What is molecular weight (mass?

In the case of non-short term weight gain, what proportion of the new mass comes from breathed-in oxygen and what proportion comes from food?

• I'm trying to better understand the process of weight gain/loss in biological and physical terms. Conservation of Mass means your body can't gain more weight/mass than it takes in, but people tend to think about weight change in terms of calories, which leads to confusion. If you ask most people whether they will gain more weight by eating 1/2 pound of chocolate vs 2 pounds of salad, they will say definitely the chocolate. To close this gap, what is the max weight you can gain from eating 1 pound of the most efficiently metabolized food, and what percentage of this weight will come from the food itself vs. the oxygen you breathe after eating it (lets say you don't drink anything)? Ps. Also interested in generalizations of this question: What fraction of mass is retained by a typical person for different types of food? What are the key physiological factors that raise or lower mass retention?

• Answer:

  Weight Gain from Oxygen Oxygen doesn't really has an effect on weight gain. Our bodies utilize it as the final electron acceptor in the process of cellular respiration, and it just forms water at the end of the cycle. Law of Conservation of Mass It's definitely true that your body can't gain more weight than what it takes in; however, that doesn't mean that your body has to turn all the mass that it takes in into fat or muscle (the two most stable forms of weight gain). Your body can only metabolize and make energy from certain compounds (and energy is what really matter since fat is stored energy and it takes energy to build muscle, so muscle, like fat, is a form of stored energy, albeit, more useful); those compounds being: sugars (4 kcal/gram), fats (9 kcal/gram), and proteins (4 kcal/gram). Keep in mind a kcal is 1000 calories, or 1 "nutritional calorie." (When you look at the "nutrition information" the caloric content is in nutritional calories.) Our bodies can't use energy from any other source besides those three compounds; fiber, vitamins, minerals, and water do not contain any calories since our bodies cannot metabolize them to extract energy from them. The 1/2 pound of chocolate vs 2 pounds of salad It's true that people will gain more weight from eating 1/2 a pound of chocolate than from eating 2 pounds of salad (assuming that all the calories are stored as fat). Chocolate is more calorically dense, meaning that within 100 g of chocolate there is more sugar/fat/protein (things that our bodies can obtain energy from) than there are in 100 g of salad. Salad contains lots of fiber and water and it doesn't have a very high caloric density compared to chocolate since a much lower proportion of its mass contributes calories. 2 pounds (~900 g) of salad contains ~150 calories (according to  http://caloriecount.about.com/calories-salad-vegetable-tossed-i21052). 1/2 a pound of chocolate contains 1233 calories (based on searching Google for "chocolate nutrition" ) and 2 pounds of chocolate contains almost 5,000 calories! Let's put this in terms of fat mass gain, assuming that any calories taken in will contribute to weight gain in the form of fat (which isn't very likely, but it keeps things simple). It's generally taken for granted that there are 3,500 calories in one pound of body fat; that means that the calories in 2 lbs of salad will make ~0.04 pounds of body fat, and the calories in 1/2 lb of chocolate will make ~0.35 pounds of body fat.  The Most Efficiently Metabolized Food Our bodies prefer to use glucose for energy, but can use other sources of energy (fats and proteins), but they must go through intermediate steps to be prepared for metabolism, while glucose is immediately ready for use. If you were to consume one pound of glucose then I believe most of it would be used by the body rather quickly, especially if you're engaging in strenuous exercise. The glucose that you don't use is stored either as glycogen or as fat. As far as the amount of weight that you'll gain from eating 1 pound of glucose, I'm not really certain. It would depend on several factors such as basel metabolic rate (BMR) and activity levels. However, glucose (as a simple carbohydrate) is metabolized very quickly, so if you're not going to use it soon then it's going to get stored as fat or glycogen rather quickly. A complex carbohydrate will take longer to digest and metabolize, and won't cause an insulin spike, so less of the calories would be stored as fat, and more would be used for energy needs. Fraction of Mass Retained This has to do with the caloric density of the food, as I described above, as well as your BMR and how much you exercise. Weight gain generally comes down to the "calories in, calories out" ideology; however, if you eat a lot of sugar (simple carbohydrates i.e. sucrose) then that will promote weight gain in the form of fat since your body quickly absorbs the sugars, finds that it doesn't need so much energy all at one time, and then it will store that as fat.