Dear Mark: Young Guy, Low Testosterone; RS for Babies; and Sprints vs Tempo Runs

White PotatoFor today’s edition of Dear Mark, I answer three questions. First up is a young guy with low testosterone. He eats the right foods, does the right exercises, and has lived the right number of years, but he’s saddled with the testosterone of a much older man and wants to know why. Next, I discuss whether resistant starch is suitable or important for infants. The parents are benefiting from it, but does that mean an infant needs it, too? And finally, I explore the value of tempo runs – tough but doable runs maintained at the anaerobic threshold pace – for fat loss. How do they compare to my favorite fat burning exercise – sprints?

Let’s go:

Hey Mark. Your site is great! I’ve learned a lot. I just have a question. Despite the fact that I lift weights (squats, ring dips, pullups) 3x a week and do jiu jitsu 2x a week, along with eating plenty of chicken, beef, eggs, etc. my testosterone at 19 years old is 270 (low end of normal). All my doc said was remove GMOs, perhaps increase my calories (I’m 145 lbs with 1500 daily calories) and carbs, and to check back in a few months. But this makes no sense to me. Any suggestions?


Thanks for the kind words! Much appreciated.

What your doc said makes perfect sense to me, actually. Well, not the GMO thing. It’s not that I’m necessarily a fan of them, but I don’t see how they’d impact testosterone unless we’re talking about GMO soy.

You’re lifting heavy using compound, full-body movements three times a week. That takes a lot out of you, and it should be increasing your testosterone levels. You’re practicing jiu jitsu, one of the more calorically-demanding martial arts (and most instructors include conditioning workouts along with skill training, so that’s even more energy you’re expending). You’re eating plenty of protein, animal fat, and getting ample amounts of dietary cholesterol from eggs that the strength-training body can leverage into testosterone, hypertrophy, and strength – so you should be enjoying healthy levels of potent testosterone flowing through your veins.

Assuming you’re taking care of the other important determinants of testosterone levels, like zinc intake (eat your oysters), vitamin D/sunlight exposure, and excessive stress/cortisol, I think I know the problem.

You’re just eating too few calories for your situation. 1500 calories is drastically low for a 19 year old male trying to support your level of activity.

In obese guys? Yeah, a low calorie diet will improve sexual function and raise testosterone. But you’re not obese or probably even overweight. The obese guys make up for the lack of calories by consuming the thousands of calories stored in their body fat as they lose weight. Their calorie intake is low, but their calorie usage remains high.

If you’re lean, you don’t have that luxury. Ultra low calorie diets – which for an active youngster, your diet qualifies – take their toll. Recall what happens to the hormonal profile of bodybuilding competitors as they approach competition. They’re dieting hard and working out harder. Their testosterone plummets. Overall, the hormonal profile of a competition-adjacent pro bodybuilder resembles that of an 80-year old man. The good news? Their testosterone recovers once competition ends and they resume eating enough food. I’m confident the same is true for you.

You’ve got everything going for you. Just eat enough to support everything you’re doing. Somewhere in the range of 2,500-3000 calories is a good target. You can always reduce it if you’re gaining body fat, but I strongly suspect you will not. Try it for a month and re-test. It’s only a month, and it’s not the end of the world if it doesn’t go well. At least then you’ll know that it’s not the amount of food you’re (not) eating.

I should note that testosterone levels among adult males have been dropping for some time, independent of age, health, or established lifestyle factors (though they may not include “low calorie intake” or “overtraining” as factors), at least in America and Finland.

Hi Mark,
Just curious if you knew of any information on how much resistant starch a 1-year old should eat per day. My wife and I are experimenting with 1 tsp of potato starch a day (with intent to increase up to 4 tbsp to see how we feel) and want to give some to our baby as well.
She gets a lot of probiotic foods (milk kefir, sauerkraut and natto) and is still breastfed.
Any thoughts would be appreciated!
Thank you,

I don’t have a specific dosage for you, but I do know that as infants are weaned they begin to display gut bacteria that can degrade resistant starch, so they can make use of it. Also, mother’s milk comes prepackaged with a type of prebiotic known as human milk oligosaccharide, so babies can definitely benefit from prebiotics.

Do infants “need” resistant starch? Maybe. Their gut bacteria still need to eat, and RS has been shown to be a good source of food for bacteria in adults and infant pigs. Yep, pigs. You’re not one, but the omnivorous pig is a fair proxy for humans. Infant pigs who eat resistant starch enjoy increased calcium, zinc, iron, and phosphorus absorption compared to infant pigs eating digestible starch.

Here’s what I’d do: once the baby’s getting the majority of his calories from solid food rather than milk/formula/breastmilk, start including tiny amounts of RS. I’m talking tiny – like a quarter teaspoon at a time – just to see how he reacts. If you’re up for chewing food (which is “weird” but also pretty normal on a global, historical scale), try a small amount of chewed green banana.

Actually, scratch that. Green bananas are awful unless incorporated into smoothies. Why not stick to whole foods that taste good? Once your kid starts eating starches, you can give him small amounts of cubed cold leftover potatoes. 100 grams of cooked and cooled potato has about 5 grams of RS, so start small.

If the kid doesn’t have any problems like extreme amounts of gassiness, constipation, diarrhea, or other symptoms of an upset stomach, the RS is probably beneficial and definitely harmless. Just go slowly.

Hi Mark,

I know tempo runs are great for increasing your race-running pace and lung capacity but are they as effective at fat-burning as sprinting is?



Great question. For those who don’t know, a tempo run is a sustained effort at or around the anaerobic threshold, also known as the lactate threshold. You’re at maximum aerobic/fat-burning capacity and juuust starting to flirt with obtaining significant energy from the anaerobic pathway. Tempo runs are tough but manageable – they’re “comfortably hard.” They’re intense enough to push training adaptations without being so unpleasant that you quit before it’s over.

There are actually different kinds of tempo runs. Most people, when they use the term, refer to 5-10 mile runs at close to race speed. But sprinters do them too. Sprinters do tempo runs for active recovery and building/maintaining the aerobic base (yes, even sprinters need an aerobic base). For a tempo day, they might run a total of 2000 meters in 100s, 200s, or 400s. Easy-ish pace, enough to get their heart rate up a bit. They look a lot like sprints, only at 60-75% intensity.

Tempo runs can get you into trouble. First of all, there’s not a switch that blocks the aerobic pathway and turns on the anaerobic pathway. It doesn’t work like that. You’re always burning bits of everything and there are no hard barriers, so it’s very easy to go harder than you’d intended and get into almost pure sugar burning territory. That’s cool if you want to do that to improve your fitness and you accept the consequences, but I’d advise against using such a run on a regular basis for fat burning/body composition alterations/health maintenance.

Don’t get me wrong: if you do them right, tempo runs are great for burning body fat. But they burn lots of other stuff, too. And if you do too many of them with too little rest in between, you’ll burn out. Tempo runs got me into trouble. My absolute intensity was significantly different than the rest of the population, but my relative intensity wasn’t necessarily higher than an enthusiastic amateur runner. I just did too much.

I’d say sprinting still gets the nod. Sprinting isn’t so much a pure burner of calories in a mechanistic sense (although it does that too). What it really does is get the “furnace” going better than almost anything else, allowing you to make better use of energy substrates without them turning into fat, and increases insulin sensitivity so glucose replenishes glycogen without requiring as much insulin. Plus, a recent study found that two minutes of sprinting resulted in the same 24 hour oxygen consumption (a marker of post-exercise metabolism) as 30 minutes of steady state endurance training (akin to a tempo run). So yeah, a tempo run is good, but sprints are just so much more efficient.

Make sure you’re doing tempo runs right and they can be very helpful. If you find yourself bonking and craving sugar/carbs afterwards, you’ve probably ventured far afield of the aerobic pathway and should go easier next time. Or just focus on sprints for fat loss and do tempo runs for fun.

Thanks for reading, folks. Got any comments or advice for the readers who asked today’s questions? Leave ’em in the comment section!

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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