Bolus Insulin

What bolus insulin covers

Bolus insulin means delivering fast-acting insulin to cover:

• Glucose from digested carbohydrate — a meal bolus.
• Bringing a high glucose level back towards target — a correction bolus.
• Or both combined.

The diagram below shows why bolus insulin is fundamentally a timing problem. Fast-acting insulin starts slowly, often peaks around 2 hours after delivery, and can keep lowering glucose for around 4 hours.

Matching bolus insulin to meal carbohydrate

The starting point for meal bolusing is counting carbohydrates accurately. Your diabetes team should be able to teach this skill — it is also covered in the GNL carbohydrate counting guide.

You will then be given insulin-to-carbohydrate ratios (carb ratios) for different times of day. These vary between people and between mealtimes.

Example carb ratios (where u = units of fast-acting insulin and g = grams of carbohydrate):

• Breakfast: 1u : 15g
• Lunch: 1u : 25g
• Evening meal: 1u : 20g

For someone eating 50g carbohydrate at each of these meals:

Grams carbohydrate in meal ÷ grams in carb ratio = meal bolus units

• Breakfast: 50 ÷ 15 = 3.3 units
• Lunch: 50 ÷ 25 = 2.0 units
• Evening meal: 50 ÷ 20 = 2.5 units

Carb ratios vary considerably between people. One major driver is physical fitness and daily activity — the fitter and more active a person is, the less insulin they typically need for the same carbohydrate intake, so ratios often look “weaker”.

Carb ratios also vary by time of day because insulin sensitivity changes through the day. Many people tend to be least sensitive in the morning and most sensitive at lunch. The point isn’t the pattern — it’s that different ratios by time of day are entirely normal.

How to assess whether carb ratios are well-matched

This test works best when life is reasonably stable, and is worth repeating more than once:

1. No food or bolus insulin in the four hours before the test.
2. Eat a balanced meal (carbs, protein, vegetables) with your usual carbohydrate amount for that mealtime.
3. Avoid a very high-fat meal for this test — high-fat meals often need extra insulin later, which is covered in the Mealtime Insulin Guide.
4. Count the carbs accurately.
5. Give the insulin before the meal (timing is covered in the next section).
6. Do only your usual activity and no corrections in the four hours after eating.
7. Review the results:
   • If glucose runs persistently high after the meal, the ratio may be too weak — many people find adjusting by ~10–20% at a time works well.
   • If glucose returns to where it started, the ratio is close to right.
   • If glucose drops after the meal, the ratio may be too strong — adjusting by ~10–20% is a reasonable place to start.

Why glucose tends to rise after eating even with a perfect carb ratio

A perfect carb ratio does not prevent all post-meal rise — and that is not a failure. It is physiology.

Compared with glucose entering the bloodstream from a meal, injected or pumped insulin is slow. It is a tortoise-and-hare situation.

Giving insulin about 20 minutes before eating tends to keep the post-meal spike under better control for many meals. Studies support this as a practical approach for a wide range of meal types.

Even with this head start, a small spike often remains. Two mechanisms explain why:

1. The portal insulin problem

People without type 1 diabetes tend not to spike as sharply after meals because insulin arrives in the portal vein at the same time as food, damping the liver’s glucose output.

In type 1 diabetes, there is very little insulin in the portal vein and relatively more in the general bloodstream. The liver’s meal-time glucose output is therefore less effectively damped.

2. The absorption speed problem

Insulin from injections or pumps absorbs from subcutaneous tissue much more slowly than insulin secreted directly into the portal system.

This combination — slow absorption plus low portal insulin — is the mechanism behind post-meal spikes. The good news is that it is addressable.

The SET framework from Dynamic Glucose Management covers how to vary pre-meal timing based on current glucose and trend arrows. The Mealtime Insulin Guide also covers practical spike-reduction approaches in detail — worth returning to after finishing Foundations.

Does injection site affect how quickly insulin works?

Yes. Fast-acting insulin tends to absorb fastest in the abdomen, second fastest in the upper arm, and slowest in the outer thigh.

Things worth exploring with your care team:

• If using a pump, many people do better keeping cannula sites to abdomen and upper arm, and rotating regularly within those areas. Switching between thigh and abdomen can create unpredictability because absorption speed changes.
• If using injections, many people find it works well to inject long-acting basal into the thigh or buttocks, and fast-acting into the abdomen or upper arm.

Lumpy areas (lipohypertrophy) should be avoided. Insulin tends to absorb poorly from damaged tissue, which can drive persistent highs and unpredictability. Regular site rotation helps prevent lipohypertrophy from developing.

Do large bolus doses absorb more slowly?

Yes. A large insulin depot tends to absorb more slowly than the same dose split across smaller depots. The mechanism is surface area and diffusion — a smaller depot has more surface area relative to its volume and disperses faster.

Splitting every dose adds unnecessary complexity. A practical approach many people find useful: consider splitting any bolus above around 10–15 units.

• By injection: split into two equal amounts at least 2 cm apart (for example, 20 units → 10 + 10).
• By pump: use a split or extended bolus — around 50% upfront and 50% over the next 15–30 minutes.