How Water Affects Your Weight

You don’t lose or gain fat overnight

…Not visibly anyway. The scales can be your best friend or your worst enemy. Is there anything more frustrating than stepping on the bathroom scales in the morning and seeing a stubborn number – or, god forbid, a higher number than yesterday?

And sometimes the opposite occurs. You might have eaten a huge, carb-filled meal the day before and you wake up lighter. What’s the deal with that?

It’s not your metabolism speeding up and chewing through more calories. It’s not a sudden drop in body fat, even if you look leaner. It’s water.

Water comprises a large percentage of bodyweight, as you probably already know. Something you might not be aware of is that water can easily move between compartments in the body. These compartments are necessary for metabolic function. They separate areas of differing pH, they isolate chemical reactions from each other, they store molecules where they’re needed. Sometimes the compartments are physical barriers like the cell wall and sometimes they’re chemical barriers that repel molecules using electrical properties, like the blood-brain barrier.

Water is able to traverse these barriers. The movement of water is called osmosis, and it’s important for diluting the concentration of chemicals (just like you might dilute a sugary drink by adding more water) that are separated into compartments by membranes like the cell wall. An example of these chemicals are the electrolytes (sodium, potassium, calcium) – these are molecules that play a role in nerve conduction, and they must be present in specific concentrations for optimal nervous system function.

How the body holds on to water

So water can move around the body – what does this have to do with your weight?

The body regulates how much water it holds. Sometimes water movement is driven by hormone signalling. Progesterone is a hormone that rises in the week or so before menstruation and can cause water retention. Sensors report electrolyte concentrations to the body, and the body either conserves water or jettisons it as a result. The physiological effects are feeling thirsty or needing to urinate.

This water-conservation process is signalled by a hormone called aldosterone, a type of mineralocorticoid. Another member of the mineralocorticoid family is our main stress hormone, cortisol. Cortisol rises in response to stress – whether it’s from training hard, financial pressure, poor sleep, extended periods of dieting and so on – and since it’s a close chemical relation to aldosterone, cortisol can interact with the aldosterone receptor. It’s like wearing a different style of shoe on the same foot – the shape is different, but the shoe still fits.

Here’s the sequence simplified: stress rises -> cortisol increases -> cortisol binds to the aldosterone receptor -> tells the body to retain water.

Since water obviously weighs something, this results in an increase in scale weight without any changes to muscle or fat tissue. And that’s not all. The extra water is mostly held in a specific compartment – subcutaneously, underneath the skin. This gives a softer, fluffier, blurrier appearance that reminds us of fat.

Stress and water weight

How can we intervene and stop this process?

Reducing stress is the answer. It’s important to realise that stress is not inherently bad. Stressors drive positive adaptations in the body and mind. We like feeling challenged mentally and we derive great benefit from the physical stress of exercise. It’s when stress piles up too high for too long that it becomes an issue, like a bucket filled to overflowing.

The scenario of training and dieting for weight loss represents a steady accumulation of stress over time. The brain interprets the physical work and lack of incoming energy as a threat to survival and responds by pumping out stress hormones to help the hunt for food. Providing more energy availability through food, then, is a tactic we can use to reduce the stress response.

Finally, we get to why having a big weekend of eating can sometimes result in a leaner look and friendlier scales. This has been called the ‘whoosh effect’ by fitness writer Lyle McDonald. Stress accumulates due to dieting, leading to activation of the aldosterone receptor and water retention beneath the skin. A day or two of more calories, fun with friends and good sleep leads to a drop in cortisol and a subsequent decrease in water retention, leaving us with lower bodyweight and a leaner look. If alcohol is enjoyed, dehydration could compound this effect even further.

If this sounds like a magical solution, bear in mind that these changes are short-term. Water balance in the body is tightly regulated, and stress-inducing activities like diet and exercise are necessary for actual fat loss.

Water, glycogen and weight gain

Sometimes a weekend of unrestricted eating can result in weight gain. The mechanisms are familiar by now – an increase in sodium means that the body must retain water to dilute the salt and ensure proper electrolyte concentrations, and this can make you heavier and bloated.

Sometimes water is not stored under the skin, but inside the muscles. Exercising muscles require a ready store of glucose to fuel muscle contraction, and this is provided in the form of glycogen. Glycogen has a relatively simple chemical structure: one gram of glucose is combined with three grams of water. In this way, both water and glucose are stored in the muscle compartment, which increases muscle size and weight.

Glycogen stores are influenced by the type of training (resistance training induces glucose uptake from the blood into the trained muscle) and the diet (dietary carbohydrate is broken down into glucose, which is then stored). Ceasing training will result in less need for glycogen depots in the muscle. Eating less carbohydrate will result in less glucose available to store as glycogen.

Why you should watch weekly weight trends

It’s important to understand that bodyweight can be readily manipulated, and that the scales are not the be-all of your weight loss journey. The volatile nature of bodyweight has important implications for weighing yourself. It means that a single measurement can’t be relied upon to see if you’re on track with your diet. Instead, daily weighing should be used, and the weekly average calculated to give a picture of the overall trend.

Once you understand how your bodyweight reacts to various situations, it can make weighing yourself much less harrowing. I expect after a weekend of high-sodium indulgence that my scale weight will be higher, and most often it is. I know that it will come down over the next few days, so there’s no need to do anything drastic in response to a short-term spike in scale weight.

Above is a chart depicting the weight loss trend of one of my clients. You'll notice how his weight swung up and down over the six week period, but the trend line show a steady weight decrease.

Finally, other measures can be used to double-check your progress. If you use several different methods to measure your body fat, you can triangulate a more precise result that you can be more confident in. For example, you could use scale weight in conjunction with photos and circumference measurements.

So, bear in mind that any short-term changes in body weight are almost certainly to do with water retention or loss. It’s no cause for alarm. We need to make sure we’re keeping an eye on longer term trends in scale weight, and given enough time we can learn how our weight responds to stressors and different foods.