MB ChB Year 1: Nutrition and Energy

WORK SESSION 9: Diabetes

AIMS

To appreciate how energy metabolism is co-ordinated and regulated.

Using Diabetes Mellitus as an example, understand the consequences of disturbing the balance between metabolic pathways.

To be aware of the aetiology, complications and management of Diabetes Mellitus.

To gain experience in evaluating data to reach a clinical diagnosis.

OBJECTIVES

To complete an annotated diagram of the metabolic pathways involved in energy metabolism.

To be able to indicate the role of insulin and glucagon in the control of carbohydrate and lipid metabolism.

To be able to recognise the major symptoms of Type I and Type II diabetes and the underlying biochemical defects.

To appreciate the major complications of diabetes and why they occur.

To know which clinical tests are used in the diagnosis and monitoring of diabetes.

To be able to use patient history and results from blood and urine tests to aid in diagnosis.

To revise the metabolic processes involved in energy metabolism

PREPARATION

Revise glycolysis, TCA cycle, lipid metabolism and amino acid catabolism.

The flow diagram at the end of this exercise outlines how the energy producing pathways are connected. Annotate the diagram to show the key enzymes involved in regulation (oblong boxes)

Answer questions 1 & 2.

Complete Table 1 summarising the actions of insulin, and put this information on the flow diagram

During the session we will look at the changes in metabolism that occur in diabetes, the diagnosis and long term problems and use the information to evaluate two clinical cases.

Bring textbooks and notes covering biochemical and medical aspects of energy metabolism and diabetes.

This work session will help you understand and revise the lecture material on metabolism of carbohydrates and lipids. You should be getting a picture of how the pathways interact and are regulated to provide an appropriate blood glucose level, and mobilise and maintain energy reserves.

SOURCES OF INFORMATION ON DIABETES

Kumar & Clark Clinical Medicine pages 959-989.

Marks, Marks & Smith "Basic Medical Biochemistry".

NIDDK website (Lots of information, readable, but be selective. Particularly good on diagnosis and complications).

INTRODUCTION

Diabetes mellitus is caused by a failure to secrete, or to respond to, the hormone insulin. In insulin-dependent diabetes (IDD, Type I DM), the b cells of the pancreas are destroyed by an autoimmune response, leading to failure of insulin secretion. In non-insulin dependent diabetes (NIDD Type II DM), there is usually still insulin secreted, sometimes in excess amounts, but the cells do not respond to the signal as effectively, i.e. there is insulin resistance. Insulin signals the 'fed' state and promotes the uptake and storage of foods. It is part of the overall control of energy metabolism. Glucagon (and adrenalin and corticosteroids) oppose the action of insulin. Normally the system is well balanced – excess energy intake is stored as glycogen or fat and blood glucose levels are kept within a narrow range. To understand what happens in diabetes and why it is a serious problem, it is necessary to understand how the system is normally regulated.

Summary of action of insulin: Table 1

Insulin has an overall effect on many metabolic pathways. This may be stimulatory or inhibitory. Complete the table below.

metabolic activiy	Stimulated or inhibited by insulin?
Glucose uptake
Glycolysis
Gluconeogenesis
Glycogen synthesis
Glycogenolysis (glycogen breakdown)
Ketogenesis
Fatty acid synthesis
Lipolysis
Proteolysis
Protein synthesis

Note that most of these actions are opposed by glucagon and hence it is the insulin/glucagon ratio that is important.

On the energy metabolism flow chart, indicate where insulin and glucagon act on these pathways (+ = stimulatory, - = inhibitory)

1. Which tissues rely particularly on glucose as an energy source?

2. Which enzyme activities are affected by glucagon in its role of maintaining glucose concentration? Indicate on the flow chart which enzymes are affected.

Using the flow chart, your notes and textbooks, answer the following questions, which relate to the metabolism of carbohydrates and lipids:

A: NORMAL METABOLISM

Explain (briefly):

(a) How insulin promotes:

(i) the uptake and use of glucose by cells (adipose and muscle).

(ii) the storage of glucose as glycogen.

(iii) the conversion of glucose to triacylglycerols.

(b) Why low blood glucose increases the availability of Acetyl CoA.

B: IN RELATION TO DIABETES MELLITUS

In insulin-dependent diabetes (IDD, Type I DM) where there is a failure to secrete insulin.

Explain briefly:

(a) Why blood glucose is high.

(b)Why glucose appears in urine

(d) What are ketone bodies? Why ketones are formed, and what danger does this present?

(d) What is HbA_1C? How can measuring its level in the blood aid in diagnosing and monitoring diabetes?

C: LONG TERM COMPLICATIONS

Despite insulin replacement therapy, life expectancy in Type I diabetics is reduced. Diabetic neuropathy and cardiovascular disease are the major causes of death in patients over 50 years of age.

So far we have looked at the immediate changes caused by poor control of glucose levels, but hyperglycaemia and fluctuating insulin levels have longer term effects as well.

Increased use of minor metabolic pathways e.g. polyol pathway - some tissues have enzymes to convert glucose (if levels are abnormally high) to sorbitol and fructose. These are osmotically active and lead to swelling. Sorbitol may be toxic.

Changes in proteins: Glycation may occur. HbA_1C is a general example of this, but other proteins, including collagen Basement membranes, blood vessels etc. ) lipoproteins, receptors and lens proteins are also very susceptible.
Also the polyol pathway uses NADPH - required to keep levels of reduced glutathione in cells. Glutathione is important in keeping the protein thiol groups (-SH) reduced and preventing inappropriate aggregation of proteins.

Changes in insulin levels: Insulin is involved in long term growth/proliferation of tissues. Levels are often raised in Type II, and even in Type 1 the administered insulin is also likely to be above physiological levels some of the time.

Though not usually as severe, these complications are also a problem in Type II.

You should be aware of the principle diabetic complications and how tight control of glucose and insulin levels may help to reduce these problems. Make sure you can explain the following:

Which factors may increase the risk of coronary heart disease in diabetes? (Consider the activity of the lipases, changes in circulating lipoproteins and the effect of glycation)

How does diabetes affect the kidney?

What is diabetic neuropathy? Why should diabetics pay particular attention to their feet?

How/Why does diabetes affect the eyes?

(i) Retina

(ii) Lens

Is there any evidence that close control of glucose levels is advantageous? Are there any drawbacks?

D: PATIENT PRESENTATIONS

Patient A: A 55 year old bus driver reported that he had cut one of his toes while trimming his nails and, several weeks later, the toe had still not healed properly and was very uncomfortable. Examination showed his big toe to be ulcerated. He was considerably overweight and admitted that he had been feeling rather tired for some time. His family had come from the Carribean and he said that 'back home' he remembered that his gradmother and a number of aunts and uncles had a weight problem and found everything 'a bit of an effort'

Patient B: A student (male, N. European, 19 years old) visited the Student Health Centre saying he felt 'really tired and out of sorts'. On questioning, he admitted to a high comsuption of beer. He was advised to 'take it easy and get more sleep', but replied that he had to keep getting up in the night to 'get rid of all that beer'. He tried cutting down the beer, but he was still going to the bathroom quite frequently, and the tiredness was getting more debilitating. He retruned to the Student Health Centre and a more thorough examination showed that his weight was beolw normal, though he claimed to be eating normally. He wasn't aware of any health problems in his immediate family.

Both patients were referred to hospital for further investigation.

The test results are given below:

E: DIAGNOSTIC TESTS FOR DIABETES

Included below are a number of tests which may be used to diagnose or monitor diabetes. The results of such tests for the two patients, A and B, are given . Use these, together with the case histories, to come to a reasoned, evidence-based diagnosis.

(i) Glucose tolerance test

The handling of glucose can be studied by the glucose tolerance test, in which a standard amount of glucose is taken by the subject and the concentration of blood glucose determined at suitable intervals thereafter. A graph is plotted of blood glucose against time. Typical results are given in the following table and graph:

Note: The indications for using this test would be a borderline fasting blood glucose and/or a persistent glycosuria (e.g. in pregnancy).

Table 2

Glucose tolerance test: blood glucose (mM)
	Before glucose	1/2 hour after	1 hour after	11/2 hours after	2 hours after
Normal	4.8	6.0	7.1	5.8	4.5
Severe (uncontrolled) diabetes	11	15	16.4	17	14.5

Table 3

Patient blood glucose Results(mM)
Patient	0	15 min	30 min	60 min	90 min	120 min
Patient A	8	8.5	10	11.5	10	10
Patient B	11	12	15	17	19	18
Normal fasting blood sample 3.5-5.5 mM

Graph 1: Glucose tolerance test - blood glucose levels after ingestion of glucose

Table 4

Diagnosis of Diabetes Mellitus - 1985 WHO Guidelines

fasting

(mM)
2 hours after

glucose load (mM)

Whole Blood
Venous
³ 6.1
³ 10.0

Capillary
³ 6.1
³ 11.1

Plasma
Venous
³ 7.0
³ 11.1

Impaired tolerance: Fasting plasma glucose £ 7.00mM, with 2 hr value ³ 7.8mM but £ 11.1mM

Impaired fasting glucose:: fasting plasma glucose ³ 6.1mM, but £ 7.0mM

(ii) Urinalysis

During the glucose tolerance test, two urine samples from Patient A and B and a normal urine sample were tested qualitatively for glucose and ketone bodies using Multistix reagent strips.

Table 5

Patient Urine Results
	Patient A		Patient B
	0	1 hour	0	1 hour
Glucose	-	+	+	+++
Ketones	-	-	+	+
Normal urine sample: -ve

(iii) Fast Hb (HbA1_c)

Blood samples were taken for the estimation of HbA_1C levels.

Table 6

Patient HbA_1C (%)
A	10
B	18
Normal	5

(iv) Insulin levels

The shape of the glucose tolerance curve is, in part, dependent upon the efficiency of insulin secretion and information about this can be obtained by measuring plasma insulin concentrations during the glucose tolerance test by an immunoassay.

Circulating levels of most hormones are very low and cannot be measured by normal chemical methods. Radioimmunoassay (RIA) was previously commonly used for assaying hormones but is now being replaced by immunometric type assays. Immunoassays are capable of extreme specificity, sensitivity and precision and can allow many samples to be assayed rapidly. Consequently they are especially useful for hormones whose concentrations are very low, or for distinguishing between compounds of very similar chemical structure.

All immunoassays rely on highly specific antibodies which bind specifically to the hormone being measured. The hormone-antibody interaction is measured by using a labelled hormone or antibody during the assay incubation and then separating the bound and free fractions. Previously, the label was radioactive (RIA), but now more frequently, fluorimetric, colorimetric, chemiluminescent or enzymatic labels are used which have a lower risk in terms of health and safety. The main limitation of immunoassays is that they do not distinguish biologically inactive molecules from biologically active molecules since both have the same antigenic determinants.

Table 7

Patient insulin results (m U/ml)

0h
1h

Patient A
60
120

Patient B
9
16

Normal
55
102

Note: This test would not be used routinely in the diagnosis of diabetes, but might be used to establish the cause of unexplained hypoglycaemia.

F: PATIENT REPORTS

Record the test results for patients A and B. Together with the patient histories, come to a clinical diagnosis. You must say why you have come to this diagnosis. Use the proforma on the following page and highlight the difference between type 1 and type 2 diabetes.

Results		Patient A	Normal	Patient B
GTT: Blood glucose (mM)	0 hr		4.8
	1 hr		7.1
	2 hr		4.5
Urine glucose	0 hr		-ve
	1 hr		-ve
ketones	0 hr		-ve
	1 hr		-ve
Glucose handling (normal/poor)
RIA Insulin (uU/ml)	0 hr		55
	1 hr		102
Insulin levels (high/normal/low)
HbA_1C (Fast Hb) % (normal/raised/high)			5

Diagnosis of patient

Patient A

Patient B

Supporting evidence from history and tests

Explanation of symptoms

Primary underlying cause

Treatment (principles only)

Flow Diagram

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Study Guide

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WS2

WS3

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WS10

Diagnosis of Diabetes Mellitus - 1985 WHO Guidelines
		fasting (mM)	2 hours after glucose load (mM)
Whole Blood	Venous	³ 6.1	³ 10.0
	Capillary	³ 6.1	³ 11.1
Plasma	Venous	³ 7.0	³ 11.1

Patient insulin results (m U/ml)
	0h	1h
Patient A	60	120
Patient B	9	16
Normal	55	102