Control of pancreatic endocrine secretions by glucose and estimating the glycemic index of foods

The glycaemic index (GI) is a ranking of carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar levels after eating. Normal fasting blood glucose level should be in the region of 3.9 - 5.5 mM (70 - 100 mg/dL). There are fluctuations during the day, with glucose levels being the lowest during the morning and rising after meals for about an hour. An increase in blood glucose levels after a meal stimulates the release of insulin from the pancreas, which stimulates target cells to increase glucose uptake and storage. The impact of food on the blood glucose concentration is referred to as the glycemic response. Foods with high GI (e.g. bananas, potatoes, white bread, rice, chips) are rapidly digested and absorbed and result in a marked increase in blood sugar levels. Low-GI foods (e.g. meat, dairy products, rye bread, nuts, olives) are digested and absorbed slowly and thus result in a slower, gradual increase in blood sugar levels. In this laboratory you will assess the ability of insulin to regulate blood glucose levels following the consumption of an oral dose of glucose either in simple or complex forms. The standard procedure involves an oral glucose tolerance test (OGTT).

Learning Outcomes:

At the end of the laboratory the student will be able to:

1. Determine the glycemic index for a test food

2. Know the normal range for plasma glucose levels

3. Understand the regulation of plasma glucose by insulin and glucagon

4. Determine the effect of ingestion of simple and complex carbohydrates on plasma glucose levels

5. Discuss the benefits of a low GI diet in terms of risk protection from obesity, metabolic syndrome and cardiovascular disease.

6. Discuss some symptoms in uncontrolled diabetes

Important:

All the exercises outlined are voluntary and should not be performed in people with known or suspected metabolic disorders (diabetes), clotting disorders or blood-borne diseases. In addition, do not volunteer if you take any medication of feel sick.

Materials & Methods:

Subjects should fast for at least 12-14hrs. Drink plenty water and avoid caffeine (tea and coffee) and juices. Do not smoke.
Ideally one subject should do an OGTT with glucose and a complex carbohydrate. However logistics prevents this strategy from taking place, therefore half the class will consume glucose while the other half will eat bread.

1. Measure your height and weight.

2. Immediately before consuming the glucose/complex carbohydrate diet, take a urine sample in order to measure urine glucose and ketone levels.

3. Obtain a blood sample from a finger-prick and measure the blood glucose and ketone levels using the Accu-check blood glucose meter provided. Use the same glucose meter each time. There is evidence of white-coat hyperglycaemia, so it is important to be relaxed.

4. At t=0 ingest the simple/complex carbohydrate, which is 50g glucose in 200ml of water or 3 slices of bread along with water to help swallowing. Check the nutritional data sheet on the bread bag to ensure the amount of carbohydrate consumed.

5. At t=15, 30, 60, 90 and 120 min obtain finger-prick blood samples and measure the blood glucose concentration. A second blood ketone concentration is taken at t=120 min.

6. Take a second urine sample at 120 min for measurement of urinary glucose and ketone concentration.

7. Enter all data into the class spreadsheet.

Prepare for your tutorial:

1. Using the class data calculate the mean plasma glucose concentrations for each time point.

2. Plot the mean values (+/-SD) for changes in plasma glucose concentration against time for both carbohydrate loads.

3. Determine the total rise in blood glucose after eating a test food (3 slices of bread) and for the standard reference food (50g glucose), by calculating the area under the curve of your graph. The spreadsheet calculates this for each individual subject. The formula for the calculation is based on the following method (Chlup et. al., 2004):

i. A horizontal line (baseline) is extrapolated across the graph from the fasting blood glucose level at t=0.

ii. Vertical lines are run from the baseline (fasting glucose levels) to where the glucose concentration is after consuming the glucose/complex diet.

iii. The incremental area under the curve (IAUC) is calculated by the summing of the triangles and trapezoids given by the length of the base times the average heights of the two vertical sides over the 120 min. The IAUC reflects the total rise in glucose after eating a test food.

Calculate the mean incremental area under the curve (IAUC) (+/-SD) for the test food and the standard reference food.

4. Calculate the GI:

GI (%) = IAUC(test food) / IAUC(Std) *100

5. From your height and weight calculate your body mass index and determine your BMI category:

BMI = Weight (kg)/[Height (m)]2

BMI Categories (http://www.nhlbisupport.com/bmi/):

Underweight = <18.5 Normal weight = 18.5-24.9 Overweight = 25-29.9 Obesity = ≥30

6. Separate the data according to BMI into the four categories (see above) and determine whether their corresponding IAUC curve differs between the groups. Is there any significant difference between the mean values of IAUC for the different BMI categories and what might this indicate?

7. What is white-coat hyperglycemia?

8. Describe the role of insulin and glucagon in the regulation of plasma glucose.

9. What is the prevailing nutritional view regarding the consumption of foods with a low GI index in terms of risk protection from obesity, metabolic syndrome and cardiovascular disease?

10. Did you note the presence of ketones in the urine at t=0 and t=120min? Explain the mechanism responsible for the presence of urinary ketone and for any change.

11. Uncontrolled diabetes is characterized by hyperphagia, polydipsia, glycosuria and polyuria. What are the mechanisms occurring to produces these effects?

Bibliography:

1. http://www.glycemicindex.com/

2. Chlup R, Bartek J, Reznickova M, Zapletalova J, Doubravova B, Chlupova L, Seckar P, Dvorackova S, Simanek V (2004). Determination of the glycaemic index of selected foods (white bread and cereal bars) in healthy persons. Biomed. Papers 148(1): 17-25.

3. Jenkins DJA, Wolever TM, Taylor RH, Barker H, Fielden H, Baldwin JM, Bowling AC, Newman HC, Jenkins AL and Goff DV (1981). Glycemic index of foods: a physiological basis for carbohydrate exchange. Am. J. Clin. Nutr. 34: 362-366.

4. Ludwig DS (2007). Clinical update: the low-glycaemic-index diet. Lancet 369: 890-2.

5. Wolever TM (2004). Effect of blood sampling schedule and method of calculating the area under the curve on validity and precision of glycaemic index values. Br J Nutr. 91(2): 295- 301.

Attachment:- lab-spreadsheet-2016.xlsx