How Many Calories Does Sulfur Have?

It sure isn't zero!

Conventional wisdom holds that we get our calories from carbohydrate, fat, and protein.

Conventional wisdom is mostly right.

However, the molybdenum-dependent sulfite oxidase reaction delivers electrons taken from the sulfur atom, mostly coming from the sulfur amino acids methionine and cysteine, and delivers them to complex IV of the mitochondrial respiratory chain, providing energy used to make ATP.

This is NOT included in the approximately 4 Calories per gram of protein figure!

That refers to the chemical bond energy in the carbon skeletons of the amino acids after their nitrogen is removed.

Guenter Shchwarz’s lab at the University of Cologne found that adding 500 micromoles per liter of sulfite onto mitochondria increases their oxygen consumption in the respiratory chain by 15%, showing that pure, inorganic sulfite is a source of energy.

Each sulfite donates two electrons to complex IV, which is half of what is required to convert one molecule of oxygen from the air we breathe into water.

According to Chapter 14 of Molecular Biology of the Cell, each four electrons delivered to complex IV generates four protons pumped and one ATP is made by ATP synthase from each 3 protons pumped. One mole of ATP provides 7.3 Calories of usable energy.

Two sulfites, then, will make one water molecule and four protons, which will then make 1.33 ATP. Thus, two moles of sulfite generate 1.33*7.3= 9.7 Calories.

As I mentioned in The Three Nutrients You Need More Of on a High-Protein Diet, the average person excretes 14-28 millimoles of sulfur in the urine, most of which is sulfate that had been sulfite until it passed through the molybdenum-dependent enzyme sulfite oxidase.

This suggests the average person gets an extra 0.07-0.13 Calories per day from sulfur. This is on a sulfur amino acid intake of, on average, 2.8 grams per day.

Elemental sulfur is roughly 32 grams per mole, so as a rule of thumb we could say that sulfur compounds have a potential caloric yield of 9.7 Calories for every 64 grams of sulfur or 0.15 Calories per gram of sulfur.

The main catabolic way of clearing cysteine does not deliver any other electrons to the respiratory chain besides those of sulfite. However, mainly in the placenta of pregnant women and in the blood vessels and nervous system of everyone, some portion of cysteine is converted to hydrogen sulfide gas to act as a signaling compound. There are multiple ways this can happen but the major ones occur through hydrolysis, where the sulfur is removed as H2S and no electrons are transferred. This removes sulfur with more electrons intact than are found in the sulfite molecule, and these additional electrons are delivered to CoQ10 of the respiratory chain, generating sulfite.

When this happens, two additional protons are pumped by complex III during the oxidation of the CoQ10, and then the electrons travel via cytochrome C to complex IV, where they generate two more protons pumped. Then the leftover sulfite delivers two more electrons to complex IV via the molybdenum-dependent sulfite oxidase as discussed above. This causes a six-fold increase in the caloric value of the sulfur.

Thus, sulfur that is used to produce H2S gas yields 0.45 Calories per gram of sulfur.

About 42% of sulfur intake comes from cruciferous vegetables like broccoli and kale and alliaceous vegetables like garlic, onions, leeks, and shallots. The sulfur in these foods is very complex and nothing can be said of it as a whole. Some of it is sulfate, which is already fully oxidized and does not have caloric value. Some of it is glucosinolates, where the sulfur is not catabolized further than the thiocyanate ion and does not have caloric value. On the other hand, catalase can generate H2S from the sulfur compounds in garlic, allowing it to feed into the pathway discussed above, generating caloric value.

As a rule of thumb, the sulfur in sulfur amino acids generates 0.15-0.45 Calories per gram depending on how it is metabolized, while the additional sulfur compounds in vegetables will have much more variable and much lower caloric yield than this.

This is so small as to have no practical utility, but I thought it was fun.