I was recently in Costa Rica again at Flo helping with and being in the acute study where we blasted meatheads with lots of volume over 4 days in a row.

The gang at Flo in Costa Rica for the 4 day acute training study

An interesting thing happened during this study related to acute changes in bodyweight.  For all the specifics, check out my Not Another Fitness podcast last week where I went over a new lesson each day.  Lesson 1, Lesson 2, Lesson 3, Lesson 4, and my #1 takeaway.

Fat loss is very topical now in early 2020 as the legions storm the gates of the local Planet Fitness to avoid the donuts on the weigh in (get it, I make funny joke), to board the treadmill to nowhere.

A new way to use the treadmill . . .

But that is not you.

You are a savvy fitness professional since you are reading this wonderful article.

What tactic can you use that is beyond the basics of “just eat less calories, bro.”

Enter scale changes stage left.

Before we get there, let’s take a step back to discuss a simple method to assess body composition progress….

….the scale.

Yes, I said the s-word.

I hear ya that using the scale to determine fat loss can be tricky.

I agree that looking at weight on the scale does not tell you where the weight comes from (lean mass, fat, water, etc.).

I’ve lost count of how many clients have stepped on the scale and lost their minds when it showed that they gained 2-6 lbs. overnight.   Low carb diets are back in vogue again (keto…kough kough), so I guarantee you will see this reported by your clients.

Get the facts if you should do a keto diet.

This can work the other way too as they “magically” lost 3 lbs. overnight as if the weight loss fairy showed up on the third Tuesday of a purple harvest moon and blessed them for their hard work of doing the pushaway of Holiday cookies.

One of the main determinations in this equation is fluid levels, but more specifically the fluid (water) that is stored with carbohydrates (aka glycogen).

How much can you move the scale based on carb intake?

I will answer that below using a 4-letter word – MATH.

For our simple back of the envelope estimations here, we will assume that the client’s fluid intake and sodium are not changing (although, they do change and more on that coming up).

Glycogen – the stored form of carbohydrates – is primarily stored in the muscles and liver.   According to classic textbook by Nancy Clark, muscle can store 1,400 kcal and the liver 320 kcal in the average 70 kg  (155 lb.) dooode bro who needs to do a muscle gain program.

Don’t ask me why physiology textbooks always use small males for their calculations.   Since glycogen is a carbohydrate, we divide by 4 kcal / gram to get 350 grams in the muscle and 80 grams in the liver.

Science in Costa Rica at Flo.

A key part is that we can’t drive muscle glycogen to zero.  At best, in trained athletes with brutal, I-saw-a-white-buffalo-in-the-sky, protocols, you could get to 40% left.  Data from Bergstrom  in 1967 (1) showed that muscle glycogen can be lowered to 1/3 of previous levels with a 3 day very low carb diet.

For simple math, let’s assume we deplete (and do not replace) muscle glycogen by 50% via restricted carbs and some ultra Slayer-inspired muscular work in the gym.

350 grams x 50% = 175 grams of glycogen

Liver glycogen can be driven to basically zero, hence the amount of glycogen used is at 175 + 80 = 255 grams of glycogen down.

If you skipped all the math for fear of having a traumatic reminiscence of 5th grade math, the total of glycogen depleted is about 260 grams.

Each gram of glycogen holds about 3-4 Xs that amount in water.

If we use 4Xs, then 255 grams of total glycogen depleted x 4 = 1,020 grams = 2.25 pounds.

Now, this is on the very conservative end since it is for a 155 lb. person depleted to only 50%.

If you are bigger mammal with more muscle, you can start to see changes of 3-4 lbs easy.  This is from glycogen alone and not  accounting for any ion / fluid changes.

Training at Flo in Costa Rica

Research Says . . .

Our back of envelope calc was fun, but what does actual research say here?

Stand back for science!

A great study done (2) looked at this very topic.

Eleven females were put on a standardized diet to reach a stable weight and then a very low carbohydrate diet for 4 days.

The average starting weight was 89.2 kg and 4 days later it dropped to 84.8 with an average loss of 4.3 kg (9.5 lbs.).  Of this, an estimated 415 grams was lost as glycogen and the rest lost as fluid.

Science Cat

The take away here is if you weigh around 200 lbs,  go super low carb for 4 days, add in some salt and even potential fluid changes from inflammation due to heavy training, you can see the scale change by 10 lbs.

When I got to Costa Rica on my last trip, I weighed in at 221.3 lbs –which may be a bit low as it was after a long travel day, lower food, 2 plane flights and 2.5 hours of sleep.

I was up early the next day for meditation, breakfast and the start of the study.  After the main data collection was done, Ryan and I lifted for 2 hours using the same protocol.

Rise, rinse and repeat like ground hog day for 3 more cycles.

Great tunes during CR lifting sessions…

Each AM we would weight ourselves.  Before the start of day 4, my AM weight before any food or drink was at 229.0.

During the study, calories were not strictly regulated per say, however we all ate at the same location with awesome food that was provided. Lots of white rice, eggs, meat, and veggies.

I also realized that after day 1, I was stupid hungry all the time.  At each of the 3 meals I attempted to eat a bit more.  I even added a protein shake or meal replacement in the afternoon.   That helps a bit, but I was still waking up in the middle of the night and murdering 1-2 quest bars.  I would find the dead wrappers next to my bed and only have a fuzzy memory of the impending doom.

Another one down…

I was secretly hoping to hit 230 by the end of the study.  I came close.

Interestingly enough, once the study ended, my bodyweight dropped to 226 almost overnight and then 224 once I got home.  I was not nearly as hungry either.

#1 Take Away

The scale can move up and down by as much as 10 pounds in larger mammals if they are doing a big change in training volume, fluid changes and dramatically altering carbohydrate intake (either up or down).

If you are doing any of these or get a wild hair up your keester and want to do all 3 in the name of science (virtually high five) like I did, then you can expect to see your weight change.

It does not mean you all of sudden turned into a fat bastard in 4 days or had that secret squirrel Russian training program with all the impending gainZ hit you overnight.

Relax, get back on the program, and it will be ok unless you got caught in the Planet Fitness new year’s stampede and ended up face down in a box of donuts.

Get some!


  1. Bergstrom J, Hermansen L, Hultman E, Saltin B. Diet, muscle glycogen and physical performance. Acta physiologica Scandinavica. 1967;71(2):140-50.
  2. Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. Am J Clin Nutr. 1992;56(1 Suppl):292s-3s.
  3. Much of this article was based on a similar one I wrote for the PTDC at https://www.theptdc.com/explain -weight-fluctuations-clients