Oddbean

Many people know carbs are 4 kCal/g, protein is 4 kCal/g, and fat is 9 kCal/g. Some people know that alcohol is 7 kCal/g How many people know the energy content of citric acid? Citric acid is right there in the Krebs cycle (ATP cycle) so it must have energy content. It is 2.5 kCal/g. Never listed on nutrition labels.

I always thought lemon juice in tea had zero calories and could be consumed during a zero calorie fast. But I saw it there in the citric acid cycle (Kreb's cycle) and got to thinking that couldn't have zero energy. So I looked it up.

the more relevant issue is THERMIC EFFECT. that is to say calorie value food is measured by BURNING it not by measuring how much of that energy can be converted into fat for storage ( since that would vary by individual ) so diesel fuel for example has 11 calories / gram but you would probably just die if you drank it on other hand protein in food has to be converted into other things before it can be stored as fat so the actual amount of energy stored is less than what is measured by burning food, and the difference is instead lost as heat this is called thermic effect of food and is the reason why it's fairly hard to get fat by eating pure protein ... most calories in meat are in fat, not protein ... and effective calories in things like lettuce are NEGATIVE because it takes more energy to process such food than the amount of energy that can be extracted from it ...

100%. Side note: diesel will just kill you, but some refined petroleum products are quite digestible, and (allegedly) palatable as a butter substitute. Lots of good research was done on this in WW2. I've never been able to look at margarine the same since. Just because its not on the label doesn't mean it doesn't make it into the supply chain.

the reason low-fat diets ( like lean chicken breasts ) are not a good idea is because you still need Omega 3 ( a fat ) and half the vitamins are fat soluble and found in fatty foods. so at the end of the day you still need to either count calories or fast.