Q. What’s the difference between fish oil and cod liver oil?

A.  Fish oil and cod liver oil are two different oils even though they both come from fish and have a similar fatty acid profile.

• Fish oil is extracted from the flesh of tuna, herring, cod and other deep-sea oily fish species. It’s rich in omega-3 fatty acids such as EPA and DHA but doesn’t have much vitamin A or D.
• Cod liver oil comes from the liver of the cod fish. It has less omega-3s but is very rich in vitamins A and D. It was routinely spooned out to children in the 1960s as a source of vitamin D in northern Europe where sunlight is limited during their long winters. And it may return to fashion again given the increasing reports of borderline deficiency of vitamin D now appearing.

Some people take cod liver oil in winter when they spend less time outdoors so don’t make their own vitamin D from sunshine, then switch to fish oil in summer so as to not take in more vitamin D than they need.

If you like numbers, cod liver oil contains less EPA and DHA (around 8% EPA and 10% DHA) than fish oil (at 18% EPA and 12% DHA).

Warning for pregnancy

If you’re pregnant, too much vitamin A can potentially cause birth defects so it’s one vitamin to watch so as not to overdose. So to be on the safe side, stick to the safe upper limit of 10,000 IU (3000 mcg) per day – cod liver oils carry a warning about not exceeding this limit during pregnancy and breastfeeding.

The manufacturer Melrose tells me that a daily dose of 4 mL (just less than a level metric teaspoon which is 5 mL) of their cod liver oil contains:

• 2,500 IU (750 mcg) Vitamin A which is the entire day's recommended intake for women but under for pregnancy and breastfeeding (the RDI is 700 mcg for women rising to 800 mcg during pregnancy and 1100 mcg when breastfeeding – see below).
• 320 IU (8 mcg) of Vitamin D which is more than the day's intake for adults aged to 50 years (which is 5 mcg) and half that of people over 50 (which is 15 mcg).

So if you're pregnant, stick to their suggested 4 mL teaspoon a day (or other label intakes) and you'll be fine.

The Recommended Dietary Intakes (RDIs) for vitamin A:

Adults

700 mcg for women 19 to 70+

900 mcg for men 19 to 70+

800 mcg for pregnant women

1100 mcg during breastfeeding

 Children

300 mcg for children 1 to 3 years

400 mcg for children 4 to 8 years

600 mcg for children 9 to 13 years

700 mcg for teen girls 14 to 18 years

900 mcg for teen boy 14 to 18 years

        mcg stands for micrograms which are one-thousandth of a milligram.

Conversions for IU and mcg for vitamin A (retinol)

IU stands for International Units.

Use this to work out how much vitamin A is in your supplement - sometimes only one figure is given in IUs and it's impossible to know if it's high or not without these quick conversions:

 700 mcg   =    1,500 IU

 800 mcg   =    2,400 IU

 900 mcg   =    3,000 IU

1100 mcg  =    3,300 IU

3000 mcg   =   10,000 IU

   Image of fish oil capsules from jcoterhals via photopin