Bolus insulin: why timing beats “more insulin” for meals and corrections

Overview

This could be the most important thing you will ever read about diabetes. Bolus insulin is how you cover meals and correct highs — and the timing mismatch between food and insulin is the key to stopping spikes, preventing hypos, and mastering Dynamic Glucose Management.

This is one of the trickiest Foundations reads. The approach here is deliberate: start simple, then layer the complexity. If you understand bolus timing, everything else gets easier.

The podcast episode is also useful: Episode 7: Fundamentals of The Glucose Never Lies.

The detail

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 into target — a correction bolus.
• Or both combined.

This diagram shows why bolus insulin is a timing problem: fast-acting insulin starts slowly, often peaks around ~2 hours after delivery, and can keep lowering glucose for ~4 hours.

Matching bolus insulin to meal carbohydrate

Your diabetes team should teach you to count carbohydrates to the gram. If not, don’t worry — we build that skill here.

You’ll then be given insulin-to-carbohydrate ratios (carb ratios) for different times of day.

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

Example: if a person eats 50g carbohydrate at each meal:

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 hugely between people. One major driver is physical fitness and daily activity: the fitter and more active you are, the less insulin you usually need for the same carbohydrate intake (so ratios often look “weaker”).

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

How do I know if my carb ratios are right?

This is a clean test when life is reasonably stable (and you can run it more than once):

1. No food or bolus insulin in the four hours before the test.
2. Eat a balanced meal (carbs, protein, veg) with your usual carbohydrate amount for that mealtime.
3. Avoid a high-fat meal. High-fat meals often need extra insulin later — covered in the Mealtime Insulin Guide.
4. Count the carbs accurately.
5. Give the insulin before the meal (we’ll cover timing properly below).
6. Do only your usual activity and no corrections in the four hours after eating.
7. Review results and adjust if needed:
   • Too weak → make the ratio more aggressive by ~10–20%.
   • About right → no change.
   • Too strong → make the ratio weaker by ~10–20%.

Why does glucose still rise after eating when the carb ratio is perfect?

If your ratio is perfect, you can still see a post-meal rise. That’s not failure — it’s physiology.

Compared with glucose entering from a meal, injected/pumped insulin is slow. It’s a tortoise-and-hare situation.

The simple solution: give insulin about 20 minutes before eating. Studies suggest this keeps the spike under much better control for many meals.

Even with a head start, a small spike often remains. Two big reasons:

1) Portal insulin problem. People without type 1 diabetes don’t spike as much because insulin arrives in the portal vein as food arrives, damping the liver’s glucose output.

In type 1 diabetes, there is very little insulin in the portal vein and relatively more in the bloodstream.

2) Absorption speed problem. Insulin from injections or pumps absorbs much slower than insulin secreted straight into the portal system.

This double-whammy (slow absorption + low portal insulin) is why spikes happen. The good news: they’re controllable.

How? Using SET from Dynamic Glucose Management. Teaser: you change pre-meal timing based on current glucose and trend arrows — but finish Foundations first.

The Mealtime Insulin Guide is packed with spike-busting tactics too.

Does injection site affect spikes?

Yes. Fast-acting insulin usually absorbs fastest in the abdomen, second fastest in the upper arm, and slowest in the outer thigh.

Top tips:

• If using a pump, stick mainly to abdomen and upper arm for cannula sites. Switching regularly from thigh to abdomen can create a roller-coaster because absorption speed changes.
• If using injections, many people do better injecting basal into thigh or buttocks and fast-acting into abdomen or upper arm.

Avoid lumpy areas (lipohypertrophy). Insulin absorbs poorly from damaged tissue, driving highs and unpredictability.

Rotate sites to prevent lumps. Here’s a simple rotation system:

Do large bolus doses absorb more slowly?

Yes. A big insulin depot tends to absorb more slowly than the same dose split into smaller depots. Same principle: surface area and diffusion.

You don’t want to split every dose. Keep it simple: consider splitting any bolus above ~10–15 units.

• Injections: split into two equal amounts at least 2 cm apart (e.g., 20 units → 10 + 10).
• Pump: use a split/extended bolus — around 50% upfront and 50% over the next 15–30 minutes.

Practical foundations

Re-read the timing and portal-insulin sections a few times. They are essential for Dynamic Glucose Management and for mastering meals in real life.

What’s next

Next up: Correction insulin.

Foundations / related content

The order shown below is recommended, but navigate as you see fit.

• What is Diabetes?
• Continuous glucose monitoring (CGM)
• Basal insulin
• Bolus insulin
• Correction insulin
• Carbohydrate counting
• Three balanced meals
• Hypoglycaemia
• Measuring success

References

• Carbohydrate counting
• Correction insulin
• Dynamic Glucose Management
• SET framework
• Mealtime Insulin Guide
• Fast and slow movers