6 November 2020

Factors Influencing Overnight Weight Fluctuations

by Stacey Rogers 0

One of the most common scenarios among both females and males is being disheartened by a scale weight reading. After nailing your new training and nutrition program you step on the scale in the morning, only to see that number flash before your eyes a whole kilo heavier than yesterday! Within a split second, you…

One of the most common scenarios among both females and males is being disheartened by a scale weight reading. After nailing your new training and nutrition program you step on the scale in the morning, only to see that number flash before your eyes a whole kilo heavier than yesterday! Within a split second, you go from super-motivated one moment to sad and defeated the next. That number is often enough to ruin your day before it has even had a chance to get started!

Before we get into talking about why you may gain (or lose) weight overnight, I want to first discuss what we mean when we refer to body ‘weight.’ Bodyweight can be divided into two distinct compartments, fat mass and fat-free mass. 

·      Fat mass is, you guessed it, fat.

· Fat-free mass is everything else that isn’t fat. This includes muscle, bone, organs, water etc. 

When we talk about bodyweight we are talking about fat mass + fat-free mass. 

So, when people say they want to ‘lose weight’ they are actually saying they want to lose fat mass. It is very important to understand this difference as weight loss can be achieved easily via manipulation of many variables, such as drinking less fluids or even cutting off a limb. Both of the previous examples would see you lose weight, but that’s not really what we are after, is it? 

You will often see this among athletes in weight class sports, for example boxing and powerlifting, where athletes will decrease fluid intake in the days and hours prior to competition in order to decrease their body weight. In this instance, athletes are not losing body fat, or muscle, they are losing a component of what makes up their fat-free mass (ie. Water) and thus this weight loss is ‘transient’ in nature, meaning the changes in their body mass are acute or short term and not the result of changes in tissue. We won’t go into the dangers of these practices if done incorrectly in this article, that is a topic for another time (read more about it HERE)

Now that we have established what we mean when we are referring to body weight, and thus scale weight, given our bodies are not machines and don’t stay the same weight forever, we can begin to appreciate the multitude of variables that can cause scale weight to fluctuate hourly, daily and weekly. 

Illustrating this we can view scale weight in the following way:

Scale weight = Actual weight + Weight-Impacting Variables

Unfortunately, scale weight doesn’t tell the whole story and “weight loss” isn’t always the best or only measure you should use to assess progress. There are numerous factors that may cause your scale weight to fluctuate from day-to-day. One that you will not see among these factors is “fat loss!” Noticeable fat loss doesn’t happen overnight. It happens over weeks and months, and can often occur in the absence of decreases in scale weight. Our current society has programmed us to believe that any drop on the scale is associated with fat loss and any weight gained is fat gain, which is not always the case. As you have learnt earlier, the scale is unable to distinguish between different body compartments. 

Understanding that weight loss does not always equal fat loss is important for anyone looking to use the scale regularly.  As you will come to learn, scale weight is susceptible to short term fluctuations, however with the correct education, you can reduce the “noise” of these fluctuations and differentiate between scale weight fluctuations by assessing the underlying trend of the data.

Before addressing such factors, it is important to acknowledge that daily weighing may not be for everyone. Understanding the appropriate weigh-in frequency for you or your clients is essential. Nonetheless, scale weight measures are an important self-monitoring tool that have been found to have strong associations with long term weight-loss success (1). That being said, scale weight measures are nothing more than an objective metric of body mass changes, and there are other forms of objective and subjective metrics that can be used, in conjunction with scale weight measures, to assess progress, such as waist circumference and girth measurements, progress photos and clothing. Using a variety of metrics that you or your client are comfortable with will not only help you assess your progress but also to influencing enjoyment, motivation and ultimately adherence long term. If you or your clients can see progress being achieved, whether this is from a body composition perspective or, in some instances, improvements to behavioural and lifestyle habits, motivation is likely to be sustained.

In this article, we will discuss the primary factors associated with daily weight fluctuations including carbohydrate intake, the menstrual cycle, training and muscle damage, food volume, hydration status, sodium, stress, meal timing and the time of weigh-ins. These factors produce acute, short-term scale weight fluctuations, which are perfectly normal and, in most cases, are independent of body fat gain (or loss).

Carbohydrate Intake

Variations in carbohydrate intake have a noticeable impact on how your scale weight fluctuates. Every gram of carbohydrate is accompanied by approximately 3-4g of water. Drastically reducing carbohydrates results in an initial, often rather large, weight drop due to primarily a loss in water weight. In one study comparing a mixed diet with low carbohydrate, high-fat diet, both of the same caloric content, participants on the mixed diet lost 2.8kg, while the participants on the low carb diet lost 4.6kg over the course of a 10-day period. Energy-nitrogen studies concluded the difference in weight loss was all accounted for by losses in total body water (2). 

This drastic weight loss is, I’m sorry to tell you, not due to your body suddenly becoming a ‘fat-burning machine.’ It is attributed to your body entering into a state of diet-induced diuresis. Firstly, carbohydrate restriction mobilizes glycogen (stored carbohydrate) stores in the liver and muscle. As mentioned, 1g of carbohydrate (glucose or glycogen) is mobilized with approximately 3g of water. The liver stores approximately 100g of glycogen, while muscle stores around 400g. Mobilizing these stores can see a weight loss of approximately 1kg (2). 

After a prolonged time spent in a very low or restricted carbohydrate state, your kidneys will also excrete salt, contributing to further water loss (we will talk about the impact of salt intake on weight shortly). This results in the generation of ketone bodies from the breakdown of dietary and endogenous (made within the body) fats. These ketone bodies are filtered by the kidneys, however, are not reabsorbed, therefore their presence in renal fluids increases sodium delivery, thus resulting in increased sodium and water loss through urination. 

Once glycogen stores have been liberated, and a new steady-state has been established for sodium, the rate of loss from low carbohydrate diets exhibits no greater promise than any other calorie-restricted diet. This is why people transitioning to low carb or ketogenic diets generally experience large weight drops in the initial days/weeks. Often this drastic weight loss will plateau off after a few weeks, and weight loss will continue at a normal, steadier rate (provided the person is in a calorie deficit). 

If we look at a real-world example of this, say an individual is consuming 300g of carbohydrates per day, then they decide to transition to a low carbohydrate diet, slashing their carbs to a mere 50g per day.  

300g carbohydrates come with ~ 3g of water per gram of carbohydrate.

300g (carbohydrates) x 3g (water) = 900g of water.

If you were to also factor in water loss from salt excretion, you’re looking at a loss on the scale of around 1-2kg (depending on the individual). This “weight loss” may also leave an individual feeling ‘leaner’ or ‘tighter.’ This loss is not fat loss, it is simply water loss from the initial carbohydrate reduction. As mentioned this initial loss will slow and a more normal rate of loss will continue after a few weeks. 

The opposite is also true if you increase your carbohydrate intake. If we use the example above, going from 50g of carbohydrates per day to 300g will see an increase of at least 900g of water plus additional water/salt.  Depending on how the lean the person is, the additional carbs may contribute to “filling them out” resulting in a leaner, tighter look. 

In some instances, certain people may experience a “softer” look after the first week of increased carbs, hence the reason it is important to allow sufficient time to assess changes after making a significant diet adjustment. Far too often people freak out at this point, claim carbs made them fat, rather than waiting out the initial fluctuation. The most plausible explanation for this is due to the transportation of glucose into the cell, which as we have established, draws water along with it, thus increasing total body water (2). Furthermore, in some people, the increase in insulin in response to higher carbohydrate intake may result in more sodium (salt) and water being reabsorbed by the kidneys, which promotes water retention (3). Ultimately in both of these situations, scale weight may fluctuate independently of changes in fat mass.

If your calories are accounted for, things will tend to level off after a few weeks. 

As a side note, this is why many people gain weight after a ‘cheat meal’ or night out. An increase in salt, water and overall food volume can result in scale weight fluctuations and variations to their physical appearance. 

Menstrual Cycle

A woman’s weight may be significantly impacted by hormonal fluctuations during her menstrual cycle. Every woman is different; however, scale weight may fluctuate anywhere from 0.5kg up to 4kg or more. In many cases, this can be attributed to large water fluctuations during her cycle. Such fluctuations may be attributed to several factors. 

  • Changes in sex hormone levels through the menstrual cycle may lead to water retention, particularly during the late luteal phase resulting in a subsequent weight increase independent of body fat changes. This is mainly explained by progesterone deficiency in the late luteal phase of the cycle (4). 
  • Changes in food intake, especially carbohydrate. One study showed an increase in total energy intake and carbohydrate in the luteal phase compared to the follicular phase. Such changes have been attributed to hormonal changes, in particular a reduction in estrogen which is linked to an increase in carbohydrate intake during this phase of the cycle (4).  
  • Changes to intestinal mobility. It is not uncommon for bowel habits to fluctuate markedly during the course of the menstrual cycle. Although not consistent among all studies, one study has suggested that small intestinal function may be more prolonged during the luteal phase of the cycle, therefore may contribute to scale weight fluctuations due to the presence of greater fecal volume (5). 

It is for these reasons comparing weights from week to week is often not the best approach as it does not take into consideration these fluctuations accompanying a woman’s cycle. It is for this reason, comparing weights month-to-month will provide a better indication of progress (ie. Week 1/Month 1 to Week 1/Month 2). 

Training & Muscle Damage

You’ve absolutely smashed your training, hit a tonne of PBs, but the scale increases and ruins your celebrations. Weight training and exercise causes muscle damage resulting in muscular inflammation in conjunction with water retention. This increase in water may result in acute weight gain and is not indicative of fat gain. When you exercise, cells are challenged to maintain their volume despite large fluctuations in metabolic demand and blood flow (6). During moderate and high-intensity exercise, water inside non-contractile skeletal muscle and other inactive tissues move out, while there is a net flux of fluid into contracting skeletal muscle, thus resulting in an increase in cell volume or swelling of contracting skeletal muscle (7). Such actions within the body have the potential to contribute to scale weight fluctuations.

Food Volume

Consuming a high volume of food (even when in a calorie deficit) increases the volume of food in your intestines and contributes to scale weight fluctuations, and may also mask fat-related weight loss. This is due to increased gastrointestinal contents. Furthermore, a higher than normal dietary fibre intake may also increase fluid in the intestines as it will draw water into the intestinal space and add bulk to the stools (8).

Bowel movements (or lack therefore) may also impact scale weight as too does urine output. It is for these reasons that it is important to standardize an individual’s daily self-weighing protocol to minimise the number of variables potentially impacting scale weight. For example, weighing first thing in the morning after voiding. 


Dehydration will cause scale weight loss due to less fluid in the body. Although variable between and within different population groups, water accounts for approximately 60% of the human body (9). And a litre of water is roughly equal to one kilogram on the scale. The consumption of less fluid not only adds less water to the body system, but it may also see the ‘unlocking’ of bound body fluids, which in turn will be excreted leading to additional losses (8). Increased respiration, urination and sweating will also lead to fluid losses from the body and thus a lower scale weight (if fluids are not adequately replaced). 

A drop on the scale in a dehydrated state or following a period of increased fluid loss is therefore not indicative of fat loss. In contrast to this, overhydration or an increase in hydration may result in a scale weight increase, however, as with dehydration the change in scale weight is not associated with changes in fat mass.

Sodium (Salt) Intake

Salt causes water to be retained within the body leading to an acute increase in scale weight. For example, if you are normally consuming a certain amount of salt per day and then one day you consume more than this, chances are the scale will show an increase. The opposite will occur if your salt intake is less than normal. The human body is a smart machine and tightly regulates the osmotic pressure (the pressure that must be applied to a solution to stop fluid movement) of body fluids via excretion and retention of electrolytes and fluid (10). This is consistent with the common health recommendation to lower sodium intake in order to lower blood volume, and therefore blood pressure. In one study, hypertensive subjects lost 1-2% of their body mass following a lower sodium diet (<500mg/day) for 5 consecutive days (8). Such results indicate a potential decrease in intravascular fluid retention resulting in scale weight changes. 

Increasing salt intake also promotes increased fluid intake in humans (11). As a result,  individuals may consume more fluids and therefore scale weight will likely increase due to more fluid in the body. Once again, these changes in scale weight are temporary and, with appropriate dietary changes, can be overcome. 


Chronic stress can cause changes to particular hormones in the body, such as cortisol. Both acute and chronic stress activates the hypothalamic-pituitary-adrenal (HPA) axis, and the corticotropin-releasing-hormone (CRH)-adrenocorticotropic hormone (ACTH)-cortisol cascade, which is synonymous with the concept of stress (12). Bodyweight fluctuations may occur when cortisol interacts with the aldosterone receptor. Aldosterone is a steroid hormone produced by the adrenal glands and is responsible for regulating salt and water in the body. A state of stress leads to ongoing activation of the sympathetic nervous system (SNS) or ‘fight-or-flight’ system, which causes the release of renin from the adrenal glands, thus activating the rennin-angiotensin-aldosterone system (12). This system plays a key role in maintaining salt and water homeostasis, and therefore blood volume, blood pressure and electrolyte balance. Any shift away from homeostasis will likely see shifts in fluid within the body. 

It is difficult to know whether this factor is in play, however good communication between coach and client will assist in identifying whether stress is an issue impacting scale weight. During aggressive or prolonged dieting phases, diet-induced stress will likely increase, thus scale weight fluctuations due to stress require consideration and appropriate management.

Timing of your last meal and weigh in

If you usually consume your last meal at 7pm, then one day you had to work late and your last meal wasn’t until 11pm, the scale may show an increase the next day due to a delay in the digestion process leaving a greater volume of food in your intestines compared to normal. As mentioned earlier, the timing of your weigh-in also may impact scale weight. 

If you usually weigh in at 6am every day, then one day you don’t weigh in until 9am, this could potentially make a difference. Ideally, you want to standardize the process of weighing in as much as possible to minimise the impact of such variables. 

At the end of the day, it is important to not be discouraged by overnight fluctuations in your scale weight. This is normal and does not reflect true fat loss progress. Focus on long-term weight trends, body measurements and progress pictures as a means to assess fat loss progress and make informed adjustments to your training and nutrition approach.


  1. Burke, Lora & Wang, Jing. (2011). Self-Monitoring in Weight Loss: A Systematic Review of the Literature. Journal of the American Dietetic Association. 111. 92-102. 10.1016/j.jada.2010.10.008. 
  2. Denke MA. Metabolic Effects of High-Protein, Low-Carbohydrate Diets. Am J Cardiol. 2001, 88 (1): 59-61.
  3. Osterberg KL, SE Pallardy, RJ Johnson, et al. Carbohydrate exerts a mild influence on fluid retention following exercise-induced dehydration. J Appl Physiol. 1985, 108 (2): 245-50.
  4. Kirchengast S. Changes in fat distribution (WHR) and body weight across the menstrual cycle. Coll Antropol. 2003, 26: 47-57.
  5. Bisdee JT, PJ Garlick, WP James. Metabolic changes during the menstrual cycle. Br J Nutr. 1989, 61 (3): 641-50. 
  6. Usher-Smith JA, CL Huang, JA Fraser. Control of cell volume in skeletal muscle. Biol Rev Camb Philos Soc. 2009, 84: 143-59
  7. King MA, LB Baker.Dehydration and exercise-induced muscle damage: implications for recovery. Sports Science Exchange. 2020, 29 (207): 1-7.
  8. Reale R, G Slater, LM Burke. Acute weight loss strategies for combat sports and applications to Olympic success. Int J Sports Physiol Perform. 2016, 12 (2).
  9. Sawka MN, SN Cheuvront, R Carter. Human Water Needs. Nutr. Rev. 2005, 63: S30-39.
  10. Reale R. Acute weight management in combat sports: pre weigh-in weight loss, post weigh-in recovery and competition nutrition strategies. Sports Science Exchange. 2018, 29 (183): 1-6.
  11. Rakova N, K Kitada, K Lerchl, et al. Increased salt consumption induces body water conservation and decreases fluid intake. J Clin Invest. 2017, 127 (5): 1932-43.
  12. Kubzansky LD, GK Adler. Aldosterone: A forgotten mediator of the relationship between psychological stress and heart disease. Neurosci Biobehav Rev. 2010, 34 (1): 80-6.

Your email address will not be published. Required fields are marked *

Send this to a friend