The answer is....

1.2 - 1.4 grams protein per kilogram per day

For example, if you weigh 110 lbs, this is equivalent to 50 kg (kilograms=pounds/2.2)

50 kg x 1.2 = 60 grams
50 kg x 1.4 = 70 grams

So a 110 lb runner needs somewhere between 60 and 70 grams protein per day.

Although carbs are your body's primary source of fuel, protein is crucial for endurance training. Protein is necessary for muscle growth and repair, hormone production, hair and nail health, immune function, and red blood cell production. During intense training, protein may be used for fuel as well. The carbon backbone of amino acids converts to acetyl-coA, which is able to enter the Krebs Cycle to produces ATP (fuel for muscles). This generally only occurs when there is excess protein after growth and repair or if their is insufficient energy from carbohydrates and fatty acids.

More important than overall protein intake is making sure to get some source of protein with every meal and snack. Have you ever found yourself hungry very quickly after a bagel and cream cheese or a big plate of pasta? Carbohydrates are digested very quickly, starting with the production of the salivary enzyme amylase. The carbs enter the bloodstream, blood sugars go up, and insulin is released from the pancreas to (1) shuttle glucose into cells so it can be used for energy and (2) to lower blood sugars back to a normal level. The greater carbohydrate load at a meal, the more insulin that is released and the further our blood sugars will fall. You may be hungry before actually having metabolized the carbs. Including a protein (or healthy fat) helps slow the release of the glucose into the blood, thus providing sustained energy.

You don't want to eat too much protein close to a workout because of the slower digestion (it takes about 4 hours for proteins to be completely digested), which may lead to cramping due to poor blood flow to the gut. You do want to include adequate protein at breakfast and after hard workouts. Breakfast is especially important for blood sugar balance. Your blood sugars are the most susceptible for peaks and valleys in the morning. You have essentially been fasting for 8-12 hours and blood sugars are low. You will most likely crave carbs in the morning because of low blood sugars and low serotonin levels (carbs help produce serotonin), but you must balance them with protein or you can find yourself hungry the whole day, crashing in the afternoon, and craving sweets after dinner. Next time you find yourself wanting a sweet after dinner or caffeine in the afternoon, take a look at the protein in your breakfast.

Breakfast options for protein - yogurt (look for low sugar varieties and/or higher protein Greek yogurt), cottage cheese, nut butters (all natural almond, cashew, peanut), all natural chicken breakfast sausages, nuts or seeds, whey protein powder, or eggs.

Milk does have 8grams protein per 8 ounce serving, but it also has 12grams carbs due to the milk sugar lactose. When consuming cereal and milk, look for an additional protein source.