Thermogenesis occurs when the metabolic rate increases above normal. Macronutrients have varying effects on the thermogenic response. When food in consumed, the metabolic rate increases above the fasting level. This rise is often referred to as specific dynamic action, or SDA. Thermogenesis also occurs in the process by which brown adipose or baby fat cells generate heat and burn calories in the digestion of foods that aren't counted on the label. Unfortunately the body changes unsaturated fat to saturated (added Hydrogen atoms) fat in response to cold, so staying cold may not be as effective in burning calories as once believed. As far as protein is concerned, it is believed that thermogenesis occurs because the body must use energy to process protein, as opposed to carbohydrates and lipids, which are more efficiently metabolized. Therefore, carbs and fats have a lower thermogenic effect.