At altitude our bodies basal metabolic rate, which is how much energy we need to lie on a bed and do nothing, is much higher. This is especially apparent in the first few days at altitude. So if your body needs more energy to do nothing, then it certainly needs more when exercising. The energy systems you use alter too. Where before you may have used a high percentage of your fat stores to fuel your endurance exercise, now your body has a greater reliance on carbohydrates.
A good idea to combat this, plus the addition of appetite suppression, is to eat high carbohydrate snacks/meal little and often. Never over-facing yourself with a large plate full but constantly supplying your body with energy. OTE energy drinks can be great for when training as they are refreshing and aid hydration, but also are supplying you with 40g of carbohydrates per 500ml bottle. This coupled with some real food (such as bananas) or easy to consume and convenient OTE energy gels can help to ensure you are reaching your carbohydrate needs.