Exercise insulin and glucose

In a non-diabetic, exercising muscles first use their stored glycogen. Glucose is then taken up from the bloodstream as required for continued exercise. As the blood glucose concentration falls, pancreatic insulin release is reduced and glucose is released from the liver glycogen stores to 'top up' the blood glucose concentration. If carbohydrate has been eaten, this will be absorbed into the bloodstream and insulin will be released if necessary to store it in the liver and/or facilitate its use by the exercising muscles. Liver glycogenolysis will cease while the glucose derived from the meal is distributed. However, if exercise continues and the blood glucose level falls, insulin release will fall and liver glycogenolysis will again release glucose into the circulation.

This process can still occur in a person with diabetes treated by diet alone, and, to a large extent in metformin-treated patients. However, as soon as sulphonylureas or particularly insulin injections are introduced, the fine tuning of glucose balance in exercise is disturbed.

In a person with insulin-treated diabetes who exercises, the effects on blood glucose and other biochemistry, such as lipids, depend on the amount of circulating insulin and how much food has been eaten. The crucial difference between the diabetic and non-diabetic athlete is that there is no fine on/off control of insulin release.

Table 12.1 The training zone

Age (years)

Training zone—heart rate (beats/minute)

60%

85%

Maximum

15

123

174

205

20

120

170

200

25

117

166

195

30

114

162

190

35

111

157

185

40

108

153

180

45

105

149

175

50

102

145

170

55

99

140

165

60

96

136

160

65

93

132

155

70

84

119

140

75

87

123

145

80

84

119

140

To derive greatest benefit from exercise the heart rate should be within the training zone for 20 minutes or longer, most days. At first aim for the lower end of the training zone—60 per cent, or lower in someone who has not exercised recently. Do not use for people with autonomic neuropathy.

To derive greatest benefit from exercise the heart rate should be within the training zone for 20 minutes or longer, most days. At first aim for the lower end of the training zone—60 per cent, or lower in someone who has not exercised recently. Do not use for people with autonomic neuropathy.

Someone who is insulin-deficient is likely to have a high blood glucose. Exercise will further increase the blood glucose as the stress hormone response releases glucose from the liver. As the exercise continues the muscles take up glucose from the bloodstream. However, this effect is unlikely to outweigh that of hepatic glucose release. Lipolysis, which occurs in prolonged exercise to provide free fatty acids as an additional fuel, may be followed by ketone formation in insulin deficiency. Any food eaten will merely serve to exacerbate the hyperglycaemia. Thus exercise may worsen hypergly-caemia and promote ketosis in an insulin-deficient person.

In someone who has a large subcutaneous reservoir of injected insulin, hypogly-caemia may ensue. As before, the exercising muscles will use their stored glycogen. The presence of insulin ensures good glucose uptake by the exercising muscles. However, high plasma insulin concentrations inhibit glucose release from the liver thereby further reducing the blood glucose concentration. Hypoglycaemia rapidly ensues. This situation can be prevented by eating carbohydrate which will be absorbed and top up the blood glucose level as exercise proceeds.

In patients taking sulphonylureas, the drugs enhance pancreatic insulin release, as well as improving tissue glucose uptake. They may thus produce hyperinsulinaemia and can cause exercise-induced hypoglycaemia.

EXERCISE, INSULIN, AND GLUCOSE 117

Liver Food Fat

Liver Food Fat

Glucose Fatty acids

At rest

Brief exercise

Longer exercise

Food

Glucose and fat

Glucose and fat

Less glucose and

absorbed into the

absorbed into the

fat absorbed as

blood

blood

some blood diverted

to muscle

Insulin

Released from the

Less insulin released from pancreas as

normal

pancreas according

glucose falls

to blood glucose

diabetic

Released from the

More released from injection site as

injection site

circulation to muscles and skin

increases

Liver

Stores glucose as

Starts to release

Releases a lot of

glycogen*

glucose ~

glucose ~

Fat

Stores fatty acids*

Starts to release

Releases a lot of

fatty acids ~

fatty acids ~

Muscle

Stores glucose as

Converts glycogen

Takes up glucose

glycogen*

to glucose for

and fatty acids and

energy ~

uses them for

* Needs insulin

energy*

~ Blocked by insulin Fig. 12.1 Exercise, food, and insulin

~ Blocked by insulin Fig. 12.1 Exercise, food, and insulin

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