Dear Mark: Diet to Get Lean, Exercise to Get Healthy?

For today’s edition of Dear Mark, I’ll be discussing the role of exercise alone in weight and fat loss. If it seems like I’ve been harping on this same topic for a couple weeks now, it’s only because it’s so important to identify both the mainstream (“You must exercise ten minutes for each Oreo you eat.”) and alternative misconceptions (“Exercise has no effect on body composition.”) about exercise. Exercise—all by itself—actually can help you lose body fat, even if it has less to do with “burning calories” than other factors.

Let’s take a look:

So basically, correct diet makes you lean. Correct exercise makes you healthy.

Yes and no. Diet is the biggest factor in weight loss. but in order for the weight you lose to be primarily fat and the weight you gain to be primarily muscle, you really should exercise.

There are a good number of studies in which an exercise intervention alone actually improved body composition. In most of these, it’s very likely that the exercising groups spontaneously altered their eating patterns. The point is that they didn’t set out to change their dietary intake, if any change occurred. They set out to exercise and that was it—and it was enough to make them leaner.

Let’s keep it to just this year alone. What are some 2018 studies into the effects only exercising can have on body composition?

In obese adolescent girls, a 12-week strength and aerobic training program (3 days a week) dropped body fat by almost 4%, while also reducing glucose, insulin, and inflammatory markers.

In obese teens, eccentric stationary cycling is better at reducing fat mass than concentric stationary cycling. But both programs lowered body fat without altering food intake.

Teens might be especially sensitive to the fat-reducing effects of exercise. For one, teen males don’t compensate for all the calories they burn during training. Even obese teens who engage in a single acute bout of high intensity interval training show reduced food reward (junk food and food in general becomes less enticing than normal) and, despite subjective appetite remaining high, eat less food than obese teens who don’t do the HIIT session.

It works in adults, too. Among young women with obesity, a 12-week circuit training program with no changes to diet reduced body fat, body weight, and BMI compared to the control group.

In obese women, high-intensity circuit training reduced body weight and body fat while increasing lean mass. Those who did not participate in the exercise program got heavier, fatter, and saw their waist circumference expand.

One recent study found that weight training was more likely than other forms of training to reduce appetite and improve weight/fat loss, thanks to its unique effects on appetite hormones. That’s been my experience, both with myself and when working with others. A regular diet of heavy strength training just works best. You’re hungry, but for the right things. Chronic cardio has a different effect—increased cravings for refined carbs. The classic sight at cafes in Malibu was always the hordes of spandex-clad cyclists and runners cramming themselves with pastries and cookies.

Even among the perpetually obese, those who exercise on a regular basis have less region-specific body fat (if you measure the fat percentage of a particular region of the body, it’s lower in obese people who exercise) and less overall body fat.

Not all studies find big changes in body weight. That’s normal. That’s to be expected. But what they do consistently find in exercise-only studies is that training reduces fat mass, even when it doesn’t reduce overall body weight.

Now, just imagine what you can accomplish with a good diet and regular training and physical activity. Oh, and throw some stress reduction, good sleep, regular circadian rhythmicity in there for good measure.

What’s your experience been with exercise alone for fat—not just weight—loss?

Thanks for reading, everyone. Take care.


Wong A, Sanchez-gonzalez MA, Son WM, Kwak YS, Park SY. The Effects of a 12-Week Combined Exercise Training Program on Arterial Stiffness, Vasoactive Substances, Inflammatory Markers, Metabolic Profile, and Body Composition in Obese Adolescent Girls. Pediatr Exerc Sci. 2018;:1-7.

Julian V, Thivel D, Miguet M, et al. Eccentric cycling is more efficient in reducing fat mass than concentric cycling in adolescents with obesity. Scand J Med Sci Sports. 2018;

Shakiba E, Sheikholeslami-vatani D, Rostamzadeh N, Karim H. The type of training program affects appetite-regulating hormones and body weight in overweight sedentary men. Appl Physiol Nutr Metab. 2018;

Thivel D, Finlayson G, Miguet M, et al. Energy depletion by 24-h fast leads to compensatory appetite responses compared with matched energy depletion by exercise in healthy young males. Br J Nutr. 2018;120(5):583-592.

Miguet M, Fillon A, Khammassi M, et al. Appetite, energy intake and food reward responses to an acute High Intensity Interval Exercise in adolescents with obesity. Physiol Behav. 2018;195:90-97.

Kim JW, Ko YC, Seo TB, Kim YP. Effect of circuit training on body composition, physical fitness, and metabolic syndrome risk factors in obese female college students. J Exerc Rehabil. 2018;14(3):460-465.

Batrakoulis A, Jamurtas AZ, Georgakouli K, et al. High intensity, circuit-type integrated neuromuscular training alters energy balance and reduces body mass and fat in obese women: A 10-month training-detraining randomized controlled trial. PLoS ONE. 2018;13(8):e0202390.

Santi A, Bosch TA, Bantle AE, et al. High Body Mass Index Masks Body Composition Differences in Physically Active Versus Sedentary Participants. Metab Syndr Relat Disord. 2018;

Drenowatz C, Hand GA, Sagner M, Shook RP, Burgess S, Blair SN. The Prospective Association between Different Types of Exercise and Body Composition. Med Sci Sports Exerc. 2015;47(12):2535-41.

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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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