Dear Mark: Higher BMI and Mortality Risk, Calories When Meal-Skipping, HIIT After Fasting

Dear Mark- BMI FinalFor today’s edition of Dear Mark, I’m answering a couple questions from you guys. First, there’s a new paper out claiming that the ideal BMI for overall mortality is an overweight one—27, to be exact. Is this really the case? Should we all pack on a few extra pounds to see us through into old age? Next, should you try to make up for all the “lost” calories when you eat in a compressed eating window? If you skip breakfast, should you try to eat more at lunch? And finally, how does one incorporate high intensity interval training on a low-carb diet that includes fasting?

Let’s go:

Hey Mark,

What do you make of this new study showing that the ideal BMI for mortality is 27, which is overweight? It seems like you hear this a lot, that the slimmer you are, the more you’re actually likely to die young.

Here’s an article about the study:


I don’t think much of it, to be honest.

It was a pretty broad study, the kind that’s been done before. They plotted BMI against mortality in a large cohort of Danes from 1976 to 2013, tracking how the “ideal” (lowest association with mortality) BMI had changed over the years. By 2013, the BMI associated with the lowest mortality had increased by 3.3 points to 27. That’s overweight.

So: start a mass gain protocol, stat?

Before you do that, let’s also look at another recent study on bodyweight and mortality. This one was a meta-analysis of 230 existing studies comprising 30.3 million people and 3.4 million deaths.

The researchers added an interesting and highly relevant wrinkle to their analysis: instead of pooling everyone together, they separated never-smokers and never-smokers without health problems. They also analyzed the effect of BMI on mortality risk in studies with long-term followups. Why do these differences matter?

Smokers tend to be leaner than non-smokers. They also tend to die earlier than non-smokers. Separating the never-smokers from the rest removes those people who are skinny because they smoke. Simply pooling everyone together and analyzing the effect of BMI wouldn’t distinguish between those who were skinny because of or carrying extra weight as a buffer against smoking-related disease.

Separating the never-smokers without health problems takes it a step further. Skinny people who die young are often skinny because of disease or the disease’s treatment. Skinny people who never smoke and have no health problems are a different breed.

Focusing on studies with longer followups gives researchers more data to work with. If you only check back in on your people for five years, you’re going to miss a lot of deaths. It’s not like people stop dying when the study ends. If you follow a cohort for 20 years, you’ll see more deaths—more data points—and get a more accurate picture of the relationship between BMI and mortality.

So, what’d they find?

Among people who’d never smoked, the ideal BMI was 23-24.

Among people who’d never smoked and had no health problems, the ideal BMI was even lower at 22-23.

And when you looked at studies of never-smokers with long followups (20+ years), the ideal BMI was even lower at 20-22.

That’s the polar opposite. Overweight isn’t healthier. If you still aren’t convinced, check out some of my other posts covering similar studies:

Is Weight Loss Impossible and Unhealthy?

Does Skinny Equal Healthy?


Love Primal! My question to you is this; When I am skipping breakfast in the morning am I trying to get all of my calories in that 7 hour window from 12 – 7 or am I supposed to cut out the calories from my breakfast that I skipped?

Part of the reason why intermittent fasting/restricted feeding windows works so well is that it leads to inadvertent calorie reduction. When you don’t eat an entire meal, it’s hard to make up for it.

Eat when hungry. Stop when full. Most studies find that breakfast skippers are hungrier at lunch than breakfast eaters, but they don’t actually eat enough to make up for the calories they missed at breakfast. This is good. This is why skipping the occasional meal tends to improve weight loss. It makes eating less easier.

Unless you’re trying to gain weight, you don’t need to force more calories down. It’s not like you’re running on empty just because you didn’t eat breakfast. You’re still burning energy, burning “calories.” And once you get fat-adapted, a process which fasting tends to accelerate, you tend to get that energy from your body fat. You aren’t missing any calories. You’re just getting them elsewhere.

Dear Mark,

I am a 52 year old menopausal woman. About 3 years ago, I lost 25 pounds with intermittent fasting and have kept it off. At 5’3″ tall, I currently weigh 125 pounds. Despite this, I have been diagnosed with insulin resistance and at the urging of my doctor, I began eating a very low carb diet about a year ago. Most days, my carb intake is easily under 40g with most of that coming from veggies and the occasional glass of red wine. I don’t eat sugar or grains and limit any artificial sweeteners.

So I recently heard about how great HIIT is for improving insulin sensitivity and I would like to incorporate it into my exercise routine (which now consists mostly of walking and some body weight exercises). I’ve done one session on my stationary bike that consisted of 20 seconds intense peddling and 2 minutes rest x 3. At the end of this, my legs were so shaky, I could hardly stand. This was after an overnight fast of about 16 hours. In your opinion, should I be eating before a session? Since I eat so few carbs, I don’t want to “carb up” before…is there any need to? Or is it completely normal to be that totally spent, given that I am just starting?

Also, after the session, my blood glucose level took a pretty big jump…probably about 30 points. Just trying to understand the physiology behind this.

Thank you for your time!

Congrats on your weight loss!

Did you ever read the post about “sleeping low“? It went up a couple months back and I’d recommend it.

In short, doing a big HIIT session after a long fast on low-carb is really tough. Your glycogen stores will be depleted (from lack of carbs in the diet and from not eating for the past 16 hours), so you won’t have a lot. That’s why your legs were shaky.

A better option is to eat a small dose of carbs before the HIIT workout. Remember that green veggies don’t really contribute to your carb intake. They provide fiber, micronutrients, phytonutrients, but very little usable glucose. You’re not gonna refill glycogen with broccoli and Swiss Chard, so instead eat a potato (maybe cooked and cooled for increased resistant starch), a banana, some mango, or maybe a bowl of berries. Something starchy and Primal to the tune of 30-40 grams of carbs.

After your HIIT session, eat low-carb until your next one. You’ll have depleted most of your glycogen during the workout, so you’ll be primed to burn fat. As long as you’re truly just walking and doing some body weight stuff, you can probably get by with just that small bolus of starchy carbs once a day or whenever you do something intense.

As for the blood glucose spike, that’s normal since sprinting is a particularly glucose-intensive activity. It burns right through muscle glycogen. Sensing this, the liver releases glucose to provide additional fuel for the body; in healthy people, type 2 diabetics, and type 1 diabetics alike, blood glucose spikes after most any exercise, but especially after sprinting. Long term, sprinting will improve glucose control.

That’s it for today, folks. Any follow up questions of your own? Let me hear them in the comments below.

TAGS:  dear mark

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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29 thoughts on “Dear Mark: Higher BMI and Mortality Risk, Calories When Meal-Skipping, HIIT After Fasting”

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  1. Brilliant, Mark! I was wondering about that BMI study, and true to form, you gave a thoughtful explanation. Guess I won’t have to pack on the pounds to ward off an early death. 😉

  2. I like the idea of “carbing up” with berries, if you have to. I’ve been debating starting some HIIT sessions, but the carb source gave me a little pause. So I’ll try it out if I get some shakes.

  3. I know that seemed like an obvious question, but I’m glad you provided a little clarification on fasting. It hadn’t really occurred to me that if you eat the same amount of calories (only later in the day) after you fasted, you may be negating some of the benefits. Not all of them, I’m sure, since autophagy, etc. during the fasting period were still probably helpful. But it makes sense to defer to your intuitive sense of hunger, which, after a fasting reset, is usually lower.

  4. What an oversight! Not separating smokers and non-smokers? Oy, vey! Now I wonder how many people are going to use that study as an excuse to avoid making some of the hard changes that could do them some good. Thanks for this, Mark.

  5. Interesting point about the blood glucose spike: that during a high intensity exercise like sprinting, the body skips glycogen and goes for released glucose.

  6. I hope everyone reads your article, Mark, so they’re not fooled by the headlines the BMI study has been grabbing. I’ll share it!

  7. I find that (as a breakfast skipper or late breakfast eater) the study on hunger at lunchtime rings true. If I skip breakfast, I may feel extra hungry come lunchtime. BUT, I find that I fill up very, very fast, so it’s pretty obvious that I’m not recouping the calories I missed by skipping breakfast.

  8. I’ve found that the easiest meal for me to skip is lunch. If I have a project to keep me occupied, I feel better and have more sustained energy if I just wait until supper. Anyone else here skip lunch occasionally? If so, have you seen any noticeable benefits?

    1. I have breakfast between 11 and 1:00 so I essentially miss the typical Lunch. Then I have one more substatial meal between 6 and 8. I may snack a little in between with berries, hard cooked eggs, primal leftovers like a few slices of steak slathered in chimichurri. I drop weight fast on this regimen as long as I spend an hour or two in the morning with moderate exercise and ingest nothing but water after 8:00 in the evening.

    2. Many years ago (in college) I skipped lunch and it worked great for me. I ate a big breakfast, and decided not to waste time dealing with lunch and instead went straight to the library to study after my morning classes. I ate an early dinner, which meant I was hungry again for breakfast the next day. I think so much depends on your routine. You need to experiment to find what works best. At that point in my life, lunch was truly optional

    3. For a number of years since Primal I’ve had a fat/protein breakfast and just not felt hungry for lunch and eat an early 5:30-6:30 evening meal. HOWEVER, the last year or so I’ve gained about 12lbs in body fat and the more I read around the subject of older females (I’m 49) and fasting the more I’m beginning to think this isn’t so good. I think the ‘stress’ of skipping is probably ‘fat storing’. Quite a few texts talk of maintaining even blood glucose with more regular eating… this doesn’t make huge sense to me when I think of Grok but I’m now having a small lunch even though I don’t feel hungry to see if it helps.

  9. And population studies should take into account genetic variability. One diet fits none; OK, very few actually. _Death By Food Pyramid_ by Denise Minger gives a breakdown of various genetic differences and consequences.

    Different Stokes for Different Folks is for example the reason for 2 vs 4 cycle motorcycles.

  10. BMI studies will always be flawed because they don’t differentiate between muscular people and fat people. At 5′ 11″ and 190 lbs., my BMI is 27, which is classified as “overweight.” But I have visible abs (not magazine cover quality, but you can see ’em). I’m not fat; I have an athletically muscular build. I can’t believe that being strong and muscular is a health detriment (I can believe it once you’re getting to the extremes that professional bodybuilders get to, but I’m not even close to that).

    Maybe it’s the case that having extra muscle mass, and therefore having a higher BMI, presents a mortality risk. But I highly doubt it.

    1. Yes, I’d also like more follow up on this point. I’m a short, stocky, breastfeeding mom, so my BMI is 26. There is a bit of fat going on due to breastfeeding, but there’s no way I’d ever be able to get my BMI down to a comfortable 20-22.

      I know Mark has posted about carrying capacity for weight, so I wonder if that comes into affect? Also, the stats weren’t separated for men and women, kind of surprised on that point as women naturally carry more fat.

      Still taking this study with a huge grain of salt as BMI is a poor marker for health.

      1. Agreed that differentiating between men and women in a study would also be really helpful. I’ve read several times that BMI alone can be an inaccurate test of health. It might be more well-rounded if bodyfat percentage was also included in the study. Although it’s kind of difficult for the average person to get a really accurate bodyfat calculation – the calipers are pretty inaccurate, and the water displacement method and egg machine are pretty costly to get done.

  11. Fasting is da bomb. I’ve been IF for a year, on weekends only, steadily increasing the hours. I now do a 20hr fast on Sat. and a 24 hr fast on Sun. I love it. I’m finally getting rid of this stubborn belly fat, that going primal alone was not capable of.

  12. Legs shaky after your first HIIT session? Totally normal for anyone. Overtime you’ll adjust to the intensity, independently of your carb intake, and you’ll feel more solid. I do my exercise in the morning so I like my carbs at night and lunch. Before I was fat adapted I tended to crave carbs after a workout. Now I mostly crave fat and protein and tend to want the carbs later. No need to rush out to fill the glycogen stores right away. They get filled pretty easily.

  13. the newer studies on BMI seem to reflect the data on hunter gatherers BMI ie where height and weight has been measured and it is possible to calculate it. From memory the range from several such studies is about 20-23.5. ob

  14. Excellent, I now have a solid goal! Six feet tall, I should get my weight down to… 150 lbs!

  15. Mark, thanks for commenting on the BMI study. The first thing I thought of when I first saw that popping up was that they were documenting mortality at time of death, not time of onslaught of disease. Many diseases are ‘wasting’ diseases and cause significant weight loss prior to death. The numbers you put up make a lot more sense.

  16. I don’t think being that thin would give me an advantage if something tries to eat me. When I weight this 190-ish I am surprisingly strong without really trying. Down toward the 170 mark I’m not sure I could crush a cougar’s trachea while kneeling on its front legs…it would just throw me. As of now, I could pretty well throw IT.

  17. BMI can be misleading when one is muscular and overweight/obese. But it’s useful because it’s simple and, unfortunately, majority of people who are overweight/obese are not muscular, they’re just fat.

    That said, a more useful metric is waist to height ratio – you want it to be 0.5 or less (if fat, you don’t want to be apple shaped because of fatty liver disease etc.)

  18. I thought it was clear and helpful 🙂 I’m a small-headed person and my partner is large-headed, and we have completely different ways of carrying weight and muscle. (We even prefer different foods types and eating patterns.)

  19. Lol – another grab for people justifying being overweight and feeling”safe” from early death – it will be more food for the maggots I guess…

  20. One more thing that makes BMI unreliable – some examples are those with a high muscular weight, but low body fat can have BMI’s up to 28 or more, yet body fat is 4%, so 2 people with a BMI of 28 could actually have radically different muscle/fat ratios. Its a well known fact that strength athletes in particular have high BMI’s but low body fat.

    The whole study uses a very broad brush, so broad in fact as to make it kind of useless.

  21. I’m 64 and in good shape. Can I fast (except for fatty coffee) and do HITT/running/weight training will fasting? What if I do a 24,48,72 hour fast.

  22. I am Female, 63 years, 5’ 4” 117 pounds BMI of 20. I lost my last 15 pounds with Keto. I do not want to lose any more weight. I want to intermittent fast to get autophagy benefits.

    My problem is I am struggling to maintain my weight because I cannot fit all my calories into my reduced eating window, especially fats. Please advise. No ode seems to address my situation.