Dear Mark: Hempseed, Too Much Omega-3, and Vitamin D’s Halflife

It’s a lovely summer morning here in Malibu. The cool sea breeze keeps the coming warmth at bay, for now, and makes me glad for the hot mug of coffee I’m clutching. I’ve still got a couple hours before heading into the office, so I’m hoping that I can get this post wrapped up and edited in time to hit the water for a bit of paddle boarding. We’ll see. I’ll try not to rush things too much. Today we’ve got a quick round of questions on a diverse set of topics: the suitability of hempseed in the Primal way of eating, whether you can get too much omega-3, and how long vitamin D lingers in the body before you need to replenish your stocks. As always, feel free to keep the questions coming and I’ll do my best to get to them!

Hi Mark,

First off, thank you for the wealth of information you provide on MDA. This is an amazing resource for someone on a budget. Secondly, I was reading your Primal shopping list and I noticed that hemp seed is not listed under nuts. I’ve heard that hemp seeds are very nutritionally dense but would they be considered Primal? Thanks again for all the great information!



I covered hemp over a year ago. Here’s the post. It’s a fairly short, quick read that I still stand by, but I’d like to add something which I failed to mention last time. Hempseeds contain no phytic acid, the mineral-binding antinutrient common to most nuts, seeds, grains, and legumes. This is extremely rare for a seed, and extremely cool because it means hemp seeds require no soaking to get rid of phytate.

Hempseeds are good tossed in a salad. They’ve got good levels of magnesium and contain ALA, the plant omega-3. Stick to whole hempseeds, rather than processed meal, and store them in your fridge or freezer to avoid any rancidity issues (since they’re high in PUFA, they’ll go bad pretty quick otherwise). Thanks for pointing out the omission, Andrew. The shopping list has been updated!

Thanks for your website and efforts. It really is excellent and I really appreciate all the work you must put in.

Is it possible to take too many omega three supplements?

Thanks again!


Yes. Remember that we talk about limiting polyunsaturated fats because they are inherently unstable and prone to oxidation (which can happen on the store shelf, on your counter, when cooking, or in your body), and omega-3s are in the same category.

Polyunsaturated fats – all of them – are fragile, and very powerful. Remember that the only essential fatty acids are the polyunsaturates, and that goes for omega-3. Be careful with the dosage, as several studies show that while a lack thereof is dangerous, an excess can also be harmful. One study found that pregnant rats with either an omega-3:omega-6 dietary ratio of 14:1 or a ratio of 0 (no omega-3 at all) gave birth to pups with impaired hearing; the excessive omega-3 group also had pups with retarded growth. Another study showed that when compared to a diet high in olive oil, a high omega-3 diet increased LDL oxidation in humans, most likely because of its inherent instability. It was overrepresented in the LDL particles, and the LDL particles were becoming more fragile.

Don’t forget why we take omega-3 in the first place. We’re not trying to cram as many in as possible. We’re trying to offset the imposing presence of omega-6 in the diet. Processed foods, vegetable oil, restaurant fare, and conventionally-raised animals all contain disproportionately large amounts of omega-6 fats. Fish is the best source of omega-3s, but few people eat much fish. And what fish they do eat is of the lean, low omega-3 variety, often covered in bread crumbs and deep fried in more vegetable oil. Adding omega-3 can help correct the skewed omega-3:omega-6 ratio, so if your omega-6 intake is abnormally high, or you’re trying to eat better after a lifetime of Standard American Dieting, taking higher initial doses of omega-3 has merit.

How much is too much? Don’t stray past a 1:1 omega-3:omega-6 ratio. If you keep your omega-6 intake low by avoiding seed oils like corn, canola, or soybean and processed junk foods, you won’t have to take much omega-3 to balance out the ratio and you won’t be in danger of taking in excess.

Dear Mark,

I’ve been following PB since the beginning of the year, and one of the most enjoyable recommendations to follow has been spending some time in the sun. I had previously been vaguely aware that sunshine promoted vitamin D production in the body, but I had no real appreciation of the scale or importance of this. Sadly, I live in the UK, with its notoriously changeable weather. Even during summer, many days may pass without a hint of sunshine. So my question is this – how long does vitamin D stay in the body? Will one longer session in the sun a week satiate my body’s needs, or should I be supplementing on cloudy days?




Well, according to one (older) study, 25OH(D) – which is the most basic measurement of vitamin D levels in the blood – has a half-life of a month. That’s a decent chunk of time, meaning vitamin D sticks around, and, provided you obtain enough during your sunning sessions, once a week might provide sufficient levels. One thirty minute session of full-body (no shirt, shorts, no sunblock) sun exposure when the sun is at its strongest, for example, can get you between 10-20,000 IUs of endogenous vitamin D production. If you get 20,000 IU a week, you’re way ahead of the game.

Of course, this all depends on your skin tone. If you lack melanin, you will make more vitamin D in less sunlight (a recent study showed that among healthy adults living in Toronto, darker skin was inversely associated with 25OH(D) levels). If your ancestors have been living in the same area for thousands of years, you’re probably adapted to that area’s UV-B availability. The only way to know for sure is to get your levels tested. Take the test, note your levels, and spend three or four months trying to obtain vitamin D from the sun alone; no supplements. You might even consider taking records of your sunbathing – time of day, hours/minutes spent, cloud presence, temperature – to really understand the conditions necessary for vitamin D production. Retest and see if your levels budge. The mechanism for storage definitely exists, so the key is finding out whether you’re making enough D in the first place.

That’s it for today, folks. Be sure to give the Workout of the Week a try if you’re game, leave a comment in the comment section, and thanks for reading!

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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