Root and tuber diets also contain bioactive protein. Yam contain dioscorin a bioactive protein which was reported to be capable of regulating blood pressure as observed in rat induced with diabetes. Angiotensin converting enzyme (ACE) inhibitory and antihypertensive activities were suspected in dioscorin as observed in hypertensive rats (Hsu, 2002; Hou, et al., 1999; Hou, et al., 2000; Hou, et al., 2001). The study of Iwu et al., (1990) on hypoglycaemic activity of dioscoretine from tubers of Dioscorea dumetorum in normal and alloxan diabetic rabbits showed beneficial effects of dioscorin in lowering blood pressure (Iwu et al., 1990; Chandrasekara, et al., 2016). Hypertension is a risk factor for cardiovascular disease, stroke and renal disease and is one of the 10 leading risk factors influencing the global burden of disease. A diet which is high in potassium appears to prevent retention of sodium and hence lower blood pressure (Buemi, 2002; Dickinson, et al., 2006 ). The abundant potassium in root and tuber diets, is also an indication of the possible role of the diets in counteracting the retention of sodium in the blood.
Root and tuber diets also have anticancer and hypoglycaemic activities. The phytochemicals present in root and tubers has been reported to exhibit anticancer effects in several types of carcinoma cell lines and animal models. The study of Huang et al., (2012) showed that aqueous extract of sweet potatoes had higher antiproliferative activity than that of ethanol extracts. The separate studies of Halton et al. (2006), Liese et al. (2009), and Villegas et al. (2007) showed that tuber diets has the propensity reduced the risk of type 2 diabetes. The study of Maithili, et al., (2011) antidiabetic activity of ethanolic extract of tubers of Dioscorea alata in alloxan induced diabetic rats showed that diabetic rats with administered yam extract exhibited significantly lower creatinine levels which could be a result of improved renal function by reduced plasma glucose level and subsequent glycosylation of renal basement membranes (Maithili, et al., 2011; Chandrasekara, et al., 2016).
The hypocholesterolemic and antiobesity activities of root and tubers has also being documented. Diosgenin, a steroidal saponin of yam (Dioscorea), exhibited hypolipidemic effects in rats as it showed pancreatic lipase inhibitory activity, protective effect of liver under high-cholesterol diet, and lowered total cholesterol level (Son, et al., 2007; Chandrasekara, et al., 2016). The study of Hwang et al., (2011) revealed the possibility of purple sweet potato to be used for the prevention of obesity due to the findings that anthocyanin fractions of purple sweet potato inhibited hepatic lipid accumulation through the induction of adenosine monophosphate activated protein kinase (AMPK) signaling pathways. AMPK plays an important role in the regulation of lipid synthesis in metabolic tissues (Hwang et al., 2011; Chandrasekara, et al., 2016).
However, some studies implicated root and tuber diets as contributing to obesity, diabetes and cardiovascular diseases. Potatoes in particular was linked to obesity because it serves as a source of consuming dietary fats. Schulze et al., (2006) in their study on dietary patterns and changes in body weight in women, classified potato diet in the Western dietary pattern (energy dense diet) and all other vegetables in the Prudent dietary pattern and reported that a Western dietary pattern, (including potatoes, and other foods high in energy) was associated with weight gain among participants who are nurses. However, King and Slavin, 2015 in their study on White Potatoes, Human Health, and Dietary Guidance argued that, all potatoes are not fried, their relegation to the Western dietary pattern seems arbitrary and judgmental. Similarly, Mozaffarian et al. (30) in their study on weight gain between 1986 and 2006 in 3 separate prospective cohorts, reported that weight change was strongly associated with an increase in the intake of potato chips, and fried potatoes by 1.69 lb and 1.28 lb respectively. Due to these studies and other similar ones, many nutrition recommendations are advising against consumption of fried potatoes and other tubers (USDA, 2010).
Root and tuber diets have over many centuries been a staple diet for people in many regions of the world. They provide varieties of dishes using different cooking methods and serve as sources of several nutrients that offer nutritional and health benefits as established by many studies published in high impact journals. Despite their numerous health benefit, which though still required well conducted intervention studies and reviews to established, dietitians, nutritionist, associations and organizations are yet recommending that consumption of root and tuber diets contribute to obesity and other diseases and therefore should be avoided. Instead of advising against the consumption of root and tubers, it will be appropriate to play emphasis on their cooking and processing methods, by educating the general populace on the negative effect of frying and providing alternative methods of preparation (such as boiling, baking, grilling, braising etc.) of root and tuber diet. Also, method of preparation can prevent the negative effect of saponins, phytate and tannins in cassava which was reported to have anti-nutrient properties. Dietary guidance should continue to emphasise the need to moderate consumption of high-fat foods, consume food rich in fiber, control for portion size, consume whole sources of carbohydrate foods, encourage healthy methods of food preparation and educate on the combination of leave vegetables and other food group such as pulses and legumes with root and tubers. Nutritionist and dietitians should develop healthy recipe from root and tubers and carry out demonstrations on how to prepare these recipes to the populace especially at their clinic and when going out for community outreach. Further intervention studies in human on the ability of root and tuber diets to perform antioxidative, hypoglycemic, hypocholesterolemic, antimicrobial, and immunomodulatory activities and other functions will be a right step in the right direction.