Making sense of USDA database Essential Fatty Acids - "18:3 undifferentiated," etc


Continuing the discussion from Essential Fatty Acids: Ratios versus Amounts:

Quoted is what I’ve determined is an accepted range of the EFAs. (If that can be disputed, please message me or comment to the linked thread!) However, while those numbers are great to have, determining the amounts in various foods has been the next challenge. Half of the ingredients on the DIY site contradict one another and the USDA database lipid breakdown seems to leave some ambiguity, particularly with the “18:3 undifferentiated” point.

My concern is this: For a given entry on the USDA database that does NOT specify individual isomers, but only lists “18:3” and “18:2” undifferentiated, etc, how can someone reliably break down the omega-3/6 fatty acids for a DIY ingredient entry?

Example: Masa Flour per 100g USDA Database
Fatty acids, total polyunsaturated: 1.724 g
18:2 undifferentiated: 1.672 g
18:3 undifferentiated: .052 g
No specific isomer breakdowns

According to the USDA SR27 doc pages 21-24 and SelfNutritionData Fatty Acids, “undifferentiated” is the sum of all specific isomers within a given lipid, with a common name given for the most typical found. In the case of 18:2, that is linoleic acid (LA), aka 18:2(n-6)c,c, aka Omega-6 PUFA. For a DIY recipe using 100g of Masa, I would put 1.67g of Omega-6 Fatty Acids, although I cannot be certain that is the specific isomer included.

Easy, right? Well… then we come to “18:3 undifferentiated” which includes both 18:3(n-3)[ALA - Omega 3] AND 18:3:(n-6)[GLA - Omega 6]! In other words - if I understand this correctly - this one point includes both Omega 3 and Omega 6 isomers! And often this is the only breakdown given, leaving me to guess which of the two omegas are actually included. I’d think this is fairly significant to understand to determine my Omega intake and ratios with any kind of accuracy. Often it may be obvious dependent upon the food source, but then again maybe not.

Should I just assume the remaining .052g of “18:3 undifferentiated” is also an Omega 6 source and add it to the total? Do I ignore it? Do I hedge my bets and add is an Omega-3 (doubtful)? I do not like this uncertainty!

When adding ingredients to a recipe using the USDA database, what does everyone look at specifically while compiling the omega serving sizes?

Essential Isomer Breakdown:


  • ALA (alpha-linoleic acid) = 18:3(n-3) / plant sources
  • EPA (eicosapentaenoic acid) = 20:5(n-3) / marine animal sources
  • DHA (docosahexaenoic acid) = 22:6(n-3) / marine animal sources
  • DPA (clupanodonic acid) = 22:5(n-3) / marine animal sources


  • LA (linoleic acid) = 18:2(n-6) / vegetable oils
  • GLA (gamma-linoleic acid) = 18:3(n-6) / vegetable oils
  • AA (arachidonic acid) = 20:4(n-6) / meat sources

… Granted, this is going deeper than may be strictly necessary, but as I’ve been doing my research, the lipid numbers on the USDA kept popping up and I haven’t been fully confident on what it all meant. I’m hoping others with a chemistry background or more general knowledge can help me make sure I understand what to record to make my master ingredient list as accurate as possible.

Let’s have some dialogue! :sweat_smile: (I searched and doesn’t seem like this has been brought up specifically… probably for a reason. ;))


18:2 and 18:3 mean the hydrocarbon chain starting from the carbonyl (C=O double bond) is 18 carbons long. The :2 or :3 means that there are two or three double bonds within the hydrocarbon tail. If the double bond is six carbons from the end of the hydrocarbon tail then this is an w-6 (or Omega 6) fatty acid. If it is three from the end then w-3, and if nine from the end it is w-9. In my memory serves me correctly anything that is :1 is w-1, :2 is an w-6, :3 would be w-9,

I don’t know if this helps but at least now you can interpret the code.


It helps to clarify at a layman level what I’ve been reading on and off for the past week, but I don’t know that it resolves my concern about “18:3 undifferentiated” being primarily comprised of either an omega-3 or and omega-6 isomer.

I’m curious if the lack of replies is due to me wording my question poorly (or over thinking to the point of creating problems) or if this is a lack of knowledge in the community… I hoped/expected some of the other DIYers to jump in with some clarification. :yum:

I don’t mean this snidely whatsoever: does no one care about this? Meaning, can the numbers be approximated close enough in a recipe that breaking it down so precisely isn’t necessary?


It’s not a poor question - just one that can’t be easily answered.

The necessary testing to differentiate alpha-linolenic acid from gamma-linolenic acid is likely complex, time-consuming, and expensive - while the information gained is marginal, at best. The test for linolenic acid is probably much easier. But I’m speculating.

I focus my attention more on the EPA and DHA part of the equation, where the quantity is more important, as well as more easily determined.