Occasionally athletes will travel to an high altitude area for training. While there is some great evidence for physiological benefits of doing this, you need to be aware of the potential effects on your nutritional status.
Exposure to high altitude results in an increase increased utilisation of blood glucose as a fuel at rest and during exercise. With a heavy training load the body requires a greater than usual amount of carbohydrate for energy. The shift for fuel usage is from fats to carbohydrate, and hence a corresponding reduction in the use of dietary fat and stored fat.
Acclimatisation at altitude causes a reduction in total body water and plasma (or blood) volume. Given that the air can be cold and dry more fluid is also lost while breathing. These increased losses necessitate greater fluid intake than at sea level to stay well hydrated.
Basal metabolic rate increases at altitude. It is important to increase energy intake to match this increase in BMR.
Living at high (>2500-m) altitude can cause AMS or acute mountain sickness. You may experience a loss in appetite, nausea and/or vomiting, and hence a reduction in your food intake. There would be only a very small risk of this in Flagstaff.
There are a few reasons to explain the common loss in body weight at altitude.
When the oxygen in the environment is low, your body needs to adapt to increase the circulation of oxygen around the body. This is partly achieved by increasing haemoglobin in the blood. There is an increased need for iron to support the production of haemoglobin, meaning an increase in dietary requirements of iron.