I mean it may at first but the body is good at adapting to changes like this for long term (Keto diet is an excellent example of this). Besides I will still be eating carbohydrates (simple and complex) just one time a day but not in the same quantity as 3 meals a day. Reserves of sugar are also stored in the liver in case the body is really desperate for it.

If you could do serious damage to yourself only eating once a day we would've died out hundreds of thousands of years ago.

Hundreds of thousands years ago life expectancy wasn’t 80yo.

That is mostly because of childhood mortality. A large number of babies were unlikely to make it to the age of five. Which significantly drags down the average life expectancy.

And I'm saying if only eating a meal a day did us harm, given the _way_ worse conditions of most of our history, we wouldn't have nearly gotten this far.

It doesn't mess with your blood sugar. Assuming you're eating a low sugar meal with high fat and complex starches, your blood sugar stays at a normal level. It does take time for your body to adjust, though.

I can wake up and go for a 15 mile hike with no breakfast and not feeling hungry.

See: intermittent fasting

There is an American meme that believes that not eating often and regularly is unhealthy. It’s untrue. Study after study shows that fasting is good for us.

Exactly. And no one wonders why the countries with the highest rates of obesity are those in which a high calorie breakfast is the norm and actively recommended.

Breakfast is the least natural meal, you should not even be hungry when you wake up, it's unhealthy and should be skipped.

I heard it said elsewhere that a mouse deprived of food dies in a matter of dys while even a slender built human can with water last approx a month before succumbing to starvation. 3 meals a day is more a modern indulgence than best practice.

Blood sugar levels are not something that needs to be babysat by a metabolically healthy person. We should be able to seamlessly move from being fueled by glucose to ketones. (Well, I suppose suddenly running out of available glucose because of strenuous endurance activity can sometimes be rough waiting for the switch to complete. Thing is, once in ketosis this kind of activity is easier as energy supply becomes more steady state)

The only broad concern I would have is being more prone to accidentally having too much calorie restriction for a prolonged period, where metabolism reduction, potential thyroid issues, and potential nutrient deficiencies can come into play based on highly individual thresholds.

I also suppose some people who fast, especially on very low carb/carnivore diets can sometimes experience the exact opposite of your concern, their blood sugar levels can become constantly elevated, seemingly induced primarily by elevated cortisol, though also seemingly effected by other complicated individual factors like the kinds of stored fatty acids they are burning.