Calculating Your Fluid Loss and Maintaining Hydration

If you want to know how your body is going to react to exercise, then you need to look at your AMR. This is your ‘Active Metabolic Rate’, which tells you just how many calories you burn during the day when you are engaging in what would be your ‘average’ level of activity. Your BMR meanwhile, tells you how many calories you burn even when stationary.

But it’s not just energy we burn during exercise, we also use fluids – and the same is true when we’re not active.

Understanding how much water our body burns through is not only interesting, it’s also very useful for knowing precisely how much we should be drinking on any given day and for making sure we stay adequately hydrated.

Insensible Loss

Inactive water loss is what the term ‘insensible’ water loss describes. That is the amount of water we lose when stationary and not urinating – it’s the water that we lose generally through the day through breathing, thermic processes etc. Daily loss should be around 500-800mls per day for adults. A more accurate calculation can be used by doctors that look at the surface area of the skin.

This means that even when you are entirely inactive and aren’t going to the toilet, you are losing water. So you need to rehydrate, even if you don’t feel that need necessarily or see any visible evidence of it.

Sensible Loss

Meanwhile, we also have our ‘sensible fluid loss’, which is water loss that we can actually see. On average, this is the combination of urine and fecal matter.

On average you will lose around 100-200ml each time you urinate – which is related with the amount you drink (and eat – as we get a lot of hydration from our food). The human bladder is able to hold up to 600ml of urine but in general, you feel the need to urinate once it reaches 150ml. On average, it is thought that an average adult will urinate six to eight times a day (although this number fluctuates quite drastically) but anything up to 10 times is considered normal and healthy, whereas some people will pee as little as two or three times.

Thus if you were to urinate seven times a day at 150ml, you would lose 1,050ml of fluid which is within the normal range of 800-2,000.

Feces meanwhile accounts for around 6% of our total sensible fluid loss from elimination. On average, this is usually around 100ml-200ml per day.

Women also lose an average of 50ml per day from vaginal secretions, whereas ejaculating will cost men 3.7ml of fluid.

Sweat

During exercise, we will lose around 0.8 to 1.4 liters of fluid through sweat per hour. However, there are again numerous contributing factors here which can affect this number. The temperature during your training will have a big impact on this for instance, as will the intensity of the training, as well as the fitness of the individual.

The best way to get a good idea of your fluid loss during exercise is to weigh yourself prior to and immediately following exercise. The amount of weight you have lost in this time can then be used to show how much fluid was lost.

And we don’t just sweat during normal activity – we also sweat throughout the day and this is thought to amount to about 1136.52ml.

Water Loss and Maintaining Fluid Balance

So if we combine all these factors, we then have the following fluid losses to consider:

• Secretions: 3-50ml
• Insensible: 500-800ml
• Urine: ~1,050ml
• Feces: ~150ml
• Sweat: ~1,136ml
• Exercise: ~1,200ml/hour

This then puts us at 3,036 liters of lost fluid per day and closer to 4,036 when we take exercise into account.

That’s then actually more than the recommended eight glasses that we are told to drink, which equates to roughly 2 liters or half a gallon. But of course the reason for this seemingly low recommendation is that we actually get a lot more fluid from our food. Food is meant to account for 20% of our water intake but for most of us, that will be way off.

And again, there are considerable differences here depending on the kind of food you eat as well as your own body type. Whereas non-starchy vegetables contain about 95% water and fresh fruits offer about 90% water, cheese or meat are much lower at around 60-70% water (though still possibly more than you would have thought!).

At room temperature, the weight of water is roughly equal to its volume, so that means you can roughly calculate how much water you’re taking in at any given time. For the average person, this will come to about 1,100ml of water.

Obviously other drinks count as well, so if you’re drinking Coca-Cola, tea or milk, then that will offer more fluids as well. The problem is that things like tea, coffee and soda drinks contain caffeine, which actually increases fluid loss by acting as a diuretic (making you pee more). Overall, you will still gain more than you lose but some of the benefit is mitigated. The same goes for alcoholic beverages.

So what have we learned from all this? Other than the sheer amount of fluid that comes in and out of our body on any given day, this reminds us just how important it is to stay hydrated and to keep drinking water. When you combine the food you eat, with the water you drink and the other fluids, you should be just about replenishing your fluids. But with that said, it’s still fairly close and if you add exercise into the mix, or if you have a particularly fast metabolism, then it might be close. So keep drinking! Why not go and get yourself a glass of something refreshing now?