Now that you know why I'm so crazy-for-beets, and why I've ditched the glycoalkaloid-laden nightshades, let's talk my obsession with sweet potatoes, the world's seventh most important food crop (Shekhar et al., 2015). In fact, in developing countries, sweet potatoes rank fifth in terms of caloric contribution to the human diet and third in terms of production value (Shekar et al., 2015).
Whereas the white- and cream-fleshed cultivars are predominately grown in the Pacific, the yellow- and orange-fleshed sweet potato is primarily harvested in the United States (Shekar et al., 2015). Compared to white-fleshed sweet potatoes, orange-fleshed sweet potatoes tend to have superior nutritional signatures, with increased levels of carotenoids, anthocyanins, flavonoids, and total protein compared to their white counterparts (Shekhar et al., 2015). Biochemical screening has indicated that white-fleshed sweet potatoes, on the other hand, have higher reducing sugar, carbohydrate, and phenolic content (Shekhar et al., 2015).
Mohanraj and Sivasankar (2014) discuss the value of the sweet potato, noting that it is "a valuable medicinal plant having anti-cancer, antidiabetic, and anti-inflammatory activities" (p. 733). Furthermore, most cultivars of sweet potatoes are fantastic sources of vitamin C, vitamin B2 (riboflavin), vitamin B6 (pyridoxine), and vitamin E, in addition to copper, iron, manganese, potassium, and dietary fiber (Shekhar et al., 2015).
Although many in the keto and low carb camps poo-poo this and other starchy tubers for their hypothetical promotion of metabolic derangements, insulin resistance, and blood sugar dysregulation, sweet potatoes have long been used in traditional medicine for the treatment of diabetes mellitus, or type 2 diabetes.
A Cochrane Database Review concluded that most human trials using sweet potato for type 2 diabetes are low quality; however, two studies with 122 participants demonstrated improvements in long-term blood sugar dysregulation as indicated by hemoglobin A1c (HbA1c) levels (Ooi & Loke, 2013). Studies have also shown that sweet potatoes ameliorate diet-induced aortic stiffness independent of body mass and composition and reduce the burden of arterial oxidative stress (Garner et al., 2017).
Further, sweet potato improved insulin sensitivity in insulin resistant rats, both by normalizing insulin signaling, and by significantly lowering levels of inflammatory cytokines and adipocytokines such as tumor necrosis factor-α, interleukin-6, resistin, and retinol binding protein-4 (Chen, Lai, Hung, & Liu, 2013).
Although there are approximately four hundred varieties of sweet potato, the deep hue of purple sweet potatoes, imparted by a class of bioflavonoids called anthocyanidins, puts it ahead of the pack. For example, an in vitro, or cell culture study, has revealed that an extract of purple sweet potatoes exerts antilipogenic (anti-obesity), lipolytic (fat degrading), and anti-inflammatory effects on fat cells, or adipocytes, and also possesses free radical scavenging effects (Ju et al., 2011).
Another study showed that purple sweet potato administered to obese mice significantly attenuated weight gain and reduced fat accumulation induced by a high fat diet (Ju et al., 2017). In addition, purple sweet potato favorably modified energy expenditure and kidney and liver function, and led to improvements in lipid profiles (Ju et al., 2017).
Purple sweet potatoes may also have clinical applications for combating neurodegeneration. In fact, in a mouse model of domoic acid-induced cognitive dysfunction, oral administration of purple sweet potato color led to significant improvements in behavioral performance on cognitive function tests such as the Morris water maze task and a passive avoidance task (Lu et al., 2012).
In this study, the purple sweet potato color also inhibited signaling through the endoplasmic reticulum stress pathway through which domoic acid engenders neurodegenerative changes (Lu et al., 2012). According to the study authors, "Purple sweet potato color significantly suppressed endoplasmic reticulum stress-induced apoptosis, which prevented neuron loss and restored the expression of memory-related proteins" (Lu et al., 2012, p. 646).
Not only that, but sweet potato color also stimulated mitochondrial biogenesis, or the formation of new mitochondria, the cell organelles that represent the energetic powerhouses of the cell, by stimulating an estrogen receptor-α-mediated signaling cascade (Lu et al., 2012). Further, the sweet potato color promoted mitochondrial synthesis by decreasing the expression of p47phox and gp91phox, proteins which contribute to the generation of the respiratory burst that causes production of oxidative-stress producing reactive oxygen species (ROS) (El-Benna, My-Chan Dang, Gougerot-Pocidalo, & Braut-Boucher, 2009).
Cumulatively, these findings point to the importance of eating the rainbow, and gravitating towards deeply pigmented fruits and vegetables to optimize healing. Researchers conclude that, "These results suggest that purple sweet potato color could be a possible candidate for the prevention and treatment of cognitive deficits in excitotoxic and other brain disorders" (Lu et al., 2012).
Chen, Y.Y., Lai, M.H., Hung, H.Y., & Liu, J.F. (2013). Sweet potato [Ipomoea batatas (L.) Lam. "Tainong 57"] starch improves insulin sensitivity in high-fructose diet-fed rats by ameliorating adipocytokine levels, pro-inflammatory status, and insulin signaling. Journal of Nutrition Science and Vitaminology (Tokyo), 59(4), 272-280.
El-Benna, J., My-Chan Dang, P., Gougerot-Pocidalo, M-A., Marie, J-C., & Braut-Boucher, F. (2009). p47phox, the phagocyte NADPH oxidase/NOX2 organizer: structure, phosphorylation and implication in diseases. Experimental and Molecular Medicine, 41(4), 217-225.
Garner, T., Ouyang, A., Berrones, A.J., Campbell, M.S., Du, B., & Fleenor, B.S. (2017). Sweet potato (Ipomoea batatas) attenuates diet-induced aortic stiffening independent of changes in body composition. Applied Physiology Nutrition and Metabolism, [Epub ahead of print]. doi: 10.1139/apnm-2016-0571.
Ju, R., Zheng, S., Luo, H., Wang, C., Duan, L., Sheng, Y.,...Huang, K. (2017). Purple Sweet Potato Attenuate Weight Gain in High Fat Diet Induced Obese Mice. Journal of Food Science, 82(3), 787-793. doi: 10.1111/1750-3841.13617
Ju, H.S., Park, H.J., Kim, M.Y., Shin, H.L., Park, K.Y., Yang, J.O.,...Do, M.S. (2011). Anti-obesity and antioxidative effects of purple sweet potato extract in 3T3-L1 adipocytes in vitro. Journal of Medical Food, 14(10), 1097-1060. Anti-obesity and antioxidative effects of purple sweet potato extract in 3T3-L1 adipocytes in vitro.
Lu, J., Wu, D.M., Zheng, Y.L., Hu, B., Cheng, W., & Zhang, Z.F. (2012). Purple sweet potato color attenuates domoic acid-induced cognitive deficits by promoting estrogen receptor-α-mediated mitochondrial biogenesis signaling in mice. Free Radical Biology and Medicine, 52(3), 646-659. doi: 10.1016/j.freeradbiomed.2011.11.016.
Mohanraj, R., & Sivasankar, S. (2014). Sweet potato (Ipomoea batatas [L.] Lam)--a valuable medicinal food: a review. Journal of Medical Food, 17(7), 733-741. doi: 10.1089/jmf.2013.2818.
Ooi, C.P., & Loke, S.C. (2013). Sweet potato for type 2 diabetes mellitus. Cochrane Database of Systematic Reviews, 9, CD009128. doi: 10.1002/14651858.CD009128.pub3.
Shekfar, S., Mishra, D., Buragohain, A.K., Chakraborty, S., & Chakroborty, N. (2015). Comparative analysis of phytochemicals and nutrient availability in two contrasting cultivars of sweet potato (Ipomoea batatas L.). Food Chemistry, 173, 957-965.