ABSTRACT
This study was conducted to measure the glycemic index and blood glucose response of staple foods consumed in Nigeria in healthy Wistar rats. The test food samples Maize (Zea mays), Wheat (Triticum spp), Acha (Digitaria exilis) and Oat (Elkris super oat) were formulated with palm kernel cake, rice husk/bran, soya beans, crayfish, bone meal, premix and salt into diets in the laboratory of the Department of Home Science, Nutrition and Dietetics, University of Nigeria, Nsukka. Proximate compositions of the flours were determined using standard method. The rats were divided into six (6) groups of three (3) rats each. Group 1 (the negative control) received (0.4 ml distilled water), while group 2 (the positive control) received 2 g/kg b.w of glucose dissolved in 0.4 ml of distilled water orally using gastric cannula. Rats in groups 3 to 6 also received 2 g/kg b.w of glucose in 0.4 ml distilled water and on a separate day received 2 g/kg b.w of the test diets of maize, acha, wheat and oat and the blood glucose level recorded at 15, 30, 45, 60, 90 and 120 min intervals. Result of proximate analysis showed that the carbohydrate content of maize diet (62.14 %) was significantly higher (p < 0.05) than oat (60.56%), acha (59.90%) and wheat (57.62%). Wheat had the highest protein content (19 %), followed by oat (16.45%), maize (15.46%) and acha (9.57%). The moisture content of wheat (6.90%) and oat (8.67%) diets were however lower when compared to acha (11.42%) and maize (10.09%) diet. Acha had the highest fat percentage (5.40%) and oat with the least (3.00%). The ash content of acha (4.33%) was higher than wheat (3.96%) and oats (2.84%). Acha (9.36%) also had the highest fibre content and its values were significantly (p < 0.05) higher than wheat (7.87%), oat (8.50%) and maize (4.52%). The mean Incremental Area under Curve (IAUC) of the test diets were significantly (p < 0.05) lower than the standard glucose (positive control). Regarding the GI, the study showed that the GI of maize (76%), wheat (69.4%), acha (57.5%) and oat (48.1%). It could be concluded that maize had a high glycaemic index while wheat and acha are intermediate glycaemic index foods and oat (Elkris Super Oat) a low glycaemic index food. Thus, the oat (Elkris super oat) having a low glycaemic index is a preferable diet in achieving better glycaemic control.
CHAPTER ONE
INTRODUCTION
The idea of glycaemic index (GI) was introduced by Jenkins et al. (1981) as a means of classifying different sources of carbohydrates (CHO) and CHO-rich foods in the diet, according to their effect on postprandial glycaemia since different carbohydrate containing foods have different effects on blood glucose responses. Carbohydrate-containing foods are graded as either having a high (GI ≥ 70), intermediate (GI = 56-69) or low (≤ 55) GI values depending on the rate at which blood sugar level rises (Foster-Powell et al., 2002), which in turn is related to the rate of digestion of starch and absorption of sugar available in that food (FAO/WHO, 1998).
The importance of glycaemic index studies is linked to the possible therapeutic and physiological effects of diets with low GI on healthy, obese and diabetic subjects. The GI has also been related to colon diseases and physical activity (Jenkins et al., 2002). Dietary management is important in achieving better glycaemic control to reduce the risk of diabetic complications and to prolong life expectancy. A major focus of nutritional management of diabetes is the improvement of glycaemic control by balancing food intake with endogenous and/or exogenous insulin levels.
Nutrition is of utmost importance in intensive diabetes management and has been described as the keystone of care (Kalergis et al., 2005). Research findings on GI indicate that even when foods contain the same amount of carbohydrate (i.e., carbohydrate exchanges), there are up to fivefold differences in glycaemic impact (Foster-powell et al., 2002). In addition, several prospective observational studies have found that the overall GI and glycaemic load of diet, but not total carbohydrate content, are independently related to the risk of developing type 2 diabetes (Salmeron et al., 1997), cardiovascular disease (Liu et al., 2000), and some cancers (Franceschi et al., 2001). Although logic suggests that low-GI diets should improve glycaemic control, the findings of randomized controlled trials have been mixed; some studies have shown statistically significant improvements (Giacco et al., 2000; Gilbertson et al., 2001), whereas other studies have not (Lafrance et al., 1998; Luscombe et al.,1999). As a result, the issue of GI has been fraught with controversy and has polarized the opinions of leading experts (Pi- Sunyer, 2002; Willet et al., 2002).
Earlier reports show there is no universal approach to the optimal dietary treatment for diabetes due to controversy about how useful the glycaemic index (GI) is in diabetic meal planning, thus further research is needed (Meyer et al., 2000). The foods (Maize, Wheat, Acha and Elkris super oat) tested in this study were selected to represent the nutritional variability that adult Nigerians consume. The prevalence of chronic diseases such as coronary heart diseases, obesity and diabetes in Nigeria are high and hence the G.I of these commonly consumed foods needs to be established. This will provide science-based information that will enable Nigerians make proper decision based on scientific evidence on what to eat and the quantity as well.
1.1 Glycaemic Index (GI) and Glycaemic Load (GL)
1.1.1 Glycaemic index (GI)
Glycaemic index (GI) is a physiological ranking of carbohydrates on a scale of 0 to 100 according to the extent to which they raise blood sugar levels. Glycaemic index is defined as the incremental area under the blood glucose response curve elicited by a 50 g available carbohydrate portion of a food and is expressed as a percentage of the response shown after
50 g anhydrous glucose taken by the same subject. GI is therefore a reflection of the rate of conversion of carbohydrate into glucose (Sangeetha et al., 2012). Glucose, a monosaccharide, induces a large glycaemic response and is often used as the reference food and assigned a GI of 100 (Venn and Green, 2007). The glycaemic index can thus be calculated from the formula:
Where IAUC= Incremental area under the blood glucose curve of the test meal.
IAUCS= Incremental area under the blood glucose curve of the standard (glucose).
Glycaemic index may be low or high. Low glycaemic index carbohydrates are those that are digested and absorbed slowly and lead to a low glycaemic response, whereas high glycaemic index carbohydrates are those that are digested and absorbed rapidly and lead to a high glycaemic response (Brouns et al., 2005). In general, most refined carbohydrate-rich foods
have a high glycaemic index, while non starchy vegetables, fruits and legumes tend to have a low glycaemic index (Ludwig, 2002).
Glycaemic index values can be interpreted intuitively as percentage on an absolute scale and are commonly interpreted as follows:
High GI ≥ 70
Intermediate GI = 56-69
Low GI ≤ 55
(Brand-Miller, 2003).
1.1.2 Glycaemic Load (GL)
In practice, the actual carbohydrate load from a normal portion size varies considerably between food products. In order to address this problem, the concept of glycaemic load (GL) was introduced (Salmeron et al., 1997), representing both the quality and the quantity of the carbohydrates in a food or a meal. GL allows comparisons of the likely glycaemic effect of realistic portions of different foods, calculated as the amount of carbohydrate in one serving times the GI of the food (i.e. GL = g carbohydrate × GI/100). Wylie-Rosett et al. (2004) stated that glycaemic load was developed as a way of comparing the glucose-raising effect of foods with widely differing amounts of carbohydrate compositions. The higher the glycaemic load, the greater the expected rise in blood glucose and insulinogenic effect of the food. Long-term consumption of a diet with a relatively high glycaemic load was shown to be associated with an increased risk of type 2 diabetes and coronary heart disease (CHD) (Liu et al., 2000).
In the illustration of the glycaemic load concept, an example of a food with a high glycaemic index but low glycaemic load is watermelon. The glycaemic index of watermelon is 75%. However, if a serving size of 80 g is recommended which denotes 4 g available carbohydrate resulting in a glycaemic load of only 3 g. Due to this concept it is thus unnecessary to exclude the above and many other fruits and vegetables with high glycaemic index values from healthy diet (Foster-powell et al., 2002). Barclay et al. (2005) categorized foods with a glycaemic load <10 as low-glycaemic load and >20 as high glycaemic load. Thus, by adding the glycaemic loads of individual foods together, the total glycaemic load of a complete meal or the whole diet can be calculated (Salmeron et al., 1997). However, there are concerns that the use of glycaemic load or glycaemic response in isolation may lead to the habitual consumption of lower carbohydrate diets (Barclays et al., 2005).
1.2 Determination of Blood Glucose
Glucose is the main source of energy for the body cells and blood lipids (in the form of fats and oils which are primary compact energy store). It is transported from intestines or liver to body cells through the blood stream and presented to the cells for absorption via the hormone insulin. Blood sugar concentration or blood glucose level is the amount of glucose (sugar) present in the blood of a human or animals (David and Peter, 2006).
The blood glucose concentration of humans or animals are measured based on international standards with respect to molar concentration. It is measured in units such as millimoles per litre (mmol/l) or millimolar (mM). In some countries such as the United States, blood sugar unit are expressed as milligrams per decilitre (mg/dl). Both units can be used interchangeably, since the molecular mass of glucose (C6H12O6) is approximately 180 mg/ml the measurement of glucose is between these two scales, which is the factor of 18. Therefore
1 mmol/l of glucose is equal to 18 mg/dl (WHO, 2006). Glucose concentration vary prior to and after each meal consumption, so that with a substantial carbohydrate load the human blood glucose levels usually remain within the normal range. Although shortly after eating a carbohydrate containing food, the blood glucose concentration could temporarily increase a bit in non-diabetics. Several factors influence an individual’s blood glucose concentration, such as homeostasis, that functions to restore blood glucose concentration back to normal.
Table 1: World Health Organisation (WHO) Diabetes Criteria
Condition Fasting glucose 2 hours glucose
Normal
Impaired fasting glycaemia > 6.1 and < 7.0 >110 and < 126 <7.8 <140)
Source: WHO, 2006
1.3 Descriptions of Test Food Sample.
1.3.1 Fonio (Acha)
Fonio (Digitaria exilis and Digitaria iburua) (Fig. 1) is probably the oldest African cereal. For thousands of years West Africans have cultivated it across the dry savannas. Indeed, it was once their major food. Even though few other people have never heard of it, this crop still remains important in areas scattered from Cape Verde to Lake Chad. In certain regions of Mali, Burkina Faso, Guinea, and Nigeria, for instance, it is either the staple or a major part of the diet. Each year West African farmers devote approximately 300,000 hectares to cultivating fonio, and the crop supplies food to 3-4 million people (Lost Crops of Africa,
1996).
Despite its ancient heritage and widespread importance, knowledge of Fonio’s evolution, origin, distribution, and genetic diversity remains scanty even within West Africa itself. The crop has received but a fraction of the attention accorded to sorghum, pearl millet, and maize, and a mere trifle considering its importance in the rural economy and its potential for increasing the food supply. Part of the reason for this neglect is that the plant has been misunderstood by scientists and other decision makers. In English, it has usually been referred to as “hungry rice,” a misleading term originated by Europeans who knew little of the crop or the lives of those who used it (Lost Crops of Africa, 1996). Unknown to these outsiders, the locals were harvesting Fonio not because they were hungry, but because they liked the taste. Indeed, they considered the grain exotic, and in some places they reserved it particularly for chiefs, royalty, and special occasions. It also formed part of the traditional bride price. Moreover, it is still held in such esteem that some communities continue to use it in ancestor worship (Lost Crops of Africa, 1996).
Not only does this crop deserve much greater recognition, it could have a big future. It is one of the world’s best-tasting cereals. In recent times, some people have made side-by-side comparisons of dishes made with Fonio and common rice and have greatly preferred the fonio. Fonio is also one of the most nutritious of all grains. Its seed is rich in methionine and cystine, amino acids vital to human health and deficient in today’s major cereals: wheat, rice, maize, sorghum, barley, and rye. This combination of nutrition and taste could be of outstanding future importance. Certain Fonio varieties mature so quickly that they are ready to harvest long before all other grains. For a few critical months of most years these become a “grain of life.” They are perhaps the world’s fastest maturing cereal, producing grain just 6 or
8 weeks after they are planted. Without these special fonio types, the annual hungry season
would be much more severe for West Africa. They provide food early in the growing season, when the main crops are still too immature to harvest and the previous year’s production has been eaten. Other fonio varieties mature more slowly—typically in 165-180 days. By planting a range of quick and slow types farmers can have grain available almost continually. They can also increase their chances of getting enough food to live on under even the most changeable and unreliable growing conditions.
Of the two species, white fonio (Digitaria exilis) is the most widely used. It can be found in farmers’ fields from Senegal to Chad. It is grown particularly on the upland plateau of central Nigeria (where it is generally known as “acha”) as well as in neighbouring regions. The other species, black fonio (Digitaria iburua), is restricted to Bauchi and Plateau States of Nigeria as well as northern regions of Togo and Benin (Lost Crops of Africa, 1996). Its restricted distribution should not be taken as a measure of relative inferiority: black fonio may eventually have as much or even greater potential than its now better-known relative.
1.3.1.1 Description
There are actually two species of fonio. Both are erect, free tillering annuals. White fonio (Digitaria exilis) is usually 30-75 cm tall. Its finger-shaped panicle has 2-5 slender racemes up to 15 cm long. Black fonio (Digitaria iburua) is taller and may reach 1.4 m. It has 2-11 subdigitate racemes up to 13 cm long. Although both species belong to the same genus, crossbreeding them seems unlikely to yield fertile hybrids, as they come from different parts of the same genus (Lost Crops of Africa, 1996). The grains of both species range from “extraordinarily” white to fawn yellow and purplish. Black fonio’s spikelet are reddish or dark brown. Both species are more-or-less nonshattering.
1.3.1.2 Distribution
Fonio is grown as a cereal throughout the savanna zone from Senegal to Cameroon. It is one of the chief foods in Guinea-Bissau, and it is also intensively cultivated and is the staple of many people in northern Nigeria. Fonio is not grown for food outside West Africa.
1.3.1.3 Nutritional Properties of Fonio
Like other millets, fonio is widely reported to be rich in amino acids but particularly in the amino acids methionine and cystine (Lost Crops of Africa, 1996; Belton and Nuttall, 2002) which supply sulfur and other compounds required by the body for normal metabolism and growth. Methionine is an essential amino acid, which the body needs for health but cannot synthesise; it can only obtain it from food. It helps the liver to process fat, and is also a
methyl donor, capable of giving off a molecule needed for a wide variety of chemical and metabolic reactions inside our body, including the manufacture of the amino acid taurine. Cystine is a major constituent of the proteins that make up hair, nails and skin, and is involved in major detoxification processes in the body (Lost Crops of Africa, 1996).
Methionine levels are higher than those found in sorghum and other millets (Bavec and Bavec, 2006) and the amino acid is reportedly deficient in other major cereals such as wheat, rice, barley and rye (EFRT, 2000). When tested, fonio also appeared to be richer than pearl and finger millets in phenylalanine, another essential amino acid (Bavec and Bavec, 2006). The body needs phenylalanine to create various brain chemicals and thyroid hormones as well as tyrosine (another amino acid that it uses to make proteins). Tests have shown that the methionine level in fonio is twice that found in egg protein, (Lost Crops of Africa, 1996) leading to suggestions that fonio might be used to complement standard diets. It’s also suggested that the high content of these sulphur amino acids would make fonio an excellent nutritional complement to legumes (Lost Crops of Africa, 1996) as most legumes are low in methionine (but high in lysine, which is lacking in cereal grains).
Fonio is reputed to be richer in magnesium, zinc, and manganese than other cereals (Bavec and Bavec, 2002). It is also significantly richer in thiamine (Vitamin B1), riboflavin (Vitamin B2), calcium and phosphorous than white rice (Lost Crops of Africa, 1996). Levels of phosphorous, an essential mineral needed by all human cells for normal function, are high (Belton and Nuttall, 2002) and tests by the Laboratory of Food Technology and Animal Nutrition in Mali indicate that phosphorous and potassium are major minerals in fonio grains. Phosphorous is found mainly in bones and is a constituent of many vital compounds in the body, including ATP, DNA, and phospholipids. Potassium is crucial to heart function and plays a key role in skeletal and smooth muscle contraction, making it important for normal digestive function, while it is also an important electrolyte. Fonio also appears to have appreciable amounts of iron and, compared to white rice, is significantly richer in this essential mineral (Lost Crops of Africa, 1996) In Nigeria Fonio (known as acha) sellers identify diabetic patients as their major customers (IPGRI, 2002) and it is also reported that doctors usually recommend fonio to diabetic patients.
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