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We all know that about 3500 calories is equivalent to 1 pound (7500 cals for 1kg).
So can a piece of food weighing one pound have more than 3500 calories?
What are the highest and lowest energy dense foods? and will consuming the former make you less full while the latter make you feel fuller for less calories? or do the calories consumed cause you to feel full?
Is there a limit to how many calories can be in 1 pound of food (including artificially made "foods")?
You seem to be equating food weight with body weight, and they are not directly related.
Yes, if you eat a pound of something, you will immediately weigh one more pound, as your body has not had a chance to digest it and process it as needed. However, that doesn't mean that you will have gained one permanent pound. The body will break down the food, distribute the end result to various places for either use or storage, and get rid of whatever is not digestible.
Weight fluctuates during the day, so the best gauge of your weight is to weigh yourself at the same time every day, under the same conditions. Track that number, and that gives you your true weight.
Also, weight gain/loss is a relationship between how many calories you need to sustain your day to day activities, and how many you eat. If you consistently eat more calories than you need for a day, then you will gain weight. If you consistently eat less, then you will lose weight. The rate at which you do so varies on how big the deficit/surplus is, how efficient your metabolism, type of calories, many factors such as these.
For your main question, 3500 calories per pound of food is very calorie dense. For example, 1 lb of peanut butter is going to be ~ 2600 calories. The higher the fat content, the closer you get to that mark. If you ate a straight pound of fat, for example, you would get just over 4000 calories. (Using the typically accepted 9 calories per gram of fat, which is also not 100% accurate). So yes, it is possible to get more than 3500 calories in a pound of food.
Kind of. Oils/fats/alcohol has 9 calories per gram. 1 pound is 453.593 grams. 9 x 454 = 4,086 calories. So eating 1 pound of oil will give you 4000 calories. However, you will be unlikely to keep all that oil in your digestive tract!
Eating hygroscopic foods will also make you 'gain' more than they weight, because they will attract water. Eating honey or lots of fiber will absorb water and 'appear' to gain weight while it's in your intestines, but you'd also have to drink liquids.
Any answer to this type of question is going to be constrained by two basic laws of physics, which are conservation of mass and conservation of energy.
Conservation of mass tells us that any weight you gain or lose is going to be equal to the difference between the mass you take in and the mass you excrete. Furthermore, the body doesn't transmute one chemical element into another, so conservation of mass can be applied separately to every element.
If you could eat a pound of food and gain more than a pound of body weight, then that additional mass would have to come from some other imbalance between consumption and excretion. But this is pretty implausible because the mass of carbon is conserved by chemical reactions. Carbon is one of the main components of organic matter, so any weight gain is going to require some gain in carbon. Eating food is the only way your body has of bringing in carbon, while it has lots of ways of excreting carbon, including exhaling CO2.
Of course you can gain or lose body weight by taking in or losing water, which contains no carbon. But this is not a way of doing any long-term change in body weight.
In terms of energy, the body has some efficiency for digesting food, which depends on factors such as what type of macronutrient you're eating, and how highly processed the food is. This efficiency is very high when you eat fats -- I've seen estimates of 97-100%. If weight gain means storing energy in fat, then at the high end of this range you're simply transporting fat molecules through your mouth and into your tissues. In that case you could gain a pound by drinking a pound of olive oil, but you can't gain more than a pound of fat -- that would violate both conservation of carbon mass and conservation of energy.
1 pound of pure fat, for example, oil, has 453.6 g x 9 kilocalories, which is 4.082 kilocalories. When metabolized, up to 5% of calories from fat can get lost due to thermic effect of food, so, theorhetically, you could get ~3, 878 calories from one pound of oil, which could be theoretically converted to ~430 g of body fat. Anyway, this does not happen at any given point of time, because some of dietary fats will be quickly burnt for current metabolic needs, before even completely absorbed.
I'm not aware of any nutrient or food additive that would have more than 9 kcal/g.
From the perspective of one macronutrient (carbohydrates, proteins, fats and alcohol) promoting the absorption of another macronutrient (and ths calories): unlike in micronutrients (minerals, vitamins), this does not likely happen, because absorption of each macronutrient is independent of each other. Macronutrients can speed up or slow down the absorption of other macronutrients, but this does not likely affect the amount of total macronutrients being absorbed at the end.
Certain nutrients, for example glycerol, can promote temporary retention of water and thus temporary increase body weight (but this has nothing with promoting calorie absorption or retention).
