This is somewhat related to the losing weight/gaining muscle question found here, but I would like to find out if there are any studies or knowledge of weight loss impeding the physiological adaptations of endurance exercise training.

There are certain adaptations that occur with endurance training, such as the enhancement of Type IIa and IIb fibers, which includes fiber growth and hypertrophy. Blood vessels grow into the muscles to aid in oxygen transport, the body gets more efficient at using fat and increases the mitochondrial count in the muscles themselves.

This study looks at the physiological and metabolic changes that occur, and would suggest that there was no weight loss in the group (Since the VO2 did not change, weight did not change as weight is central to the VO2 calculation), so it is possible to increase endurance in the short term without weight gain/loss. Also, it is possible to get more fit while losing weight in untrained people.

However, over the long term there are certain physical changes that occur. Specific to cycling, the upper legs and calves can get quite large. You see this most markedly in sprinters, but also in endurance cyclists, just not quite as pronounced. Common sense and the above referenced question would indicate that the muscle growth associated with endurance training might be hampered with weight loss, but would this also impact the changes that occur internally? (Fiber density increases, mitochondrial increases, blood vessel growth). And in the case of untrained people, is there a middle ground where no more fitness/endurance can be gained without gaining weight in the short term? Any information is welcome, but studies are preferable.

  • I like this question. Looking at it in the oppositie direction, you can often eat through plateaus while strength training... wonder if the same can happen for endurance training.
    – user4644
    Commented Feb 7, 2013 at 1:42
  • I started to write an answer, but it would help considerably if you explain why you are interested in the topic? Are you trying to lose weight and gain fitness at the same time? What is your current fitness level? Commented Feb 7, 2013 at 5:58
  • Actually, JohnP is one of the most active and knowledgable members of physical fitness SE, and im guessing that this is quite an academic question. Ill be asking my friends from medschool for some meaningfull input on the matter
    – K.L.
    Commented Feb 7, 2013 at 12:38
  • @EricGunnerson - I'm interested because I don't think it's an area I've heard about, and because I do endurance sports. Every spring all the triathletes I know are losing winter weight while ramping up mileage and intensity preparing for the season. Intuitively I don't think there is an effect on fitness, but I haven't seen anything one way or the other. It may be that we should all concentrate on fitness first and then weight instead, but there is nothing that I can find to prove or disprove it. My current fitness level is 40k TT right at 1:00, and ~ 40min 10k.
    – JohnP
    Commented Feb 7, 2013 at 14:07

1 Answer 1


First off, I wouldn't be surprised if you can't find a study that addresses this - most fitness-related studies are done by grad students, and the topic they pick needs to be easily addressable within the time they have. Further, I think it would be hard to design a control for this study; if you built one that forced people to maintain weight you would probably add a lot of confounding factors.

As you probably already known, increased fitness = training stress + recovery. As long as you are meeting your body's recovery nutrition requirements well, then your fitness improvement should not be compromised if you happen to lose some weight along the way.

It's a fine line, however, and if you are in calorie deficit and not getting your recovery nutrition right, or trying hard to lose weight, you will compromise your improvement. WRT muscle strength improvement, long exercise can easily lead to muscle catabolism, where you end up breaking down your muscles instead of building them up.

Pro cyclists training for stage races with significant climbing spend their time walking this line, but they are generally more concerned with power/weight ratio rather than pure power (though power is important in the TT stages, if you lose significant time in clmbing stages the TT stages don't matter). Low weight is less of a factor on single-day races, and I think it would be the same for triathletes.

Somewhere in one of Carmichael's books ("food for fitness?"), he states that it's very hard to lose weight and increase fitness at the same time, though whether he has any credibility any more is an open question.

So, I wrote all that, but I think the best chance of getting an better answer to your question is to drop an email to Joe Friel - he regularly answers questions like this on his blog.

  • Thank you for your answer and suggestions. I'd probably email Philip Skiba over Friel, I personally am not confident in Friel's interpretations of physiology, from the books/blog entries I've read from each of them. I am interested in the cycling portions that you mention, as early season cycling is often referred to as "racing into shape", which would imply that you can increase fitness/lose weight, or at least maintain/lose. And, as you point out it probably will be a fine line to walk.
    – JohnP
    Commented Feb 7, 2013 at 16:22

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