Plain Language Summary
Keto Flush Explained
When you start the ketogenic diet, certain changes happen in your body:
- Body water decreases
- Glycogen stores shrink
- Salt is expelled from the body, creating a dehydrating effect
- Lean body mass 1Lean Body Mass (LBM) is the weight of your body that is not fat. It typically includes water, tissue, and bones. decreases
These changes happen when carbohydrates are limited in supply and because ketones expel water. Body weight will decrease as a result, and we can refer to this as wet weight. This dehydration may make you look deflated because muscle mass contains a lot of water. Less water and glycogen in the muscle reduces its size.
We coin these effects: keto flush.
However, keto flush is temporary, preventable, and reversible.
Preventing Keto Flush
You can prevent keto flush by:
- Supplementing electrolytes (such as sodium and potassium)
- Drinking more water
- Doing strength training to retain muscle glycogen and water
- Taking creatine
- Habituating to the diet. There is evidence that glycogen stores slowly refill, and that the issues of keto flush therefore dissipate with time.
Reversing Keto Flush
It is easy to reverse keto flush. Simply consume more carbohydrates and exit ketosis. This will lead to glycogen supercompensation where glycogen stores are filled beyond their normal non-ketogenic levels. You should expect to gain a couple kgs of wet weight.
Losing Muscle Mass?
When keto flush takes place, you will naturally “lose” lean body mass. But, does that imply that muscle mass is lost on the diet? Not necessarily, because water and glycogen are part of lean body mass in most methods that assess body composition.
Hence, we should not assume that this is muscle atrophy (unless you want to equate a transient reduction in wet mass with atrophy).
Weight Loss vs. Fat Loss
Keto flush has several implications. One of those is that you should not trust the weight scale entirely. A rapid drop in weight does not imply that you’ve only lost body fat. You should expect to lose 1–5 kgs of wet weight during the first weeks of a ketogenic diet (depending on how many steps you’ve taken to prevent keto flush). You can still burn fat effectively during keto flush, and the ketogenic diet is generally good at reducing hunger.
What is Keto Flush?
The Role of Glycogen
Glycogen is the body’s stored carbohydrate. It is mainly found in muscles (~350–700 g) and the liver (~100 g) (Knuiman et al., 2015). The body uses glycogen during exercise, and the more intense the exercise, the more glycogen is used (Knuiman et al., 2015). 4 grams of water are stored alongside every gram of glycogen.
We can reduce glycogen stores by:
- Restricting carb intake (i.e. the ketogenic diet)
Glycogen stores shrink on the ketogenic diet
Since the ketogenic diet restricts carbohydrates, glycogen and water stores drop. This can be seen in the figure and table below:
Resting muscle glycogen levels are more than halved on a keto diet (white bars) vs normal diet (striped bars).
Figure by Bogardus et al., 1981.
Per the table below, glycogen stores are reduced by 20–54% after 1–6 weeks on the ketogenic diet.
|Study||Glycogen||Lean Body Mass/ Fat-Free Mass|
|Phinney et al., 1980 |
Untrained, obese subjects
|Week 1: |
0.87 mg/100 g wet wt (−43% from baseline)
1.04 mg/100 g wet wt (−21% from baseline)
|Phinney et al., 1983 |
|Week 4: |
0.76 mg/100 g wet wt (−54% from baseline)
|Bogardus et al., 1981 |
Untrained, obese women
|Week 1: |
0.81 mg/100 g wet wt (−54% from baseline)
0.86 mg/100 g wet wt (−51% from baseline)
|Lambert et al., 1994 |
|Week 2: |
Approx. 0.7 mmol/kg
|Krotkiewski et al., 2000 |
|Week 2: |
−40% from baseline
|Week 2: |
|Harber et al., 2005 |
|Week 1: |
−20% from baseline
Does glycogen come back?
Please read the relevant sections on habituation to the diet and carb refills [Internal links]
Water and Salt Loss
“[Diuresis] — an increased excretion of urine” - Merriam-Webster
In other words, the ketogenic diet has a dehydrating effect. The dehydration happens in part because more salt is released from the body, alongside glycogen and water (as described in the previous section) (Denke, 2001; Miller et al., 2004; Pogozelski et al., 2005). We can see this effect in the figure below, where ketosis leads to a large reduction in the total body water of the participants.
Figure: Gomez-Arbelaez et al., 2017 (edited for clarity).
Participants were obese patients.
Per the table below, total body water is reduced by ~1–5 kg after 1–8 weeks on the ketogenic diet.
|Study||Total Body Water||Lean Body Mass/ Fat-Free Mass|
|Vargas et al., 2018 |
Resistance trained men (8 weeks)
|Yang and Itallie, 1976 |
Obese men, metabolic ward (10 days)
|Approx. −4.8 kg |
(estimated from graph)
|Johnstone et al., 2008||Week 4: |
|Week 4: |
|Sawyer et al., 2013||Week 1: |
−0.84 kg (men)
|Week 1: |
−1.13 kg (men)
|Gomez-Arbelaez et al., 2017 |
|Maximum ketosis |
Out of ketosis
|Maximum ketosis |
Out of ketosis
|Colica et al., 2017 |
|Week 3: |
Table: total body water is reduced in ketogenic diet studies.
Glycogen may be part of the LBM calculation, so it is important to also look at LBM for reduction in wet weight caused by keto flush.
“Ketone bodies are filtered by the kidney as nonreabsorbable anions. Their presence in renal lumenal fluids increase distal sodium delivery to the lumen, and therefore increase renal sodium and water loss” - Denke, 2001
Does Keto Flush Lead to an Illusion of Rapid Fat Loss?
No doubt, you can use the ketogenic diet to reduce hunger and eat less. This in turn will help you burn body fat.
However, it is important that you’re aware of keto flush. The first time you start on a ketogenic diet, you will likely experience rapid weight loss. This might feel like the diet is burning off fat, but the weight loss is also due to glycogen, salt, and water losses.
Several research teams expect people to lose anywhere from 1–5 kg of wet weight in the first week(s) of the ketogenic diet (Kreitzman et al., 1992; Bilsborough and Crowe, 2003; Miller et al., 2004). In other words, weight that is not fat mass.
You can see studies that reported water loss in the table above [internal link].
Why Keto Flush Makes You Look Deflated
Fat-free mass is mostly water. On average, it is ~73-74%, but the full range in humans is ~69–81% (Wang et al., 1999; Roumelioti et al., 2018). So it should come as no surprise that muscle mass also consists of water and protein. Though, it also contains fat (intramuscular triglycerides), glycogen, and other substances.
When muscle hydration changes, so does muscle size (Nygren et al., 2001; Bone et al., 2016). And as we know, the ketogenic diet reduces intramuscular water and glycogen. Hence, keto flush can make muscles look smaller, but it could also have the positive effect of reducing general bloating.
4.2 kg of Fat-free mass is lost on a ketogenic diet. Figure by Gomez-Arbelaez et al., 2017
You can Prevent and Reverse Keto Flush
Habituation to the diet
We have reason to believe that the body adapts to the ketogenic diet. With time, glycogen stores may return to normal.
“... initial depletion of muscle glycogen induced by the [ketogenic diet] may be at least partially reversed through habituation to the diet.” - Schick, 2016
For example, keto-adapted endurance athletes had normal muscle glycogen levels in Volek et al., 2016. During exercise, their glycogen utilization was similar to the control group, but their glucose oxidation was much lower.
"... the LC athletes had similar rates of glycogen repletion compared to the HC athletes, despite receiving a negligible amount of carbohydrate after exercise (4 vs 43 g) ..." - Volek et al., 2016
We also see that glycogen stores refill over a couple of weeks on the ketogenic diet, such as in Phinney et al., 1980. However, this is not supported by all studies.
One of the mechanisms here is that carbohydrate oxidation decreases so that glucose can be more readily taken up and be converted to glycogen (Harber et al., 2005). The body seemingly adapts by preserving glycogen stores (Schick, 2016).
Strength Training to Retain Glycogen and LBM
Many studies report that the ketogenic diet leads to loss of lean body mass. The good news is that this is to a large extent water and glycogen.
One way to partially prevent glycogen loss, is to do strength training. This is because it leads to glycogen and water gains in both the short and long-term.
“[strength training] induces ... a rise in cellular hydration.” - Riberio et al., 2017
Per the graph below, we see studies where subjects were able to maintain or increase lean body mass with strength training on a ketogenic diet. However, several of the studies have major methodological and data reporting issues. Some studies also find that lean mass is reduced, even with strength training.
Carb refill and supercompensation
We can increase low glycogen stores by eating more carbohydrates. This will lead to a supercompensation effect. Check out the figures by Toomey and Wilson below to see this effect in action.
Figure by Toomey et al., 2017.
Figure by Wilson et al., 2017.
The keto group (red) increased carb intake between weeks 10 and 11, and supercompensated glycogen stores. Hence, a very large increase (+3 kg) in lean body mass.
Salt and Creatine
Finally, you should consider increasing mineral intake and supplementing creatine. Creatine leads to greater intramuscular water retention. You will also need to drink more water if you’re going to do this.
A Call to Action for Researchers
Fat Loss is Superior to Weight Loss
While the weight scale has its use, it is also limited. The scale does not distinguish between fat mass and fat-free mass. It cannot inform you of how much body fat participants have lost. It can give you a proxy, but it is imperfect.
Therefore, researchers should favor fat loss estimation over weight loss in ketogenic trials. Studies, meta-analyses, or reviews should be careful about making conclusions about a diet’s efficacy from weight loss data. Especially if there is no fat loss data available, and if the comparison is between a glycogen reduced group, and a normal group.
How To Measure Changes in Muscle Mass in Ketogenic Studies?
DXA is not perfect (Toombs et al., 2012).
As we’ve discussed at length in this article, the ketogenic diet will dehydrate the fat-free mass component of the body, thus reducing its water percentage, which is typically assumed to be constant at ~73–74% (Wang et al., 1999; Toomey et al., 2017; Roumelioti et al., 2018).
When DXA calculates lean soft tissue, it uses assumptions about tissue hydration, and when those assumptions are violated, the estimations become erroneous (St-Onge et al., 2004; Toomey et al., 2017). Notably, FFM hydration is not constant (St-Onge et al., 2004), and it can be manipulated by diets, fasting, or exercise (Knuiman et al., 2015; Toomey et al., 2017; Bone et al., 2017).
“... Fat-free mass is routinely measured by BIA or DEXA; however, these methods have hidden drawbacks. For example, an important assumption of the DEXA measurement of fat-free mass is that it contains 73% water” - Roumelioti et al., 2018
In fact, it can be questioned whether glycogen-reduced subjects should be compared to their own baseline or even to other groups consuming high-carbohydrate diets (Tinsley and Willoughby, 2016). Several researchers now call for control of hydration and glycogen levels (Toomey et al., 2017).
Suggestions for how to make the comparison fair:
|Carb refeed the participants a couple of days before the scans/measurements.||Quick, easy.||Glycogen supercompensation will distort LBM|
|Wait for long-term adaptation of glycogen stores (duration unknown)||A more accurate comparison between groups.||Long studies are expensive and may have issues with participants dropping out|
|Slowly increase carbohydrate intake over 2+ weeks before scans/measurements.||May potentially prevent glycogen supercompensation.||Requires stringent control of carbohydrate intake.|
|Measure LBM in the ketogenic diet group 2 weeks after they have initiated the diet. Then, compare week 2 (glycogen reduced baseline) to week [x] at the end of the trial.||Avoids the issue of comparing participants with normal glycogen levels to those that are reduced.||If real decreases in LBM/protein content occur in the first 2 weeks, these will not be picked up when week 2 is compared to the end of the trial.|
|Estimate TBW separately using a 4 component model||Ability to control for water losses.||Expensive and more time consuming. Even if TBW is measured, glycogen is part of LBM and thus still affects the measurement.|