption and summing the values from all food. The average GI was calculated for each participant by dividing each participant’s glycemic load score by her total daily intake of carbohydrate. The GI and GL values (Table 1 in the original article) of our article were the participants’ average dietary GI and the global GL, but not the GI and GL of any single food items. Nonetheless, GI is neither the exposure variable nor a covariate in the current data analysis, therefore, our conclusion on the association between pre-pregnancy potato consumption and risk of GDM was not affected by measurement errors in GI, if any, in this study.
Impacts of potential confounders on the potato and GDM association
Higher consumption of potatoes, in particular French fries, could be an indicator of a low quality diet. Indeed, in our study, we observed an inverse association between overall diet quality as indicated by the Alternate Healthy Index with potato consumption. Therefore, it is not surprising that women with higher consumption of potatoes also consumed less protein, less magnesium, less vitamin C and E and more saturated fat, compared with women with lower consumption of potatoes. In order to appropriately and efficiently account for the confounding by the interplay of various dietary components, we adjusted for overall diet quality in our analysis. The associations with risk of GDM were moderately attenuated but remained significant after adjustment for overall diet quality. In sensitivity analyses, we also adjusted for different types of food that were significantly related to GDM risk previously and observed similar results.
We respectfully disagree with the suggestions of a further analysis by excluding an adjustment for age. Since advanced age is a well-established risk factor of GDM and it is associated with consumption of potatoes in this population, it should be adjusted for as a potential confounder.
French fries vs. baked, boiled, or mashed potatoes in association with GDM risk
The risk of GDM associated with different types of potato (comparing the highest category with the lowest) such as baked, boiled, or mashed potatoes cannot be directly compared with the risk estimates for French fries, because the amount of consumption in the comparison group for estimating RRs of French fries and other types of potato is different (Tables 3 & 4 in the original article). To examine the independent associations between different types of potato preparations and GDM, we included both types simultaneously in the same multivariable model and observed almost identical magnitude of the associations with risk. The mutually adjusted relative risks for GDM associated with each increment of one serving/week consumption were 1.08 (1.03 to 1.14) for baked, boiled, or mashed potatoes and 1.08 (1.03 to 1.14) for French fries. These findings were presented in the original article.
Residual confounding from other unmeasured factors and potential measurement errors based on self-reported information
In our paper, we have acknowledged that as in other observational studies, measurement error in self-reported exposures (potato consumption) is inevitable. Because the pre-pregnancy dietary information was captured prospectively, the measurement errors would most likely be non-differential with regard to the disease outcome, and such non-differential misclassifications in exposure could have attenuated the observed associations toward null. Furthermore, the use of cumulative averages of dietary intakes for participants who filled in more than one pre-pregnancy food frequency questionnaire reduces random error within an individual. We have addressed these points in the original article. We also acknowledged that although we have adjusted for many potential confounders, we are unable to completely rule out the possibility of residual confounding by measurement error in the adjusted factors. However, many self-reported measures in the cohorts’ FFQ have been validated in the NHS II or similar populations, including diet (10), physical activity (11), and body weight (12). We carefully controlled for major risk factors of GDM to minimize confounding.
Lastly, because of the observational nature of our study, we are careful not to claim a causal relation between potato consumption and risk of GDM. However, consistent associations between higher potato consumption and risk of GDM and type 2 diabetes across multiple studies can enhance the evidence for causality.
In summary, based on data from a large prospective cohort study, we observed that pre-pregnancy potato consumption was significantly and positively associated with the risk of incident GDM. Substitution of potatoes with other vegetables, legumes, or whole grain foods was associated with a lower risk of GDM. These findings should be confirmed in other populations and potential mechanisms should be examined in intervention studies and randomized controlled trials.
References
1. Bao W, Tobias DK, Hu FB, Chavarro JE, Zhang C. Pre-pregnancy potato consumption and risk of gestational diabetes mellitus: prospective cohort study. BMJ 2016; 352:h6898
2. Ooi CP, Loke SC. Sweet potato for type 2 diabetes mellitus. The Cochrane database of systematic reviews 2013;9:CD009128.
3. Niwa A, Tajiri T, Higashino H. Ipomoea batatas and Agarics blazei ameliorate diabetic disorders with therapeutic antioxidant potential in streptozotocin-induced diabetic rats. J Clin Biochem Nutr 2011;48(3):194-202.
4. Bhupathiraju SN, Tobias DK, Malik VS, et al. Glycemic index, glycemic load, and risk of type 2 diabetes: results from 3 large US cohorts and an updated meta-analysis. Am J Clin Nutr 2014;100(1):218-32.
5. Schulze MB, Liu S, Rimm EB, et al. Glycemic index, glycemic load, and dietary fiber intake and incidence of type 2 diabetes in younger and middle-aged women. Am J Clin Nutr 2004;80(2):348-56.
6. Zhang C, Liu S, Solomon CG, et al. Dietary fiber intake, dietary glycemic load, and the risk for gestational diabetes mellitus. Diabetes Care 2006;29(10):2223-30.
7. Jenkins DJ, Wolever TM, Taylor RH, et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr 1981;34(3):362-6.
8. Miller JB, Pang E, Broomhead L. The glycaemic index of foods containing sugars: comparison of foods with naturally-occurring v. added sugars. Br J Nutr 1995;73(4):613-23.
9. The University of Sydney. Glycemic index database. Secondary Glycemic index database. http://www.glycemicindex.com
10. Willett WC, Sampson L, Browne ML, Stampfer MJ, Rosner B, Hennekens CH, et al. The use of a self-administered questionnaire to assess diet four years in the past. Am J Epidemiol 1988;127:188-99.
11. Wolf AM, Hunter DJ, Colditz GA et al.Reproducibility and validity of a self-administered physical activity questionnaire. Int J Epidemiol 1994;23991-999
12. Rimm EB, Stampfer MJ, Colditz GA, Chute CG, Litin LB, Willett WC Validity of self-reported waist and hip circumferences in men and women. Epidemiology 1990; 1:466–473
Competing interests: No competing interests