I got a bit confused when looking up nutrition values for milk versus kefir made from that milk. For some reason many tables list higher values of some nutrients (e.g. potassium and magnesium) than is in the source form, i.e. the milk.

Can someone shed light on how this would happen?

I realize that lactose from the milk gets turned into carbon dioxide and alcohol, but where does it take the additional nutrients from if the only nutrition for the kefir grains is the milk and air?

  • 3
    Are the serving sizes the same?
    – Aaronut
    Jul 6, 2011 at 1:45
  • Can you edit the question to provide links to an example of this? I've not seen it when looking up kefir stuff.
    – BobMcGee
    Jul 6, 2011 at 4:42
  • 1
    I have never had homemade kefir. Does it produce a whey like yogurt does? Maybe it is measured after the CO2 evaporating somewhat and the whey thrown out.
    – rumtscho
    Jul 6, 2011 at 7:35
  • 2
    My point was that if they take 100g milk and make kefir out of it, and the CO2 evaporates and they throw out the whey, they are left with less than 100g kefir. Which means that the Mg and K are more concentrated. This would explain small differences, but not big ones. There is no USDA nutrition entry on kefir, but the difference in milk vs buttermilk is 10 vs 11 mg Mg, 132 vs 151 mg K, which is plausible as a concentration issue. And then, the real values vary from cow to cow, season to season, etc. With a low number of data points (USDA has 12 for buttermilk), a difference is expected.
    – rumtscho
    Jul 6, 2011 at 14:10
  • 1
    @rumtscho is correct. 100g of kefir takes more than 100g of milk to make. It is the same reason why protein and Ca is typically much higher in cheese than in milk. The is mass loss from CO2 alone. Whey may or may not be separable depending on the age of the kefir. A one day old kefir at 20C will likely not have any. If you are only taking out the kefir "grains" for nutrition analysis, then the mass loss has to be well over 50%.
    – user110084
    Jun 8, 2017 at 18:21

2 Answers 2


It could just be the difference in milk sources. Different cows, receiving variable types of feed, produce milk with varying nutritional content. They probably only tested kefir using milk from one or at most two sources. In comparison, the figures for normal milk could be drawn from hundreds of dairy farms across the nation.

I would expect there to be changes in the carb/protein/fat content of the milk, as well as vitamins produced/consumed by the kefir. Specifically, the carbohydrate level should be lower from lactose being digested into lactic acid.

  • Agreed. The USDA lists ~60 data points for milk and 12 for buttermilk. So I doubt that the milk data is all that good too. But the kefir is probably even worse. Add to that that the stuff makes up only 1/10 000 to 1/1000 of milk, and you see how the measurement can be imprecise. I bet that for any nutrient, "has x g of carbohydrates" is as false as saying that my body temperature right now is exact 37.00 °C.
    – rumtscho
    Jul 6, 2011 at 16:57
  • @rumtscho: yeah, I just got mainly confused by the fact that some nutrients seem to be only in either or that the amount should differ that much. Bob's answer is actually quite logical, though. Would perhaps make more sense to see in relation what milk and resulting kefir contain. I bet that would be more interesting. The milk I buy lists at least some of its ingredients ... so I would expect some changes but not really drastic ones :) Jul 6, 2011 at 17:18

There are many products that the nutritional value changes with aging and kefir is one such. You are growing good bacteria and yeast while fermenting and this adds to the milk's value. Also if you do a secondary fermentation you increase some of those values (like the B vit.) ck

  • Not so. You cannot create new mass and new elements that were not the system to begin with. In some cases, you have to include gases from the atmosphere, but this is definitely not the case with milk to kefir, you lose mass as @rumtscho commented
    – user110084
    Jun 8, 2017 at 18:16

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