Catching flu will probably result in you being bed-bound for a few days with a headache, cough, nasal congestion, and a temperature.
Unfortunately, almost everyone experiences a bad case of common cold or flu every year and are struck down with these common symptoms.
This can be very unpleasant, and just to add insult to injury, this can sometimes lead to weight gain. Our bodies respond to infection by triggering various internal processes to fight off illness and recover, which can also affect how we feel - and our weight.
However, no need to fret! There is an explanation for this, and rest assured it’s rarely a result of fat gain.
So, why is there sometimes a jump on the scales upon recovering from flu? Is this anything to worry about? All will be discussed in this article!
Key Takeaways
- Flu can cause temporary scale changes from water shifts, inflammation and lower activity—not true fat gain.
- Focus on recovery: fluids, electrolytes, rest and easy nutrition until appetite and energy return.
- Resume training gradually; avoid “catch-up” extremes that prolong fatigue.
- Weight stabilizes as inflammation and hydration normalize post-illness.
- Preventive habits—sleep, micronutrients, hand hygiene—reduce downtime.
The Dreaded Seasonal Flu and Common Cold
Flu will often get better on its own, though it does require some self-care strategies to assist with the recovery process!
Some symptoms you may experience if you have a bad cold or flu include:
- A high temperature
- Muscle aches
- Feeling exhausted
- Sore throat and dry cough
- Loss of appetite
- Difficulty sleeping
- GI issues. (1)
The majority of the time, we are advised to rest, drink plenty of fluids, keep warm, and take over-the-counter medication to help ease the symptoms. If you are taking prescription drugs, consult a healthcare professional before combining them with over-the-counter medication to avoid potential interactions.
Additionally, ensuring you get enough protein in your diet can support your recovery and help your body manage the effects of illness.
The Normality of Weight Fluctuations
Before answering the main question here - does having flu make you gain weight? - it’s important to distinguish the difference between weight gain and fat gain. Body fat percentage is a key measure for assessing health, as it provides more insight than weight alone.
It’s important to note that when talking about weight gain, we are solely referring to weight. While our fat mass makes up part of our weight, weight gain and fat gain are still different in many ways.
Fat gain simply refers to a gain in fat mass. This occurs when an individual consumes more energy (food) than what they expend over a consistent period, meaning their caloric intake exceeds their energy expenditure.
On the other hand, weight gain simply refers to a gain in weight. While this can occur when we gain fat, it can also occur due to many other factors, such as changes in muscle mass, which can increase weight even if body fat remains the same. (2)
Some factors that cause weight fluctuations include:
- Medication
- Menstrual cycle
- Bowel movements
- Carbohydrate intake
- Salt intake
- Exercise
- Illness. (3)
As most people experience, our weight is sensitive. If you weigh yourself in the morning and again in the evening, the number on the scales will be different. It may even change from hour to hour.
The above factors can cause weight fluctuations mostly due to changes in water weight.
Fluid Retention: The Hidden Culprit Behind Sudden Weight Changes
Have you ever noticed a sudden jump in your body weight, especially when you’re feeling under the weather?
One of the most common reasons for this is fluid retention, sometimes called water retention or oedema. This happens when your body holds onto extra water, leading to noticeable weight changes that can feel like unexplained weight gain overnight. (4)
Fluid retention can be triggered by a variety of factors. Hormonal changes, certain medications (like those for high blood pressure), and medical conditions can all lead to your body retaining more water than usual.
Even your eating habits, like consuming too much salt or not drinking enough water, can cause your body to hold onto fluids.
For example, increasing your salt intake over a couple of days - a common instinct when you're unwell and craving salty fatty snacks - may increase your weight as salt causes your body to retain water.
Water weighs enough to impact the scale, so even being well-hydrated or dehydrated can make a noticeable difference.
It’s important to remember that this type of weight gain is not the same as gaining fat mass. While fat gain happens when you consistently eat more calories than you burn, fluid retention is a temporary shift in your body’s water balance.
It doesn’t mean you’ve gained extra pounds of fat, and it often resolves once your body returns to its normal routine.
Other Reasons for Weight Fluctuations
Stress, poor sleep, and changes in hormone levels can also play a role in weight gain. Metabolism slows with age or certain illnesses, which can make it easier to gain weight even if your habits haven’t changed.
What you ate recently and your overall diet can also influence weight changes, as different foods and eating patterns affect water retention, metabolism, and body composition.
This is why it’s important to take the number on the scales with a pinch of salt!
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Does Having Flu Cause Weight Gain?
As we have established, many factors can cause weight fluctuations, such as...
A Change in Routine
If you think about the recovery strategies you adopt when you become ill, you are essentially changing your usual routine. Illness often leads to a more sedentary lifestyle or sedentary lifestyle during recovery, which can contribute to gaining weight or make it harder to lose weight and focus on losing fat. (5)
A Change in Appetite
Your appetite is off and you’re consuming different foods. You’re not as active due to tiredness. You may even be taking medications, or be suffering from GI issues such as diarrhea. Illness can also affect your ability to manage your weight, and may affect blood sugar levels, especially if you have underlying medical conditions like insulin resistance.
Poor Sleep
Interrupted sleep is common during illness or flu, which can have a knock-on effect on your hormones, disrupting the ones that regulate appetite and increasing your cravings for sugary and high-fat foods. It can also increase your levels of cortisol, the stress hormone, which can also increase your demand for high calorie foods.
A 2022 Harvard study says that snacking late at night slows our metabolism, makes you hungrier in the daytime, and increases body fat. (6)
Fewer than seven hours of sleep a night is associated with weight gain and obesity. (7)
All of these minor lifestyle and nutrition changes can have a big impact on your weight. Illness often leads to temporary weight changes, and some weight may be lost as you recover and return to your usual habits.
What’s more important to remember is that this is temporary! Once you get back into your normal routine, eating patterns, and activity habits, your weight will return to normal. If you’re seeing progress in other ways, such as feeling better or regaining strength, you’re on the right track even if your weight fluctuates.
The Bottom Line
Many of us are struck down with cold and flu symptoms or stomach flu every year, which can put a real strain on our daily life for up to a week or two.
When you’re unwell, your body goes into recovery mode. This is where you may be making small nutrition and lifestyle changes to speed up and support the recovery process. Back those up with Performance Lab Immune.
As a result of these changes, you may notice changes in your weight, too.
This is very rarely fat gain and normally nothing to worry about. Once you return to your normal routine and eating habits, your weight will likely go back to normal. However, some people may not lose the weight as quickly if there is an underlying issue.
If you experience sudden weight gain, persistent weight changes or you are concerned about your weight fluctuations due to illness, especially if accompanied by other symptoms, it could be a sign of an underlying medical condition.
In such cases, it is important to consult a healthcare provider for personalized advice and proper evaluation.
References
- Centers for Disease Control and Prevention. (n.d.). Signs and symptoms of flu. U.S. Department of Health & Human Services. https://www.cdc.gov/flu/signs-symptoms/index.html
- Cleveland Clinic. (2023, June 27). The difference between muscle weight vs. fat weight. https://health.clevelandclinic.org/muscle-vs-fat-weight
- Cleveland Clinic. (2022, October 20). 7 possible reasons why your weight fluctuates. https://health.clevelandclinic.org/weight-fluctuations
- Better Health Channel. (2023, March). Fluid retention (oedema). Victorian State Government. https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/Fluid-retention-oedema
- Park, J. H., Moon, J. H., Kim, H. J., Kong, M. H., & Oh, Y. H. (2020). Sedentary lifestyle: Overview of updated evidence of potential health risks. Korean Journal of Family Medicine, 41(6), 365–373. https://doi.org/10.4082/kjfm.20.0165
- Harvard Health Publishing. (2023, February 15). Hidden causes of weight gain. Harvard Medical School. https://www.health.harvard.edu/staying-healthy/hidden-causes-of-weight-gain
- Bacaro, V., Ballesio, A., Cerolini, S., Vacca, M., Poggiogalle, E., Donini, L. M., Lucidi, F., & Lombardo, C. (2020). Sleep duration and obesity in adulthood: An updated systematic review and meta-analysis. Obesity Research & Clinical Practice, 14(4), 301–309. https://doi.org/10.1016/j.orcp.2020.03.004
