Our body weight fluctuates daily even with consistent nutrition, tracking and eating the same macros every day.
Long term changes in body weight are mainly driven by losses in body fat (or muscle), however, short term changes - i.e. day to day fluctuations, are more likely driven by changes in body water and food volume brought about by acute changes in liquid intake and food volume variation. There are numerous factors that influence body weight and how it fluctuates, unfortunately, weight fluctuations that do not fit the goal trend have been shown to consistently decrease motivation in individuals that are dieting (Spreckley et al., 2021). These weight fluctuations can be enough to cause an individual to break adherence to a diet or even stop a diet altogether.
Knowing this, weighing athletes daily to get a weekly average can be very useful to assess diet effectiveness. Weekly averages are more likely to even out the daily fluctuations for a more accurate representation of the overall trend of body weight as well as indicate any actual changes in body mass.
SO WHY DOES BODY WEIGHT FLUCTUATE?
Assuming macros are kept the same, there are a few reasons why body weight fluctuates:
Food is mass. We eat food every day and more often than not, food choices vary. This fluctuation in the literal mass of food we eat each day will mean that our weight fluctuates day to day due to the volume of undigested food we have in our GI tracts from the day before (if we’re weighing in upon wake).
Given the amount of water we are made of, a large amount of our body weight is dictated by fluid dynamics. If we’re retaining more water or have drank more water, knowing 1ml water = 1g, a litre of water can theoretically increase body weight by 1kg if we haven’t urinated since drinking it. Whilst sodium is vital, more than normal can cause individuals to retain more water than they regularly do - leading to a temporary increase in body weight through water retention.
You can expect slight fluctuations in body weight depending on the type of exercise you perform the day prior. E.g. typical powerlifting training can cause a slight increase in body weight due to the micro tears created in muscles due to resistance training. The process of repair and recovery involves retention of water/an inflammatory response and can cause a slight increase in body weight because of this.
On the flip side, resistance training can deplete glycogen stores. Therefore, if training volume is increased by a notable amount and carbohydrate intake kept the same, body weight can decrease due to depleted glycogen stores. This weight will increase with water and carbohydrate intake to replenish glycogen stores.
Most uterus owners notice some degree of bloating from fluid retention immediately before and during menstruation. Studies have shown that fluid retention peaks on the first day of menstrual flow. It’s lowest during the mid-follicular period (the middle phase of the cycle) and then gradually increases over the eleven days surrounding ovulation.
HOW TO ADAPT TO WEIGHT FLUCTUATIONS
Whilst we can’t entirely control all aspects of our lives that influence acute body weight fluctuations, we can definitely collect data in a way that allows us to more effectively interpret the overall trends in bodyweight.
Weigh yourself daily. Daily readings give you more data points to create an average from and reduces the skew from a possible ‘high’ or ‘low’ day.
Weigh yourself under the same conditions. Ideally, upon wake, after going to the bathroom. This consistency will make it easier to obtain accurate readings of bodyweight.
Remember, weight loss and weight gain are almost always not linear! If we can expect acute fluctuations in body weight we’re better equipped to embrace them and continue with a diet whether the goal is weight gain or weight loss.
Spreckley, M., Seidell, J. and Halberstadt, J., 2021. Perspectives into the experience of successful, substantial long-term weight-loss maintenance: a systematic review. International Journal of Qualitative Studies on Health and Well-being, 16(1), p.1862481.