Dear Mark: Salmon and Mercury, Fruit and Sugar, plus Seniors Gaining Muscle

SalmonFor today’s edition of Dear Mark, I’ll be covering three topics. First, salmon and mercury. We’re often told to watch out for mercury in wild fish, but the story is actually more complicated than that. Learn how to tell if wild fish is safe to eat and whether wild salmon should be limited. Next, I discuss the differences between fruit sugar and sugar, or, rather, fruit and sugar. Can fruit be a part of a healthy Primal way of eating despite the sugar content? And finally, I give a few tips to an older woman who’s interested in gaining muscle and strength. I go over exercise, protein intake, as well as the ulterior benefits of resistance training.

Let’s go:

Dear Mark,

I really love fish, especially salmon, and I know they are a great source of Omega 3 so I generally feel good when I’m chowing down on ’em (especially drowned in grass-fed butter!) But I have recently started worrying about possibly eating too much mercury due to my huge fish intake. There have been times when I would have a piece for lunch EVERY DAY. I have read the FDA’s guidelines and such but would like to here from someone I trust about it. My question is how often can I eat fish, especially salmon, without worrying about mercury content? Is once a day way too much? How about 4 times a week?

Thanks for your time


The thing to realize about mercury concerns is that the selenium often found in fish binds and renders inert the mercury found in fish. As long as the fish contains more selenium than mercury, you don’t have to worry about mercury toxicity.

I’ve got good news and not so good (but not necessarily bad) news for you. First, the mixed news. Selenium-mercury ratios vary wildly, and official numbers don’t always hold when you look at individual samples of a given species. With many species sold in the United States, even the safe ones, there is high variation and low predictability with regards to selenium and mercury content. However, a study on tropical fish, squid, crabs, and shrimp in the Atlantic Ocean off of Brazil found that most seafood studied had high enough selenium to mercury molar ratios to make them safe for consumption, and find that the longer a fish, the lower the selenium to mercury ratio (higher is better). Still, only seven samples out of 683 taken had unfavorable selenium to mercury ratios.

The good news for you: wild Alaskan salmon is consistently low in mercury and high in selenium, making it totally and completely safe.

If you ever get the hankering for shark steaks or king mackerel belly (both very high in mercury with inadequate amounts of selenium), or you’re just worried about the selenium-mercury ratios in the fish you eat, you can always take a selenium supplement alongside the fish. Research shows that selenium supplementation can offset mercury toxicity due to environmental exposure. Residents of a mining town in China with a history of mercury exposure were either given a selenium-producing yeast or an inactive yeast; the ones who got the selenium-producing yeast had increased urinary excretion of mercury and lower excretion of toxins (indicated less mercury toxicity). Supplements aren’t really necessary, in fact, as long as you’re getting enough selenium from your diet. Selenium is richest in Brazil nuts, egg yolks, mushrooms, wild salmon, halibut, and animal kidneys.

As for frequency of consumption, the mercury isn’t a problem as long your selenium outpaces it. If you’re eating wild salmon, you’ll never accumulate enough mercury to be a threat. There are other reasons to limit fish, though. First, although omega-3 fats are vital for your health, they don’t need to be eaten in excess. They’re fragile things that can become oxidized and too many are harmful. Second, overfishing is a valid concern, on both an environmental level (a loss of fish has negative impacts on the marine environment) and a personal level (a loss of fish due to overfishing means less fish available for me or you to eat). A 4-8 ounce serving of fatty fish two or three times a week is probably ideal, assuming you’re avoiding seed oils and processed foods rich in omega-6 fats, which throw off our omega-3:omega-6 balance. That said, if you’re eating a piece of wild salmon every day, you could do a whole lot worse. I wouldn’t worry too much, and I personally wouldn’t stress the mercury.


I would love to know the truth about eating fruit and how the sugar affects our bodies. How does it differ from white sugar and how much should we really be eating?



Fruit differs from sugar in several important ways.

First, fruit sugar is a mix of fructose, sucrose, and glucose, with some trace amounts of a sugar alcohol called sorbitol appearing in certain fruits (apples, pears, and many stone fruits, for instance). Sugar, if we’re talking about table sugar, is sucrose – a 50/50 mix of fructose and glucose. The fructose in a peach is the same as the fructose in table sugar. The sucrose in an apricot is identical to the sucrose in table sugar. Where fruit differs is in the ratio of fructose to sucrose to glucose. Depending on the fruit, it might be 22% fructose, 30% sucrose, and 48% glucose, or 30% fructose, 30% sucrose, and 40% glucose. It could really be anything, and the ratio will differ from sample to sample even within a species of fruit, whereas refined sugar is always 50% glucose, 50% fructose.

Fructose is metabolized differently than glucose, being shunted toward the liver for processing. This makes it potentially more hazardous to the liver than glucose, especially when consumed in excess. A recent review covered the fate of fructose in human metabolism and suggested that fructose only modestly contributes to de novo lipogenesis – or the creation of fat from carbohydrate.

While that’s true, the authors give a rather large caveat: “The time periods of liver de novo lipogenesis from sugars and the factors influencing it are not completely understood, and are impacted by the concentrations and tracer characteristics of the various substrates drawn from lipid precursor pools. De novo lipogenesis may also occur in adipose tissue or muscles, but there are no adequate methods available to quantitate it.”

In other words, we only kinda sorta understand de novo lipogenesis in the liver, and we know almost nothing about de novo lipogenesis in fat tissue, nor we do have any way to measure it. They conclude the section on de novo lipogenesis with this assessment: “The influence of fructose consumption on plasma lipids and de novo lipogenesis remains controversial and understudied.”

Meanwhile, studies indicate that fructose overfeeding is way worse than glucose overfeeding in the overweight, increasing LDL particle count, oxidized LDL, and triglycerides while triggering accumulation of visceral/organ fat. Another study in overweight humans found that short term overfeeding with “simple sugars” triggered de novo lipogenesis and increased liver fat in proportion to the increase in de novo lipogenesis. So it seems that the overweight and obese who overeat fructose do experience enough de novo lipogenesis to increase liver fat.

Okay, but that’s just in the overweight and obese, you might say. It doesn’t happen in healthy people eating a normal, hypocaloric diet. True enough. Remember, though: it’s not like the overweight/obese are some niche, fringe group. They’re a whopping 70% of the adult population in America! Most of the United States, then, is likely susceptible to fructose overfeeding. Most of those people are working sedentary jobs, getting inadequate or poorly designed exercise (while overreporting the activity they do get), and eating more sweets and drinking more soda than they require. They aren’t eating hypocaloric diets. They aren’t regularly depleting their glycogen (in fact, obese people tend to have saturated liver glycogen stores). Quite frankly, they aren’t as healthy as they could be. I’m not convinced fructose is harmless in that (sizable) portion of the population. I’m convinced that eliminating simple, refined fructose from their diets is an important step toward getting healthier.

Your question wasn’t about fructose or glucose, though, so I won’t get too far into it. Let’s save it for another time. My point, after all that potentially scary stuff about fructose, is that just because fructose might have some issues doesn’t mean we need to condemn fruit. Fruit is not just fructose, glucose, or sucrose:

Where people vindicate fructose, they should be vindicating fruitIt’s quite clear that fruit is metabolized differently – and more favorably – than pure sugar, or foods containing refined sugar, even in overweight people for whom regular old refined fructose would be devastating. While the overweight/obese would be best served avoiding refined sugar, research suggests that they can handle modest amounts of fruit (read: not 30 bananas a day) pretty well.

I would like to hear something pertaining to the older generation and how they can gain muscle mass. Most everything for 50+ people is for those who are out of shape and could use a lot of improvement. I’m not out of shape, but would like helpful hints on putting on muscle mass at my age, using Primal [Blueprint] diet.


The Primal way of eating will certainly help build muscle – lots of protein, healthy fats (especially animal fats), and adequate amounts of carbohydrate tailored to your activity level will tend to do that. But our skeletal muscle responds primarily to how you use it. Just eating right won’t build muscle alone. If you don’t use it, it won’t increase and will actively atrophy. That’s why bed rest patients lose muscle mass and why your arm looks all spindly when the cast comes off. If you do use it, you will maintain your muscle. If you use it to lift heavy stuff, you will build muscle.

After all, humans are humans, whether young or elderly. We all lay down muscle the same way in response to the same stimulus, just at a different pace. Youngsters with more testosterone can gain mass a lot easier than eldsters, but that doesn’t mean you can’t do it. And even if you can’t pack on tons of muscle (you probably won’t be getting huge), you will maintain your lean mass, which is essential for growing old with grace and ability.

Now, I wouldn’t just rush headlong into an intense lifting program. You don’t want to get hurt. Seek out a trainer for an assessment of your strengths, weaknesses, and limitations.

Alternately, go for something like Dr. Doug McGuff’s Body By Science. It’s designed to be as safe and effective as possible with minimal time commitment. Check out his guest post here, where he lays out the basics of his program, or his blog, where he regularly posts workouts.

There are other reasons to lift weights as you age, of course. Lifting heavy things keeps your skeletal muscle insulin sensitive (able to accept nutrients and synthesize glycogen), and seniors with insulin sensitive muscles are less likely to convert carbohydrates to fat and develop fatty liver. It also increases bone mineral density (yep, bones are living organs that respond to stimulus just like muscles do) and improves balance, both of which are protective against falls and fractures.

You’ll also need more protein than your doctor or the USDA probably recommend, since seniors are a bit less efficient in converting it into muscleProtein supplementation works wonders particularly in the seniors, with a recent study showing that it can help frail, elderly women increase muscle mass when paired with resistance training. I’m partial to Primal Fuel, but any high quality whey isolate will do the trick. Just watch out for excessive sugar and bad fats in whichever product you choose (that’s why I use coconut fat in mine). Shoot for at least 1.5 g protein/kg bodyweight. More wouldn’t hurt, either.

That’s it for this week, folks. Thanks for reading and keep the questions coming!

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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