Trying to lose weight can be frustrating, especially when the scale refuses to budge. Or even worse, if it goes up. Regardless of how often you weigh yourself, there is inevitably a point in most people's weight loss journeys where the scale number moves in an unpredictable direction. You're not quite sure why this happens, if you’re doing anything wrong, or if this is something you should even worry about.
Many people unfortunately overreact when they see scale weight jump up, and decide to make knee-jerk changes to their eating habits, training, or lifestyle, believing that this will remedy the issue. Well-meaning as these changes may be, they are usually not the best course of action. You may think that if you're trying to lose weight and the scale goes up, there must be something wrong with your diet, how you’re training, or something else that is impeding your success. I’m here to tell you, not necessarily.
The literature on this topic looks to be somewhat lacking. Despite the fact that daily scale fluctuations seem to be a significant source of stress for people attempting to lose weight, I found very few well-sourced articles that break down the science of what exactly drives these changes. Most articles on this topic contain a lot of anecdotal evidence, and few citations.
The goal of this article is to help you better understand why day-to-day weight fluctuations occur, so you can use this knowledge to empower yourself the next time you step on the scale. A scale is simply a tool to help you in your journey. The usefulness of this tool - like any tool - depends on if you know how to best utilize it to its utmost effectiveness.
Day-To-Day Weight Changes
Day-to-day weight fluctuations can be impacted by a variety of factors. There are some factors that have been espoused as having a larger effect than they actually do by many social media gurus (the gurus always lie). Let’s address these potential causes one-by-one.
1.) Sodium (Salt) Intake. The data on whether or not increasing your sodium intake on its own has a meaningful impact on daily weight fluctuations is surprisingly mixed. In fact, one study found that acute increases in sodium intake did not increase total body water retention, or body mass (1).
This stands contrary to the common narrative promoted by many health coaches and fitness trainers that increased sodium intake can make you “retain water”. On the surface this idea seems to make sense. If you eat a lot of high sodium foods, the sodium will draw water into your cells, causing you to hold onto water weight, and your weight may go up. Over the long-term, this certainly seems to be true. Over a two month span, lower sodium intake has been associated with a decrease in total body water weight (5).
However, the extent to which this has an effect on day-to-day weight changes is less apparent. What we do know is that acute sodium intake increases thirst (4). Even zero calorie drinks have weight, so an increase in fluid intake due to thirst would increase your weight temporarily.
What do I think? I think that managing and manipulating sodium intake in order to help weight loss is probably pointless. As long as you are not consuming excessive amounts of sodium (though there is still great debate over what constitutes “excessive" (2)), or have underlying conditions that can be negatively impacted by too much sodium, I probably wouldn’t micro manage this. And I definitely wouldn’t manipulate day-to-day sodium intake in order to try and get a more favorable number on the scale.
2.) Water / Fluid Intake. If you consume more water, your body weight goes up. Water does not have calories or nutrients, so it cannot contribute to body fat stores. But it still has weight. Water weighs about one kilogram per liter, or a little more than 8 pounds per gallon. Water intake can be highly variable from person to person, or even from day to day in the same individual (19).
These sporadic changes in water intake, therefore, can have a significant effect on daily weight fluctuations. However, this is not a bad thing. No one would ever seriously recommend that you decrease your water intake to lose weight. In fact, weight loss over a longer span tends to correlate well with higher water intake (6). It is therefore a complete waste of time, and probably hurtful for long-term weight-loss success, to attempt managing daily weight changes by lowering water intake.
3.) Food Volume. Food volume is usually used to describe the actual weight, or sometimes physical size, of the food that you eat. Foods that have fewer calories per pound of weight have a higher food volume. Eating foods that have higher food volume will help you lose weight in the long-term (3). However, the immediate aftermath of eating foods that weigh more is that your weight actually goes up.
Consider this. One pound of potato chips has roughly 2400 calories. One pound of broccoli has roughly 150 calories. You could eat three pounds of broccoli and still only have consumed a fraction of the calories in a single pound of potato chips. However, eating three pounds of broccoli would create an immediate increase in your body weight that is much larger than eating a single pound of potato chips. This doesn’t mean that broccoli is bad for weight loss; quite the opposite, actually. Consuming more food volume keeps you feeling fuller for longer, which can also be helpful for long-term weight loss success (3).
Foods usually have higher food volume because they contain more water, and less fat. Water doesn't have any calories. So, if your scale goes up tomorrow because you ate a lot of water heavy, low calorie foods (such as fruits & vegetables), that’s not a bad thing. That’s a good thing.
4.) Menstruation. There is conflicting data on exactly how the menstrual cycle affects weight fluctuations. One study shows that women’s self perceived water retention, or “bloating” during menstruation definitely exists, but it does not coincide with expectations set forth by other data (7). Other studies have shown that the phases of menstruation have a large effect on appetite and food cravings, possibly attributing the weight changes to increases in calorie intake (8). Another study that looks at this more in-depth concludes that while the menstrual cycle does have an impact on water retention, this effect is likely very small. The study also suggests that perhaps another cause for increases in water weight during menstruation is increased thirst (9), which often results in more fluid intake.
Regardless of whether or not these transient changes are caused by water retention, increased calorie intake, increased water intake, or all of the above, one thing is clear. The menstrual cycle can have an effect on daily weight increases. However, the degree to which this matters in the long-term is up for debate.
Having worked with many women on weight-loss goals, it is important to know that your weight is likely to shoot up during that time of month. But, my experience is that those who keep a level head and continue to consistently eat at a caloric deficit during this time will tend to experience a large weight drop at the end of this cycle.
5.) Stress. One idea that I hear thrown around quite a bit is the hypothesis that higher stress levels can cause weight to shoot up, even in the absence of any of the other factors we have discussed, or are about to discuss. There seems to be a correlation between stress and obesity (10). There also seems to be a link between water retention and stress levels. Stress has been shown to affect salt and water balance of the body (21). One study found that psychological stress can decrease sodium secretion, and also decrease urine volume (19). This would, logically, lead to acute increases in weight due to water.
Stress’s effect on daily weight fluctuations seems to be entirely water based. However, managing stress is an important part of a successful weight loss program. In fact, one randomized control trial found that incorporating a stress management program into your weight loss strategy can produce better results than not doing so (11).
Suffice it to say, stress management is an important contributor to overall health and wellness. One of the worst feelings in the world is when you have a particularly stressful day, and also manage to curb food cravings encountered due to this stress, all while maintaining a calorie deficit. You then weigh yourself the next morning and still see the scale go up. I’m here to assure you that this is okay, and does not mean you lost progress or gained fat. Science supports this. While temporary increases in stress are probably not a good thing, the associated weight increases caused by this are nothing to be alarmed about, unless the increased stress is chronically causing you to over consume calories.
6.) Training. Exercising, specifically intense bouts of weight training, are sometimes touted as a potential cause of transient weight fluctuations. Exercise on its own has not been shown to increase body weight. Quite the contrary. Exercising regularly has been shown to have a moderate effect on long-term weight loss (22). However, can an intense bout of exercise immediately increase your bodyweight?
I don’t think so, but it is not that cut and dry. Exercise increases thirst (12). It is likely that you will drink more water if you train longer and more intensely. This will cause your weight to go up in the short-term However, there is no evidence to show that exercise on its own will increase your weight, absent increased fluid intake and other factors.
7.) Supplements. Certain supplements, specifically creatine, have been shown to cause the body to retain fluid (13). If you begin supplementation of creatine, you will likely see a small immediate increase in body weight over the next several days due to this. Creatine is the only commonly taken supplement I could find that is shown to have this effect. Other supplements, such as caffeine, can have the inverse effect (23). Caffeine has actually been shown to increase urine production, causing the body to expel some extra water weight.
However, this is again, just water weight. Water weight fluctuations, unless present in very drastic amounts, are generally not harmful. I would not worry about creatine, or any other supplement’s effect on water retention. This is, of course, assuming that said supplements are consumed in reasonable quantities within published guidelines.
8.) Urine & Stool. Food can stay in your gastrointestinal tract for up to 95 hours (4 days) after you consume it (16). This can be a large contributor to your body’s weight increasing from one day to the next. The speed in which solids and liquids make their way through your gastrointestinal tract is variable. For example, different types of fiber can change digestive transit time (24).
Even if you are eating at a calorie deficit, it is possible for these food related short-term increases in digestive transit time to cause your weight to go up in the short term. But does this mean anything, and should you try to manage these changes?
Simply put - no. If you are consuming adequate fiber, and an otherwise health promoting diet, you needn’t worry. Trying to manage food volume in your digestive tract is a horrible waste of time.
9.) Carbohydrate Intake. The consumption of carbohydrates has a mild effect on fluid retention. In fact, one study found that fluid retention due to an increase in carbohydrate intake was significant after an intense bout of exercise (15). However, there are other factors to consider. If you are simply shifting calories from protein or fats over to carbohydrates, while maintaining the same level of calorie consumption, and training, I would expect the level of fluid retention from increased carbohydrate intake to be minimal. If you were to also change other factors we talked about, such as food volume and fluid intake, I would expect the change in body weight to be more pronounced.
There is also the big elephant in the room that we need to address. Something that increasing carbohydrate intake can also go hand-in-hand with, if you are not paying attention. Something that can increase your bodyweight both transiently and over time, if you are doing it wrong. I am talking about, of course...
10.) Calorie Intake. The largest contributor to your bodyweight is how many calories you consume (14). In fact, the majority of the scholarly sources I could find regarding weight fluctuations address calories directly. If your weight goes up, it is quite possibly because you have consumed excess calories that caused this to happen. Though this is not always the case, if your weight increases or is not budging for several weeks, then it is very likely that you have over consumed calories (17).
So what should you do about this? If you are having trouble losing weight, there are some resources I can offer you to help. Working one-on-one with a coach can help get you moving in the right direction, and is effective at helping long-term weight loss outcomes. I wish I knew someone who was a qualified coach? Oh right, I am.
You can also check out my Instagram and Tik Tok, where I provide frequent weight loss tips and help. Consider subscribing to the Jonny Reps Fitness newsletter (you get a free home workout program for doing so). Or, you can even book a free initial consultation with me directly through my website.
Why Does Any Of This Matter?
Many of these causes of daily weight fluctuations (with the exception of the last one) are things that you don’t really need to worry about controlling. If you keep eating at a calorie deficit, continue training, lifting, and moving, it is only a matter of time before the scale moves in the right direction. I always say, if you do enough things right, success isn’t just possible, it is inevitable.
What causes the largest weight drop from one day to the next will not be what leads to the best weight loss success outcome long-term. I hope this article has effectively illustrated that many of the habits that cause transient weight increases are actually side effects of adopting a healthy lifestyle.
Check back next week for part 2 of this article, where I discuss weighing frequency, helpful and healthy strategies for self weighing, and the mental health aspects of all this. See you then!
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