|View from the conference centre, May 2016|
Many hormones are involved in keeping blood glucose levels stable with exercise, including insulin, glucagon, growth hormone, cortisol and adrenaline. For someone with T1D, insulin is delivered in a very non-physiological way via subcutaneous fat rather than into the hepatic bloodstream from the pancreas. It is also thought that glucagon production by the pancreas becomes less efficient over time following a diagnosis of T1D. Each of these hormones has multiple effects at different organs (brain, muscles, liver, pancreas etc.) and all interact with each other. This complex situation means that the tight regulation of blood glucose with exercise that happens automatically when the pancreas is working properly is almost impossible to achieve with a broken pancreas.
The study day
The course was a single day, but they packed a great deal into it. Speakers presented slides with graphs and evidence and whizzed through topics at such a pace that I could barely keep up let alone take comprehensible notes. The slides were supposed to be available after the event, but I don’t think they have appeared yet, a month later. My scribbled note “good slide explains this bit” will have to wait for interpretation later.
The first speaker talked about ‘normal’ exercise metabolism, the second introduced T1D into the metabolic picture, and the third session was presented by paediatric and adult Dietitians. After a break there was more detail about managing blood glucose before, during and after exercise. The workshops after lunch gave us the chance to think about case studies and individual scenarios.
Overall I think everything was included that needed to be included, but much too fast, and the main focus was on serious athletes and people who were going to be running or cycling or weight lifting or at least going to the gym regularly. There was very little about the unfit or overweight person who might be starting with walking up a flight of stairs rather than taking the lift, or trying to increase their level of activity for weight loss or fitness rather than competing for an Olympic medal. Gardening, DIY, housework and shopping are the more common types of activity that I encounter in my caseload.
I did a little brainstorm for this blog entry just listing all the issues that pertain to the subject – the list was 2 pages long. So what shall I include here? Of course, this particular blog post probably isn’t going to be of much interest to you unless you have Type 1 Diabetes and you want to know about managing your blood glucose while exercising, and I think I may have fewer than one reader in that particular category. No, this blog post is for me, to enable me to assemble my thoughts and produce a reference point for that future day when I might have to advise a patient on this subject.
Fuel for activity
So, first to recap the basics. Dietary carbohydrate is digested into glucose which moves into the blood to be transported around the body. Insulin allows blood glucose to be taken up by cells in the body where it is metabolised into energy or stored as glycogen in muscle and liver. Excess glucose is converted into fat in the form of triglycerides (a triplet of linked fatty acids) and stored in the liver, muscle and in fat cells. High levels of insulin promote this storage process and inhibit the release of glucose or fat into the blood from fat and liver cells.
When energy is needed for activity, the most accessible sources are muscle glycogen and blood glucose. The hormone glucagon prompts the liver to very quickly start converting its stored glycogen into glucose (glycolysis) and send it out into the blood. Triglycerides in the muscles are also easily accessible and are used as fuel (fat oxidation). It takes a bit longer for new glucose to be manufactured in the liver (gluconeogenesis) and for the liver to break down triglycerides into free fatty acids and send them out to be used as fuel (fat oxidation). Insulin levels need to be low for all these processes to work efficiently.
If exercise is more intense (anaerobic) there is more reliance on carbohydrate as fuel; if exercise is less intense but goes on for longer (aerobic) there is a shift towards fat as the main fuel. Obviously exercise drains glycogen stores in muscles and liver, and these are ‘topped up’ afterwards using dietary glucose (fat stores don’t need to be topped up!) Non-diabetic metabolism manages all the hormone levels so all this takes place with blood glucose maintained between 4 and 7 mmol/L at all times.
The main difference that makes things difficult for someone with T1D is that insulin cannot be regulated up and down in a physiological way. It is certainly possible to adjust insulin levels according to various ‘rules’, but adjustment is crude and doesn’t reflect the metabolic state minute by minute.
There are also a couple of scenarios when it is not advisable to exercise. If your blood glucose is high (over 14 mmol/L) then it is possible that you don’t have enough insulin on board, and the official advice is that you need to check for ketones. If blood glucose is high without ketones then a small correction dose of insulin might be all that is needed, but if ketones are present then the full correction dose should be given and exercise postponed until ketones have gone. The majority of people with T1D don't have a meter that will measure blood ketones, however, so this advice is moot.
The other situation when you might choose not to exercise is if you have had a hypo in the last 24 hours, because this makes a hypo with exercise even more likely. If it wasn’t a serious hypo needing third party assistance then you might go ahead bearing in mind the need to be extra vigilant. If the hypo was within an hour before planned activity you would be advised to wait for 45-60 minutes after your blood glucose level has stabilised before exercising.
Changes in blood glucose and insulin
The level of your blood glucose will fluctuate according to:
- the duration, intensity and type of activity
- the type and amount of food and snacks eaten or drunk before, during and after the exercise
- the level of stress and competitiveness
- your level of fitness or previous training
- hydration status
- the time of day
The level of your blood insulin will fluctuate according to:
- the timing of insulin injections/infusion
- the amount and type of insulin injected/infused
- the site of the injection or cannula
- the ambient and body temperature.
Poor ‘reproducibility’ was highlighted in the study day, meaning that the same exercise for different people or even for the same person on different days may have very different effects on blood glucose levels. With all these variables it’s not surprising that matching blood glucose levels and blood insulin levels in order to manage T1D and exercise is a minefield.
So this is the landscape we're working in, with different sources of fuel and the action of hormones all interacting, and we have to try to maintain blood glucose levels without going low or high using tools (carbohydrate and insulin) that are about as precise as trying to steer a car at full speed with just your elbows on the steering wheel. At some point you're probably going to crash.
So having set out the scale of the problem, how can it be managed? Look out for part 2 in the series, coming soon!