~A day's worth of paleo food-sourced magnesium without supplementation:

• 2 oz. nuts (macadamia, almond, walnut...) - 16-38% DV
• 8 oz. broccoli - steamed, sauteed, raw - 8% DV
• small can sardines or salmon - 9-38% DV
• 4 oz. kale with four eggs - 12% DV
• 10 g. raw cacao nibs in your (one cup) coconut milk smoothie - 29% DV
• salad - red leaf lettuce (1/3 head) + scallions (1 oz.) + lemon juice (1/2 lemon) - 5% DV

  = 79-130% DV

As a lazy paleo, I just take Now Foods, Magnesium Citrate, 200 mg tablets, twice a day which I happily purchase from iHerb.com.

EDIT: It looks like Dr. Kurt G. Harris of PaleoNu.com has started taking Mg Citrate as well. http://wholehealthsource.blogspot.com/2010/02/magnesium-and-insulin-sensitivity.html?showComment=1267032364308#c1405753174744572544

For the money, magnesium citrate powder is the best deal. I like the "Natural Calm."

Avoid supplements with magnesium oxide - it's very difficult for our bodies to absorb much of it.

By the way, the bible concerning magnesium IMHO is "The Magnesium Miracle" by Carolyn Dean, MD, ND.

i don't like taking supplements unless someone can tell me why paleolithic people could get enough of the nutirent, but moderns will have trouble. i understand the argument for vita d supplementation (like office job, not enough time out in the sun) or iodine supplementation (iodine depleted soils), but what is the argument for magnesium supplementation? also, how do you know if you are deficient in the first place? some people get leg cramps and take that as a sign, but i don't have that problem. i would like to take as few supplements as possible. Ps is my rep score getting higher?

Mg supplementation is a good idea for many reasons: it can take a long time to correct deficiencies; your kidneys can leak Mg for a number of reasons, including borderline-high blood sugar; many conventional foods are low in Mg, etc, etc.

The most absorbable sources tend to be the amino acid chelates, such as Mg Glycinate, Mg Taurate, Mg Aspartate, etc. Mg Citrate is also good, not because it's particularly well absorbed, but because it's inexpensive, so it's easy to take a lot of it (Natural Calm is a good source, and it also tastes OK).

Mg oxide is the poorest form, and should be avoided.

Mg Chloride is in-between. There's a time-release form of it called Slow-Mag that can be a pretty good option. It's also available as a liquid, which works well (but tastes terrible!)

In case you're interested, I've posted a bunch of info and references about Mg at my blog.

Instead of ingesting a magnesium supplement, magnesium can be absorbed by soaking in a tub with epsom salts dissolved in the bath water.

Maybe that method coupled with eating magnesium rich foods could be enough to top up magnesium to an adequate level.

one possibility you are missing: water. On Stephen's last magnesium post some people in the comments mentioned home-made MG water. There is also Apollinarus, Noah's (also know as Adobe Springs), and probably others that have a fair amount of Mg. This Mg advocacy site claims absortion rates of 50%: http://www.mgwater.com/benefits.shtml. Other studies demonstrate absorption.

There is also Magnesium 'oil', which is supposed to absorb very well.

I get my magnesium from having 1-2 cans of spinach per day, fresh spinach is too expensive. (I make it into a creamy soup, to make it relatively edible). Even so, I still only just exceed the RDA and no other foodstuffs come close for magnesium.

I've never been entirely convinced by the magnesium in drinking water argument. Looking at various naturally bottled spring/mineral waters, I've not seen any that contain appreciable amounts of magnesium (though it seems to vary substantially).

I can't help but notice though, that magnesium is one of the most prevalent minerals in seawater. Seaweeds are also very magnesium rich, so perhaps, like iodine, this is another case where our history as shoreline dwellers is important.

This answer focuses on the dietary sources of magnesium.

I've had this concern for a while too, considering many magnesium sources are found in carb-abundant foods (ie 1 cup crude bran= 230% RDA). here's my answer= HEMP SEED.

check out the nutritional profile= http://nutiva.com/nutrition/charts/organic-shelled-hempseed/

for 180 calories, you get 13.5 g fat, 2g carb (1 g fiber), 11g protein, and Magnesium 48% RDA. you also get a nice boost of iron- 16%, Zinc- 23%, and not to mention that hemp is a balanced source of Omega 3-6 EFAs.

it's pricy, but if i'm lagging in magnesium/ iron, that's what i take. i see it as more of a supplement than anything

Other good sources of Magnesium include=

1. Cocoa powder provides 499mg of magnesium (125% RDA) per 100 gram serving or 429mg (107% RDA) per cup. Dark baking chocolate provides 327mg per 100 gram serving (82% RDA), or 95mg (24% RDA) per square.

2. Halibut has 182mg (46% RDA) per 6oz serving

3. Avocado has 58mg (15% RDA) per avocado (201 grams)

4. Spinach cooked has 79mg (19% RDA) per half cup (90 grams)

5. Brazil nuts have 19mg (5% RDA) in a SINGLE kernel or nut. And 1 serving of nuts is 1 oz (6 brazil nuts) so that's 30% right there.

*Other nuts and seeds have appreciable amounts of magnesium, like sunflower seeds, sesame tahini, and almonds, but they're also loaded with PUFA, which some Paleos might want to watch. Flax seeds provide 392mg (92% RDA) per 100 gram serving or 39mg (10% RDA) per tablespoon, but whether the PUFA content is worth it is up to you. However, i might note that Flax has more Omega-3 than Omega-6, so you're not getting much Omega-6 anyway.

Other things that interfere with Magnesium absorption=

People with Gastrointestinal Disorders - Most magnesium is absorbed through the colon so people with gastrointestinal disorders like Crohn's disease are at high risk for a magnesium deficiency.
People with Poor Functioning Kidneys - The kidneys should be able to regulate magnesium in the blood, excreting less when stores are low, however, excessive loss of magnesium through urine can occur to people on specific medications, poorly managed diabetes, and alcoholics.
The Elderly - As we age the amount of magnesium we absorb decreases as the amount we excrete increases.
People Consuming high amounts of Fiber - Eating large amounts of fiber has been shown to interfere with the bodies ability to use magnesium. However, more research needs to be done to confirm how much fiber affects magnesium.30,31
People on a low protein diet (*Controversial) - Eating less than 30 grams of protein a day may adversely affect magnesium utilization
People taking Certain Medications
    Diuretics: Lasix, Bumex, Edecrin, and hydrochlorothiazide
    Antibiotics: Gentamicin, and Amphotericin
    Anti-neoplastic (Cancer) medication: Cisplatin
    Zinc Supplements

I think what is important here is Magnesium appears to be very safe to supplement with, and even some of the most conservative bloggers such as Dr Kurt Harris from PaleoNu have started supplementing (or replacement as he calls it).

Kurt G. Harris MD said... Very timely, I've been planning to post on Mg for some time. I am adding it to the arsenal as replacement in my dietary scheme. I take Mg citrate daily.

one possible reason why we need so much Mg but don't get enough in food: the Aquatic Ape theory. As mentioned above, Mg can be absorbed through the skin. If the most ancient ancestors lived at the seashore, they could have gotten Mg transdermally every day.

If you've ever seen any swimming babies, that idea gets some creedence. The Russians seem to have pioneered that decades ago. Here's a newer rendition:

swim like tadpoles

AFAIK, when they get older, they seem to lose the innate ability.

Note that since most MG is intracellular, a serum Mg test supposedly won't tell much - unless you're critically low.

There is a posting at Nephropal by a patient, BillyE, of Dr Tourgeman. He uses Mg Oil in the form of Magnesium chloride in a spray bottle to use topically to absorb Mg. He says Mg taken orally is only absorbed 20% as opposed to the Mg Oil Spray which is absorbed much better.

http://nephropal.blogspot.com/2011/03/magnesium-stepchild-of-medicine.html

I am sure Dr Tourgeman screens BillyE's posting since the doctor owns the site and has brought BillyE along during his repair of chronic kidney disease, diabetes II, and obesity.

Remember, magnesium-rich plant sources like spinach, kale, or swiss chard derive their magnesium from the soil. If the soil is depleted of magnesium- due to conventional farming practices (fertilizers, poor crop rotation, etc )then any food source grown there will be deficient as well.

Also, magnesium is a water soluble mineral- so cooking can leech the magnesium content right out of the food.

Be mindful of this when calculating magnesium consumption based solely from plant food.

Also, I find the RDA to be very deceiving. Magnesium fuels over 325 enzyme related functions in the body- from energy production to cell rejuvenation- so a person who is physically active (producing a lot of energy, say someone training for a marathon or ironman ) or lives/works in a high stress environment, will have a greater magnesium requirement than someone who isn't.

Providing the body with sufficient magnesium requires self-analysis and bit of research. It's a bit more complex than the commonly accepted RDA "one-size-fits-all" solution.