Fasted running: what your own heart rate says about it
You run your usual morning loop before breakfast, at your normal easy pace. Your heart rate sits a few beats higher than usual, and the last kilometres feel heavier. Is that your fitness, or the breakfast you skipped?
In short
- Your body then has fewer carbohydrates ready to use and draws more of its energy from fat.
- It does not help you lose extra weight.
- After a run or a ride of at least twenty minutes, the review sheet asks "Fasted?", with No and Yes.
- A measurement needs at least four easy sessions on each side.
1.What fasting does in your body
Fasted here means you train without having eaten in the hours before, usually in the morning before breakfast. Your body then has fewer carbohydrates ready to use and draws more of its energy from fat.
That has a price you can see. Burning fat costs a little more per litre of oxygen than burning carbohydrates, and a smaller store gives your body an extra push. Both lift your heart rate at the same pace. In the second half of an endurance run, your heart rate often climbs further when you are fasted than after breakfast.
How much that adds up to differs from person to person. For one runner it barely shows, for another it is a handful of beats. That is why a general rule tells you little here, and your own measurement tells you a lot.
It does not help you lose extra weight. You burn more fat during the session, but whether your weight goes down is decided by your energy balance over the whole day and the whole week: what you eat against what you use.
2.The "Fasted?" question
After a run or a ride of at least twenty minutes, the review sheet asks "Fasted?", with No and Yes. When you save your own recording, this sheet opens straight away. When several activities come in at once, the question is on the batch sheet. No is already selected and counts when you save. If you trained fasted, tap Yes.
Your answer changes nothing about your minutes, your distance or your load: the session is done and counts as usual. If you choose Yes, the session detail shows the line "Fasted · Yes" next to your perceived effort.
3.The "Fasted" block in the session detail
Open a fasted session with Open details and analysis, and the Fasted block sits directly below Energy system. It sets your easy fasted sessions next to your easy sessions after eating, per sport. Tempo and interval work is left out, because there the effort decides and not the fuel.
Heart rate at the same pace says how many beats higher or lower you sit when fasted than after eating, at the pace you ran that day. On the bike it is at the same power. Heart-rate drift sets side by side how far your heart rate climbed in the second half, fasted and fed.
Last 4 weeks shows whether the gap is getting smaller. When it does, it says your body is getting used to training fasted. Sessions counts how many of each side are in the measurement.
A measurement needs at least four easy sessions on each side. Until then the same spot shows how far you are, for example "3 of 4 fasted sessions", with what you can mark after your next easy session.
| Line | What you see |
|---|---|
| Heart rate at the same pace | How many beats higher or lower you sit when fasted than after eating |
| Heart-rate drift | How far your heart rate climbed in the second half, fasted and fed |
| Last 4 weeks | Whether the gap is getting smaller |
| Sessions | How many sessions of each side are in the measurement |
4.What your plan does with it
If fasted is the exception for you, your fasted sessions do not count towards your aerobic efficiency, your endurance pace band or the way your expected effort adapts to your own ratings. One fasted endurance run with a higher heart rate does not drag those down. If you run half or more of your sessions fasted, that is your normal state and everything counts.
If your last two quality sessions in a sport were both fasted, and both felt harder than your plan expected, your next quality session carries one line: eat something with carbohydrates 1 to 2 hours before. Easy sessions do not get that line.
Beyond that your plan stays the same. Whether you train fasted is your choice.
Frequently asked questions
Why is my heart rate higher when I run fasted?
When fasted, your body draws more energy from fat, and that costs a little more per litre of oxygen. On top of that, a smaller carbohydrate store gives an extra push. Both lift your heart rate at the same pace.
Will I lose more weight if I train fasted?
No. You burn more fat during the session, but your weight follows your energy balance over the day and the week. What you eat and what you use decide it.
Does a fasted session count less in my week?
No. Your minutes, distance and load count as usual. A fasted session is only left out of your aerobic efficiency and your endurance pace band, as long as fasted is the exception for you.
Why don’t I see the Fasted block?
It only appears on a session where you chose Yes, and only once an easy fasted session has been counted for that sport. Answer the "Fasted?" question on the review sheet after your easy sessions, and it will appear.
In the app
Further reading
Can you lose weight by running or walking without wrecking your training?
Losing weight and training go together as long as your deficit stays small. You aim for half a kilo a week, eat enough protein, leave hard weeks alone and read your weight as a trend.
Why your heart rate in zone 2 sometimes tells you almost nothing
In the easy range your heart rate responds little and late. It also creeps up on its own during a long run. Why a zone boundary says little there, and what to steer by instead.
Recovery run: why you run it slower than you think
A recovery run feels easy and your heart rate stays low, and yet you often run it too fast. Read why your heart rate says little there, why pace is what counts, and how Trimetra steers your recovery run.
Trimetra sets your fasted sessions next to your fed ones, so your own heart rate shows you what running fasted does for you.