Estimated read time: 4.5 minutes, (about as long as you can hold off breakfast after a fasted run).

Hey Performance Nerds! Jonah here. 🤓

I used to program fasted runs. Training low means starting a session on purpose with low fuel (glycogen).

Many studies found it helps the muscle adapt. I stopped programming them anyway.

I still run them. Just not for that.

Here's what you'll learn today:

(Augie chases squirrels fasted every morning, then eats breakfast and sleeps for four hours. His race results remain unpublished.)

The gel I race with and keep recommending because the formula makes sense: BETA FUEL delivers 40g of carbs per gel through a 1:0.8 maltodextrin-to-fructose ratio.

That ratio is the point. It helps you use multiple carb transporters, push higher intake, and keep the gut calmer when fueling gets serious.

Stryd gives runners power, pacing, and mechanical metrics that can make training decisions clearer when pace and heart rate do not tell the whole story.

See this week's full Stryd training tip at the end of this newsletter.

Glycogen is the carbohydrate your muscles and liver keep on hand. It comes from the carbs you eat.

Your body burns it first when the pace picks up. Training low means starting with less of it.

You see three methods most often:

Most runners picture a low-carb diet.

A low-carb diet cuts carbs every day, for weeks. Training low cuts them around chosen sessions only.

Start a session low and your muscle notices. Its low-fuel sensors fire harder.

The signal to build more mitochondria gets stronger. These tiny engines burn fat and carbs with oxygen.

A 2018 review in Sports Medicine collected the evidence (Impey et al., 2018).

Most one-workout studies found stronger signals. Oxidative enzyme content rose in 78% of the training studies.

They help muscles use oxygen for energy.

Fat oxidation, your rate of burning fat during exercise, climbs too. Roughly 20 to 35 percent, depending on the protocol.

Anyone selling you train-low isn't making it up.

In that 2018 review, only about 37% of studies showed a performance improvement.

A 2021 research review pooled nine studies in well-trained endurance athletes (Gejl and Nybo, 2021). Their verdict: planned low-carb training doesn't make trained athletes faster.

The results stay mixed for three reasons:

One controlled cycling trial is what moved me (Hearris et al., 2019). Same session, run three times, at three glycogen levels.

Time to exhaustion fell from 44 minutes, to 36, to 18. The signaling was comparable across all three.

In that trial, cutting carbs further bought no extra signal. It only cost work.

Two trials favor training low.

A 2016 sleep-low study improved a 10-km run by about 3%. A 2021 cycling trial raised threshold power by 5.5%.

The 2016 group also lost three times as much fat mass. That's why reviewers discount it.

The 2021 trial called itself a feasibility study, done at home during lockdown. But one feasibility study still doesn't outweigh nine pooled studies.

I'm not saying never experiment. If training time is tight, an occasional easy session low on carbs may give the muscle a stronger signal.

We still don't know if that makes you faster.

Running low can drop your pace and power at the same effort, and it raises how hard it feels.

Low carbs and low energy availability aren't the same thing.

Energy availability is what your body has left from food once exercise takes its share.

Prolonged or severe low energy availability can lead to Relative Energy Deficiency in Sport, or REDs (Mountjoy et al., 2023).

The consensus says very low carb intake can shift bone, inflammation, and iron markers. Some shifts showed up even with enough energy available.

The short-term evidence points to three shifts:

Six studies since 2019 reported at least one of these shifts.

Several of those studies lasted six days or fewer.

They tested male endurance athletes eating less than 3 g of carbohydrate per kg daily.

Those carb intakes are well below the table's recommendations.

Training low can make you more likely to underfuel.

If you're already short on daily energy, I don't program it.

The phrase isn't mine. That same 2018 review carries it.

The authors who mapped the case for training low wrote the framework I use instead. It means matching carbs to the session in front of you.

Not low all the time. Not high all the time.

I still run some easy sessions under an hour fasted because I don't feel like eating.

When I meet daily calorie and carb needs, I usually have enough stored fuel.

I eat before hard sessions and long runs. Those are the sessions worth protecting.

(I miss having a cooler answer.)

Training low does raise fat oxidation. That part isn't in dispute.

It still hasn't always made trained athletes faster.

Your capacity to burn fat tracks the mitochondria you carry. Regular training helps build them over time.

Carbs help you finish the session, recover, and come back for the next one.

Under-fueling can reduce the work you complete. Repeat that pattern and training quality can slip.

I want to become a better fat and carb burner to produce the most amount of energy I can in a race.

I need a lot of training, enough fuel, and good recovery. I would rather fuel the work and protect training quality.

So my fueling follows one rule. Don't let it make the training worse.

The daily numbers come from a 2016 joint position stand by three organizations.

Easy pace burns glycogen slowly. An easy hour usually asks for less fuel than you already carry.

Let the session decide the carbs.

Bottom line: Stop chasing fat oxidation. Start finishing the sessions that make you faster.

Jonah

Welcome to the prove you’re a nerd section. Each week, I ask a question about a common running science myth.

Answer correctly, and you’ll be entered into a weekly raffle to win a package of Jonah’s favorite supplements.

Last week’s poll asked: “Which muscle group handles some of the largest forces in running while helping support and propel you through late stance?”

Most of you identified the muscle group doing some of the hardest work under the body.

The correct answer?

A. Calf muscles

The calf complex supports the body and contributes large forces during late stance as you move forward into the next stride. The quadriceps, glutes, and hamstrings all matter, but the calf muscles best match that support-and-propulsion role.

Here’s how the votes shook out:

A. Calf muscles: 167 votes, about 76 percent
B. Quadriceps: 9 votes, about 4 percent
C. Glutes: 32 votes, about 15 percent
D. Hamstrings: 11 votes, about 5 percent

Total votes: 219

Why C was tempting:

The glutes are large, powerful muscles and contribute to propulsion. The question’s late-stance support role is what points more directly to the calf complex.

Bottom line?

Your calves are not accessory muscles. They are part of the main engine supporting and propelling each stride.

I've been doing my heat training on an indoor treadmill lately.

My usual easy pace is 8 mph, about 7:30 per mile.

For this heat session, I set the treadmill console to 7.5 mph, which should have been 8:00 per mile.

It felt extremely fast. So much for easy.

The session fell apart, and I wasn't wearing my Stryd.

I ran it again with my Stryd on.

Stryd read 7:15 per mile. The belt was moving about 10 percent quicker than the console claimed.

Peer-reviewed testing found belt speed drifting from the target. Within-stride dips hit roughly 2 to 10 percent on most machines, worse on worn ones.

My gap sat at the top of that range.

Stryd measures displacement from one stride to the next, not belt speed.

Why it matters:

This week's issue is about carbohydrate protecting your ability to repeat quality sessions. Going accidentally hard spends that same capacity.

Wear your Stryd on your next treadmill run and compare pace to the console.

Try it on your next run