: How long before carbs wear off? I have a question about carbs for the first hour of a workout. I want to maintain 45 grams of carbs per hour of work out. I can eat right before the workout and be good for the first hour.

As the comments hit on, you could take this question various ways. I'm also going to have to dive deep in my brain back to my exercise physiology classes, but I think I can give something relevant scientifically and practically.

Scientifically

There are 4 calories per gram of carbohydrate. You want to use 45 grams per hour:

45 * 4 = 180 calories per hour

Now, this makes the assumption all you burn during your cycling session are carbs. That's not how it works. Burning macronutrients is a dimmer switch. You're never completely burning one and completely not burning others. For instance, when Usain Bolt runs a 100 meters, it's not like his body completely stops using fat.

That said, as the intensity of the activity goes up, carbs (glucose) are more and more utilized.

--> For someone wondering about a keto diet i.e. "How can a person use glucose without eating any carbs?" the body can make glucose through other means. Gluconeogenesis.

Let's say you are 80 kilograms (175 lbs).

You are cycling at an intensity roughly equivalent to pretty fast jogging. We can call that 7 METS.

One MET is equal to a relative oxygen consumption of 3.5 mL/kg/min.

1 liter of oxygen consumed burns 5 calories.

To convert to calories burned over an hour then,

(7 METS * 3.5 mL per kg per min * 80 kilograms * 5kcal per liter * 60 minutes) / 1000 ml per liter
Equals 588 kcal

Or just use this calculator :).

First point: 45 carbs per hour is a very leisurely pace. The above is only for a somewhat intense jog. Walking a mile is about 100 calories. At 3 miles per hour of walking, you're at 300 calories after an hour.

Barring the person being very out of shape, you're just not going to feel any ill effects of a lack of carb intake from going at or less than a walking pace, regardless of when you last ingested them.

Practically

I very, very rarely give specific nutrition advice to my personal training clients for before their sessions. It's one of the most common questions I get, "What I should eat before I come in?" yet my response is almost always "You'll have to play around with that and see what works."

Some clients do great with their 6am session not eating anything beforehand, while others realize they need to get up 15 minutes earlier and at least eat some oatmeal, otherwise they feel like death during a workout.

--> This assumes we're doing something fairly cardiovascularly demanding. If we're moseying around lifting weights, you're unlikely to need anything to eat regardless.

Anecdotally, once a person hasn't eaten within about 4 hours of a session is when they can feel negative effects. Again, if doing an intense workout. My experience says this is probably more about blood sugar levels than it is depleting glycogen. Many can adapt to eating less before a workout just fine, and muscle glycogen does not deplete that fast. You're likely going to need at least 24 hours (assuming daily life isn't strenuous) for this to be a factor.

So unless you're fasting / eating zero carbs for a full day before your bike session, you should have plenty of carbs (glycogen) within you.

Overall, your average person does not workout hard enough to worry about hitting a wall or bonking from physical activity. You're typically having to do hours of activity at a person's limit for it to matter.

However, if you want to look at this more, that's where I'd go. People generally "bonk" around the 20 mile mark of a marathon, and that has been well studied.

Though I'd also stress to notice how variable even that is amongst people.

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