Anything that causes negative energy balance (such as calorie restriction, fasting or exercise) may lead to lower body temperatures. That’s if the negative energy balance is not compensated with sufficient fuel food intake. But lowering of the body’s temperature isn’t necessarily a bad thing. Interestingly, it is one of the most notable markers of longevity. Regardless, to avoid that side effect and keep your body warm, do not water fast during the winter months. Have warm soups and hot drinks during the undereating phase. Avoid selenium and soy containing products as these may inhibit your thyroid. Add dark chocolate (no sugar added) to your diet. Cocoa has been shown to boost nitric oxide and increase circulation which may help increase your body heat.