Insulin, explained: What it does and why it matters
Before I met my partner, I knew remarkably little about insulin. I knew it had something to do with diabetes, of course, but I couldn’t have explained what it actually did, let alone the difference between basal and bolus insulin.
These days, those words are part of my everyday vocabulary. I know why insulin matters, how different types are used and, perhaps most unexpectedly, what it smells like.
Because insulin has a smell. A surprisingly distinctive one, at that. It’s a little like encountering a particular scent for the first time: once you’ve learnt to recognise it, you’ll notice it again immediately.
It’s one of those small things nobody necessarily tells you about insulin. And it reminds me how much there is to learn about something that, for millions of people, is simply part of everyday life.
So whether you’ve recently been diagnosed, someone close to you uses insulin, or you’re simply curious about how it works, let’s start with the basics.
What is insulin, and what does it actually do?
Insulin is a hormone produced by the pancreas, an organ that sits behind the stomach. Its job is to help the body manage glucose, a type of sugar that provides energy for our cells.
When we eat foods containing carbohydrates, our digestive system breaks many of them down into glucose, which enters the bloodstream. In response, the pancreas releases insulin.
You might have heard insulin described as a key that unlocks our cells, allowing glucose to enter. It’s a useful starting point, although the reality is a little more complicated. Insulin helps certain cells take up glucose, encourages the body to store energy for later and signals the liver to reduce the amount of glucose it releases into the bloodstream.
And insulin isn’t only needed when we eat. Our bodies require energy around the clock, including while we’re sleeping or going about our day between meals.
In someone without diabetes, the pancreas continuously adjusts how much insulin it releases to help maintain glucose levels within a healthy range.
For someone living with diabetes, that process doesn’t work in quite the same way.
What happens when the body doesn’t produce enough insulin?
Although different types of diabetes have different causes, they share a common challenge: the body has difficulty regulating glucose in the bloodstream.
In type 1 diabetes, the immune system attacks the insulin-producing cells in the pancreas. As a result, the body produces little or no insulin. People with type 1 diabetes need insulin treatment to survive.
In type 2 diabetes, the body becomes less responsive to insulin, a process known as insulin resistance. Over time, the pancreas may also struggle to produce enough insulin to meet the body’s needs. Some people manage type 2 diabetes without insulin treatment, while others benefit from or require it.
Insulin may also be prescribed for other forms of diabetes, including some cases of gestational diabetes.
When there isn’t enough effective insulin, glucose can build up in the bloodstream rather than being regulated as it should.
For people who need insulin treatment, the aim is to provide the insulin their bodies cannot produce in sufficient quantities or use effectively.
And that brings us to two words you’ll hear frequently in conversations about insulin: basal and bolus.
Basal and bolus insulin: What’s the difference?
These terms can sound technical at first, but the underlying idea is fairly straightforward.
Basal insulin is the background insulin your body needs throughout the day and night.
Even when you’re not eating, your liver releases glucose into your bloodstream to keep your body supplied with energy. Basal insulin helps regulate this process.
Think of it as the background support that continues while you’re working, resting or sleeping.
Bolus insulin is an additional dose of insulin given for a particular purpose, most commonly to help manage the rise in glucose after eating. Depending on someone’s treatment plan, a bolus may also be used to correct glucose levels that are above their target.
The distinction is particularly important for people using a basal-bolus insulin regimen or an insulin pump.
Someone using injections might take long-acting insulin for their background needs and rapid-acting insulin around meals.
Someone using an insulin pump generally receives small amounts of rapid-acting insulin throughout the day to cover basal needs, with additional boluses when required.
The goal is similar, but the way insulin is delivered differs.
Of course, knowing what basal and bolus mean doesn’t make managing them effortless. Insulin requirements can change with food, physical activity, illness, stress, hormones and other everyday circumstances.
It’s one reason diabetes management often involves considerably more thought than outsiders might realise.
Why are there different types of insulin?
Insulin isn’t a single, interchangeable medication. Different formulations have been developed to work at different speeds and for different lengths of time.
Some are designed to act relatively quickly, while others provide insulin over a much longer period.
Rapid-acting insulin
Rapid-acting insulin is commonly used around mealtimes or to correct elevated glucose levels.
It generally begins working relatively quickly, although the exact onset, peak and duration depend on the formulation. Some newer ultra-rapid formulations act sooner than conventional rapid-acting insulin.
Short-acting insulin
Sometimes called regular or soluble insulin, short-acting insulin generally takes longer to begin working than rapid-acting formulations.
Because of that difference, the timing of doses around meals may also differ.
Intermediate-acting insulin
Intermediate-acting insulin, such as NPH insulin, provides background coverage over part of the day or night.
Its action is less even than that of some longer-acting formulations, and it generally has a more noticeable peak.
Long-acting and ultra-long-acting insulin
These formulations are designed to provide background insulin over an extended period.
Some work for approximately a day, while others last considerably longer. Their duration and whether they have a noticeable peak vary by product.
Mixed insulin
Mixed or premixed insulin combines different insulin components in one preparation, providing both shorter-acting and longer-lasting effects.
It can suit certain treatment routines, although it may offer less flexibility around meal timing than other regimens.
The important thing to understand is that different insulin types are not automatically interchangeable. Their timing, concentration and action profiles matter.
Choosing an insulin formulation and deciding when and how much to take are decisions to make with a diabetes healthcare team.
How is insulin delivered?
When people picture insulin treatment, they often imagine injections. And while injections remain common, there are several ways insulin can be administered.
Insulin pens
Insulin pens are widely used and can be disposable or reusable, depending on the product.
They allow people to dial up a prescribed dose and inject insulin into the fatty tissue beneath the skin, usually in areas such as the abdomen or thigh.
For some people, insulin pens are a straightforward part of their daily routine. Others find injections uncomfortable, inconvenient or emotionally difficult. Both experiences are valid.
Insulin pumps
An insulin pump is a small device that delivers insulin throughout the day.
Rather than relying on a separate long-acting insulin injection for background coverage, most pumps use rapid-acting insulin delivered in small amounts continuously.
Additional insulin can be delivered for meals or glucose corrections.
Some pumps work alongside continuous glucose monitors (CGMs) as part of an automated insulin delivery system. These systems can adjust insulin delivery based on glucose readings, although users still have responsibilities such as managing meals, responding to alerts and maintaining the equipment.
My partner uses an insulin pump, and living alongside that technology has given me a much greater appreciation of how much goes into everyday diabetes management.
What looks like a small device is part of a much bigger routine involving supplies, glucose readings, decisions and the occasional technical frustration.
And while technology can make certain aspects easier, it doesn’t remove every challenge.
Other delivery methods
Insulin can also be delivered through syringes, and certain specialised formulations or delivery methods are available in particular countries and clinical circumstances.
The best option depends on medical needs, availability, personal preferences and the advice of the treating team.
Does insulin have a smell?
A slightly unexpected question, perhaps, but yes — many insulin preparations have a recognisable medicinal smell.
That characteristic scent comes largely from ingredients such as phenol and metacresol, which are used as preservatives in various insulin formulations.
Not every insulin preparation smells exactly the same, and the smell isn’t a reliable way to judge whether insulin is safe or effective.
But for me, it’s become one of those familiar details of living alongside someone with diabetes.
It’s funny how something you once knew almost nothing about can become recognisable in ways you never anticipated.
A few common misconceptions about insulin
Despite being an essential and widely used treatment, insulin is still surrounded by misunderstandings.
Some are simply confusing. Others can contribute to unnecessary worry or stigma.
“Needing insulin means you’ve failed at managing diabetes”
Absolutely not.
For people with type 1 diabetes, insulin is essential because the body can no longer produce enough of its own.
For people with type 2 diabetes, insulin may become an appropriate treatment for many reasons, including changes in insulin production, illness, pregnancy or difficulties achieving glucose targets with other treatments.
Needing insulin is a medical consideration, not a judgement on someone’s effort, discipline or lifestyle.
“Insulin is only used when diabetes becomes severe”
Not necessarily.
People with type 1 diabetes require insulin from diagnosis, while people with type 2 diabetes may be prescribed insulin at different stages and for different reasons.
Sometimes insulin is used temporarily, such as during certain illnesses or periods when glucose levels are particularly difficult to manage.
The decision is based on individual clinical circumstances rather than a universal progression from one treatment to another.
“Insulin always causes weight gain”
Some people experience weight gain when beginning or adjusting insulin treatment, but it isn’t inevitable.
When previously elevated glucose levels improve, the body may retain and use more of the energy it consumes. Other factors, including treatment routines and the need to manage hypoglycaemia, may also play a role.
Weight changes deserve individual, non-judgemental discussion with a healthcare professional rather than assumptions about someone’s eating habits or behaviour.
“Insulin causes diabetes complications”
Insulin itself is not the cause of complications such as diabetic eye disease, kidney disease or nerve damage.
Over time, prolonged elevated glucose levels can contribute to these complications, alongside other factors such as blood pressure and cholesterol.
Insulin treatment can help reduce the risks associated with high glucose levels, although it also needs careful management because too much insulin can cause hypoglycaemia.
“Insulin means you can’t live a full life”
Insulin treatment can require planning, adjustment and attention. There are days when that feels relatively manageable and others when it is exhausting.
But using insulin doesn’t mean giving up travel, work, relationships, good food, exercise or the things that make life enjoyable.
People build full and varied lives around insulin treatment every day.
That doesn’t mean diabetes is always easy. It means a diagnosis and its treatment are only part of a person’s story.
What about low blood sugar?
One of the most important things to understand about insulin treatment is that insulin can lower blood glucose too far.
This is known as hypoglycaemia, often shortened to a hypo.
It can happen when there’s more insulin acting in the body than is needed at that moment. Possible contributing factors include changes in food intake, physical activity, alcohol, illness or the timing and amount of insulin.
Symptoms can include trembling, sweating, hunger, dizziness, confusion and difficulty concentrating. Severe hypoglycaemia can cause seizures or loss of consciousness and requires urgent assistance.
People using insulin should have an individual plan for recognising and treating hypos, including access to appropriate fast-acting carbohydrate and, where prescribed, emergency glucagon.
If you’d like to understand more about what a hypo can feel like, our article When blood sugar drops: What a hypo really feels like explores the experience in greater detail.
Understanding insulin is one thing. Living with it is another.
Learning what insulin does is relatively straightforward once someone explains the basics.
Understanding what it means to rely on insulin every day is something else entirely.
It’s the practical decisions that accompany meals, the supplies packed before leaving home, the glucose readings that don’t always behave as expected and the need to think ahead even when life is busy.
It’s also the technology, the changing routines, the accumulated knowledge and the personal preferences that develop over years of experience.
Before meeting Lee, I didn’t know the difference between basal and bolus insulin. Now I understand the terminology, recognise the equipment and even know that distinctive smell.
But I’ve also learnt that understanding the science is only one part of understanding diabetes.
The rest comes from listening to the people who live with it.
And perhaps that’s the most useful place to begin.
This article is intended for general information and understanding. Insulin is a prescription medication, and its type, dose and timing should always follow an individual treatment plan agreed with a qualified healthcare professional. Never stop, switch or adjust prescribed insulin based on general online information.