Why do people who participate in marathons tend to have smaller muscles?

Because they more focus on muscle endurance then that of muscle strength.

Explanation:

Long distance runners need their bodies to better focus on their muscles to all work together at the same time to help them all keep a steady pace, go a long distance and more. Muscle strength is more focused on individual muscles themselves, have one or a few used at once. That is to build up certain muscles that will help with other things, not long distance running.

so if you eat a 2000-calorie diet, you should aim for about 225 to 325 grams of carbs per day.

but if you need to lose weight, you will get much faster results eating around 50 to 150 grams of carbs.

in fact, a popular alternative called the low-carb diet has been shown to be much more effective for weight loss than the high-carb diet that has been recommended for the past few decades.

this diet restricts your intake of carbohydrates like sugars and starches (breads, pasta, etc.) and replaces them with protein, fat and healthy vegetables.

studies show that low-carb diets reduce your appetite and make you eat fewer calories and lose weight pretty much effortlessly, as long as you manage to keep the carbs down (1trusted source).

in studies where low-carb and low-fat diets are compared, the researchers need to actively restrict calories in the low-fat groups to make the results comparable, but the low-carb groups still usually win (2trusted source, 3trusted source).

low-carb diets also have benefits that go way beyond just weight loss. they lower blood sugar, blood pressure and triglycerides. they raise hdl (the good) and improve the pattern of ldl (the bad) cholesterol (4trusted source, 5trusted source).

low-carb diets cause more weight loss and improve health more than the calorie restricted, low-fat diet still recommended by many people. this is pretty much a scientific fact at this point (6, 7, 8trusted source).

explanation:

there are many studies showing that low-carb diets are more effective and healthier than the low-fat diet that is still recommended by many people.