[vhfpmr (OP)]

When you store energy as fat, how much is left by the time it's retrieved again?

[evan_au]

It depends on how far you have to carry it.
It takes more energy to carry around energy in the form of body fat, "saved up for later".

[Colin2B]

One way of looking at efficiency is to measure how much energy is lost when we store calories and then reuse them. If we look at the energy required for the digestion, absorption, transport, and storage of food, it accounts for approximately 10% of daily calorie burn.
This assumes that calorie intake is at the maintenance level ie no weight gain or loss. If you consume more calories than you need the excess doesn't all go to fat because the body begins to create more heat, so the process becomes less efficient.

[vhfpmr (OP)]

Hmm, it takes energy to carry a battery around too, but that's not what's normally meant by its efficiency. Like charging and discharging a battery, what's the loss of converting from blood sugar to fat, and back to blood sugar again? The energy used in digestion is a loss that's external to the storage system, and common to whether the energy is stored or used immediately. The premise of the question is that some of the surplus energy is lost as heat, otherwise the efficiency would be 100%, and the system would contravene the laws of thermodynamics.

[greengo]

Since the fat gram holds 9 cals, I should say it is a very efficient fuel.  When accessing fat energy stores, the body needs carbohydrates to "spark" the fat burn.  When in carbohydrate deprivation, the brain and body must utilize either protein or fat energy to function.  The brain and nervous systems will not operate in an 'efficient" manner when deprived of carbohydrates and have to run on fats.  Usual mental abilities are slowed down and a general irritability is experienced.  In this sense fat energy is not efficient. 

[adneybradyxz]

body weight is effect on our body so eat healthy food

and add some spices
i am also add a food an spices.

you know about spices and its healthy benfits

[Colin2B]

The energy used in digestion is a loss that's external to the storage system, and common to whether the energy is stored or used immediately.

Sorry, missed this. Seeing your other post jogged my memory.

Ok, I can now see what you are asking. Just an aside, although digestion is common to both storage and usage  there are different pathways for carb and fat, hence different efficiencies.

If you are just considering blood sugar then if you have 100 extra calories in glucose in your bloodstream, it will take 23 calories to convert the glucose into fat and then store it. Compare this to 100 extra calories of fat floating in the bloodstream it will only take 2.5 calories to store it. So the body prefers to store fat because it is more efficient.
Of course it is possible to store limited amounts of glycogen in muscle, but fat is good for bulk storage.

If you are interested in the broader subject there is a lot of research published on efficiency of foodstuffs fed to animals, pigs are similar to human.