Acute vs. Chronic effects – understanding what you read

Here’s a very common problem people have when reading training articles or scientific studies – they don’t take the time to differentiate between acute and chronic effects and ask to themselves which is actually important in a particular context.

Now, just for a tiny bit of background to set some context, acute effects are short-term effects.  Depending on what we’re talking about, they could take place over a matter of minutes, hours, or even up to a day.  Chronic effects are long-term effects.  They’re typically the long-lasting outcome of a particular intervention, whether it be training, dietary, etc.  These take place over a matter of days, weeks, months, or years usually.

So, when you read an article, ask yourself, “is this person describing an acute effect or a chronic effect, and do I actually care about an acute or chronic effect in this scenario?”

For example, studies show you lift heavy and you get a short term spike in testosterone production.  Or that if you train with short rest periods, you get a spike in growth hormone output.  So what do we do?  Do we have unbridled excitement because we just unlocked the secret to gainz, or are we skeptical of the usefulness of these results?  If you picked “skeptical,” you’re correct!  Those studies are reporting acute changes.  The chronic changes we’re interested in are increases in muscle mass or strength.  You might assume that higher levels of testosterone or growth hormone post-workout would mean more muscle and strength, but you can’t say that for sure without evidence from studies looking at chronic effect taking place over the course of weeks or months.

Another good example is pre- and post-workout protein consumption.  Plenty of studies show that getting protein around your workout increases protein synthesis and decreases protein breakdown acutely, but evidence from a massive new meta-analysis shows that protein timing doesn’t affect chronic muscle growth in response to training very much at all – simply getting enough protein throughout the day is sufficient for the same anabolic response.

Of course, there’s some overlap.  Acute changes CAN add up to chronic changes, but they don’t have to.  This is an important distinction to make.  In both of the examples I used, looking at the acute evidence, it would certainly be worth testing whether those short-term changes would add up to long-term results.  However, at best, acute evidence is only good for generating hypotheses to be answered by longer-term research.

Of course, the way research is done creates some more problems.  It’s cheap(er) to do research on acute mechanisms.  You draw some blood, put people through a workout or feed them some food or supplement, draw some more blood a couple more times, analyze the samples, and that’s that.  Research about chronic effects, on the other hand, requires months of commitment to a study, a lot more sessions or meals, a lot more blood draws, a lot more data collection and analysis, and (probably most importantly) a lot more money.  For this reason, you’ll always see people getting a little TOO excited (myself included, honestly) about really compelling studies showing acute mechanisms, by which we might surmise some long-term outcome, because unfortunately, we may never see the longer term study due to logistical and funding issues in the scientific process.

Now, don’t think I’m saying acute effects are useless by any means.  Sometimes we’re not interested in the effects on adaptations weeks or months in the future.  Sometimes we’re concerned about what will improve my performance for a competition I have tomorrow, or even in a couple hours.  This is when acute effects are hugely important.  For example, we know that glycogen resynthesis occurs most rapidly after exercise, and takes place relatively quickly.  So if you’re competing in a sport with multiple games in one day, or doing something like a CrossFit competition with multiple WODs in one day, consuming carbs directly after your first game/event will have a substantial impact on your performance in endeavors taking place without a matter of hours.

The takeaway is simply that you need to ask yourself what you’re interested in when you’re reading information about the effects of some training program or some dietary intervention or supplement – are you interested in getting better long-term, or are you interested in optimizing your performance here and now?  And, based on your answer, is what you’re reading addressing the proper time frame, whether it be acute or chronic, that you’re interested in?  By asking those two questions, you’ll avoid a lot of false starts and frustration by putting information and studies in their proper context.