Most people think they want to lose weight, but actually, they want to lose body fat (not their precious lean muscle mass).
Getting fit and healthy is not just about the weight on the scale.
Rather than recording just your weight, it’s better to track your body composition to:
- ensure the changes you make are achieving the results you hoped for, and
- empower you with the data to fine-tune the process.
Bioimpedance scales make many nifty estimates of your body composition. But, unfortunately, these numbers often create more confusion than clarity.
This article addresses:
- the meaning of each measurement on your bioimpedance scale,
- the accuracy of bioimpedance scales,
- their strengths (and limitations), and
- how to actively manage the numbers if they are not moving in the desired direction.
- How do bioimpedance scales work?
- Are bioimpedance scales accurate?
- What is the most accurate way to measure body fat?
- What about DEXA?
- Athlete mode vs normal mode
- It’s the long-term trend that matters!
- Body weight
- How to change your body weight
- The pros and cons of tracking your weight
- Body fat percentage
- Fat-free body weight (lean mass)
- Body mass index (BMI)
- Body water
- Basal metabolic rate (BMR)
- Subcutaneous fat
- Visceral fat
- Muscle mass
- Skeletal muscle
- Bone mass
- Waist-to-height ratio
- Keeping it simple
- Further reading
How do bioimpedance scales work?
Bioimpedance scales run a weak electrical signal through your feet and around your body. The resistance (or impedance) generated by your body, combined with your weight, is used to ESTIMATE your body fat and several other valuable parameters.
Electrical current moves quickly through salty water (like your blood) while fat is less conductive. So, a higher level of resistance correlates with higher body fat.
Your muscle is like a sponge, filled with approximately 75% water. Meanwhile, fat contains about 10% water. So, if you are relatively lean, an electrical pulse will travel more quickly than if you have more body fat.
This short video explains bioimpedance analysis in more detail.
Calibrated with data from DEXA scans, your bioimpedance scale says, ‘You look like someone with a body fat of X.’ While the measurements given by your scale are only estimates based on correlations, they can still be helpful.
Are bioimpedance scales accurate?
No! Bioimpedance scales are not perfectly accurate.
But no method of measuring your body fat is perfectly accurate.
Every form of measurement has some level of error (or noise). But we can still use the data if it is reliable. While we’d love to get accurate and reliable data all the time (see the target in the top left of the graphic below), we can still use inaccurate but reliable data (target in the bottom left) to track the trends, especially if we can easily collect the data regularly.
What is the most accurate way to measure body fat?
The most accurate method to measure body fat is burn calorimetry. This method determines the calories and macros that appear on food nutrition labels.
When we burn a substance, we can determine its glucose, fat, protein, and water content based on the heat and gases produced. But most of us aren’t (or shouldn’t) be looking for that level of accuracy to determine our body fat.
Your weight, body fat, or any other measurement you make is an estimate that allows you to track changes and trends.
The data is not perfect, but it can help you make adjustments to ensure you continue to move toward your goal.
What about DEXA?
One of the most accurate ways to measure your body fat (while still alive) is a DEXA (Dual X-ray Absorptiometry) scan.
A DEXA scan measures the fat, water, and bone volume in different areas. This data allows us to train the algorithms used in the much cheaper bioimpedance scales.
But even DEXA is not perfectly accurate. There is still some variability between machines. Across the interwebs, you will see plenty of bodybuilding folks who get lean, fork out the money for a DEXA scan, and then don’t believe the results and compare themselves with others. “Bro, you’re not 8.2% body fat. You look much more like 6.1%. Don’t worry about it, bro. You look great just the way you are. Put down that pizza – it’s not that bad”.
The other downside of a DEXA scan is that it’s expensive. DEXA technology measures bone density in people who have osteoporosis. It’s unlikely most people will regularly go to a specialist clinic and spend a hundred dollars to track long-term trends.
That’s where bioimpedance scales come in; they function as DEXA-Lite for the masses. Once we accept that we can’t have perfect accuracy, we can get on with tracking the trend and changes in our body composition.
Athlete mode vs normal mode
Before you start tracking, you need to decide whether to use your scale’s “athletic” or “normal” mode. “Athlete mode” is calibrated with more muscular people. “Normal” mode is calibrated with most of the population who have less muscle and more fat.
The best way to decide which setting is right for you is to compare the number you see on your scale with what you see in the mirror vs what you see in the body fat comparison photos below.
You could also use the US Navy Body Fat Calculator (which tends to correlate well with DEXA) to estimate your body fat using several measurements on your body. The Navy Body Fat Calculator tends to be accurate but not as conveient to do on a regular basis.
Once you have selected the option that most closely resembles your mirror image, you can get on with tracking the trend. If you end up changing between athlete and normal mode, you can go back and delete the data recorded in the other mode to clean up the long term trend data.
Note: If you decide to switch between normal/athlete mode, you will need to delete measurements taken on the other mode. To do this in the Rhenpho app, go to trends, tap on the clock in the top right of the screen, select the day you want to delete, then click on the trash can.
It’s the long-term trend that matters!
The primary benefit of using a bioimpedance scale over other methods of measuring body composition (e.g., DEXA, bod pod, or underwater weighing) is that you can do it every day at home. In addition, regular measurements are reassuring that you’re on the right track over the long term.
Modern scales typically have a high degree of reliability, meaning you can step on them repeatedly and get the same reading.
The margin of error at different weights (according to the Renpho user manual) appears in the table below.
Most people will want to see the weight on the scale decrease (like the example below). However, underweight people who want to “bulk up” will want to see their weight trend up (ideally without excessive fat mass gain).
But the Holy Grail that everyone wants to achieve is improved body recomposition (i.e. losing fat while gaining muscle). Wise food choices and resistance training, especially at the start of your health journey, begin the process. But once the “newbie” gains are past and they have more training experience, most people find “periodising” their diet and training more beneficial (i.e. splitting time into blocks with definite periods either focusing on gaining strength or losing body fat).
How to change your body weight
The main factor that affects your weight over the long term is HOW MUCH you eat. However, changing your body composition is also heavily influenced by WHAT you eat; you must burn the fat and feed the muscle.
While energy is always conserved, it’s not as simple as managing calories in vs calories out. Due to our satiety response to different foods, it’s hard to change how much we eat without also changing what we eat. That’s why we encourage people trying to lose weight to prioritise nutrient-dense foods with a higher percentage of protein.
The pros and cons of tracking your weight
Some people can be intimidated by the number on the scale and use it to judge themselves. In addition, because we pay more attention to negative news, the short-term fluctuations can make them feel like a failure, especially if they are not making the progress they want.
Still, if you are trying to lose weight and get healthy, it’s best to weigh yourself daily and watch the long-term trend. Try not to over analyse the day-to-day fluctuations. Remember, you can’t force the numbers to go in the direction you want but, you can manage the inputs you know lead to the changes you want to see.
According to an analysis of data from the US National Weight Control Registry, regular weighing is one of the most common habits of people who have lost weight and kept it off. Seeing the trend in your weight can help you be more mindful of your eating and activity levels and act if required before things get out of hand.
Body fat percentage
Your body fat percentage is the fat to non-fat weight ratio on your body (i.e. lean mass).
- Body fat % = body fat / total weight
Your lean mass includes your muscle, organs (brain, heart, liver, etc.), stored water, glycogen, and bones.
- Body fat (in kg or lbs) = weight x body fat percentage.
- Lean mass = weight — fat mass.
Knowing your lean mass can help estimatehow much protein you need to consume. The more accurate calorie and protein calculators (including our Optimising Nutrition Macro Calculator) use lean mass (rather than total body weight).
The table below shows the ranges of body fat percentages for men and women.
You ideally want to see your body fat percentage going down over time, but this may not always be the case when you lose weight. What matters is that you lose more body fat than fat-free body weight (or lean mass).
The ‘secret’ here is to ensure you consume adequate protein during the weight loss process and ideally do some resistance training to tell your body that you need to keep the metabolically expensive muscle in a calorie deficit.
Fat-free body weight (lean mass)
If you are losing weight, you want to see your fat mass trending down over time.
Occasionally we see people in our Nutritional Optimisation Masterclass gain lean mass (fat-free mass) when they significantly increase their protein intake (even without making changes to their activity).
But the reality is, most people tend to see some decrease in their lean mass (or fat-free body weight) as they lose weight. So, the key here is to keep the loss of lean mass to a minimum while losing weight.
Preservation of lean mass can be achieved by feeding your muscle (with adequate protein) and resistance training (telling your body that your expensive to maintain lean mass is still required). It sounds simple. But it’s not easy to keep overall energy intake low enough to promote weight loss while also consuming sufficient protein to maintain precious lean mass. The more aggressive your energy deficit, the more you need to prioritise protein.
- slowly dial up your protein target if you are losing lean mass, and
- decrease your calorie target if you are not losing weight at the desired rate.
While we see some people able to lose weight at up to 2% per week over the short term, most people find a rate of 0.5% more sustainable. Maintaining a lower but consistent rate of weight loss is usually better than going all out and finding yourself in a rebound binge that undoes all your hard work.
As your calorie target comes down and your protein target increases, your protein % rises. Our satiety analysis shows that the percentage of protein in our diet has the strongest correlation with how much we eat.
Body mass index (BMI)
Your Body Mass Index (or BMI) is your weight (in kg) divided by your height (in metres) squared. BMI is a simplistic indicator of your metabolic health.
Because BMI is easy to measure, there is plenty of data available to show the relationship between BMI and many factors such as longevity and your risk of dying from any cause (e.g. heart disease, cancer, etc.).
However, as with most things in nature, optimal is not found at the extremes. A BMI of between 25 and 30 will give you the best chance of living a long and healthy life. Of course, you don’t want to be underweight, which leads to frailty, sexual/hormonal dysfunction, and lowered immunity. But the likelihood of being underweight in our current society with its excess of food availability is small, and most of us struggle instead with being overweight.
Because BMI doesn’t consider how muscular you are, it may have limited benefit, particularly if you don’t have an average amount of muscle mass. So instead, it’s your excess body fat that’s cause for concern.
Body water includes your blood, lymph, and extracellular fluid. Body water typically makes up 50 – 65% of the weight for men and 45 – 60% for women.
Your weight can fluctuate from day to day based on what you ate or drank, your activity levels, or when you last went to the toilet.
- A lot of resistance training can increase your weight due to temporary inflammation in your muscles, attracting more water.
- Consuming more electrolytes (sodium, potassium, magnesium, etc.) will cause you to hold more water.
- Lowering your carbohydrates will reduce the amount of water stored with the glycogen (i.e. stored glucose) in your liver and muscles.
Some weight-class athletes use techniques (e.g. sauna, hot baths, not drinking water, and avoiding electrolytes) to quickly shed water weight, but this is the opposite of what you need to do to be healthy and lose body fat over the long term.
Basal metabolic rate (BMR)
Once a bioimpedance scale has determined your weight and lean mass, it quickly calculates a theoretical basal metabolic (BMR). Your BMR is the amount of energy (in calories) that your body needs to function (not including exercise).
Your BMR is used to set your daily calorie target in food tracking apps like Cronometer. However, in the Nutritional Optimisation Masterclass, we prefer people track their current intake and make small changes based on their differences in weight, body fat, and lean mass. If you are losing weight, you don’t need to drop your calorie target. But if your weight loss slows, you can decrease your calorie target a little bit for the coming week. If you’re smaller, you will need less energy to maintain that weight.
If you are losing weight, you will likely see your calculated BMR decrease. However, to minimise this drop, it’s essential to prioritise adequate protein and some level of resistance training.
Subcutaneous fat is the fat stored beneath your skin, effectively on the outside of your body.
While it’s unsightly, and we’d prefer to be rid of it, subcutaneous fat is considered a healthy fat because it’s located where it’s meant to be.
We get into trouble when we overfill our subcutaneous fat (or exceed your Personal Fat Threshold), and it overflows into our visceral fat. Some people are “blessed” with the ability to store a lot of subcutaneous body fat before it floods more sensitive areas. Meanwhile, others with a lower Personal Fat Threshold often develop diabetes at a much lower BMI.
Once our subcutaneous fat runs out of storage space, any excess energy from our diet backs up into our bloodstream as elevated levels of free fatty acids, ketones, and glucose (i.e. Type 2 diabetes).
Keep in mind that these numbers are just estimates based on correlations. Once we get down to this level of granularity, these numbers should only be used to highlight areas of concern to manage your risk and monitor trends. If you are concerned about these number and want to know them accurately, you should get a DEXA scan.
Visceral fat is the fat stored inside your muscle wall and around the organs in your abdominal cavity. While you need a minimum amount of visceral fat to survive, an excessive amount of fat stored in your liver, heart, and muscles can be dangerous.
Some people who look lean carry a lot of fat around their organs, commonly known as TOFI (or thin on the outside, fat on the inside). If your visceral fat levels are high, you likely also have elevated blood sugars because your body is over-fuelled with energy (i.e. energy toxicity).
In our Data-Driven Fasting Challenge, we use pre-meal blood sugars to guide meal timing. If your blood glucose is high, it means your fuel tanks are still full, and you can wait a little longer before eating. This data tends to be super effective in targeting a reduction in visceral fat (which is only used for fuel once you have depleted the excess glucose in our bloodstream and liver).
If your blood sugars are elevated, you’ll likely benefit from dialling back your dietary carbohydrates, particulalry added fructose (e.g. soft drinks and food sweetened with high fructose corn syrup which is converted to fat in the liver). But high levels of visceral fat also correspond with an overall high levels of body fat, so dialling back dietary fat will also be helpful.
Don’t go too hard too early. Most people experience optimal long-term results when they progressively dial back their carbs (to stabilise blood sugars to healthy levels) AND lower dietary fat to burn the fat off their bodies, not their plates.
Aggressive crash dieting leads to rebound binging and feelings of failure. Once progress slows, you can make further tweaks. Dieting usually gets challenging, but there’s no need to make it harder than it needs to be too early.
The muscle mass shown on your bioimpedance scales represents the total weight of your muscle, including skeletal muscle, your heart, your smooth muscle and the water contained in them. You ideally want to see this trending upwards over time (as shown in the example below).
It’s your muscle that is metabolically active and uses energy. If you lose muscle mass, your metabolic rate slows, and you will have to cut your energy intake to prevent weight gain continually. Muscle mass tends to decline with age, so resistance training and consuming adequate protein are investments in your future metabolic health.
The chart below shows my current muscle mass, along with fat-free body weight (or lean mass). Over the past three years or so, I’ve been working consistently (with progressive overload resistance training and adequate protein in my diet) to build strength. My lean mass is 82.5 kg. Once subtracting the mass of my bones (4.2 kg), we have 78.4 kg of total muscle mass (including my heart, liver and the water contained in them). 52.9% of my muscle mass is attached to my skeleton. This is what is used to move my body and lift weights.
In the chart below from Karen, you can see when she started CrossFit and building muscle, her muscle mass slowly trended up.
But the reality is, when you build muscle, it’s hard not to add some additional body fat. Over time, you can optimise this process by prioritising protein with just enough energy (from fat and carbs). However, it’s natural that we crave more food to recover from intense workouts. The ‘secret’ is to dial back the energy from fat and carbs to the point that you feel recovered but aren’t gaining excess body fat (i.e. a dirty bulk).
I initially developed the Data-Driven Fasting system to help me lose some extra fluff after an intense strength building phase. Using pre-meal blood sugars to ensure a negative energy balance, we can reduce the excess glucose and fat in our body without the hassle of tracking calories.
Skeletal muscle is the component of your muscle attached to your bones and involved in moving your body and lifting heavy things (i.e. excluding your organs like your heart). If you are consuming adequate protein and doing resistance training, you want to see your skeletal muscle mass increasing.
Your bone tissue consists of bone minerals (calcium, phosphorus, etc) and bone matrix (collagen, fibre etc). Like muscle, consuming adequate minerals and protein and resistance exercise is crucial to maintaining a healthy bone mass.
Keep in mind that these numbers on your bioimpedance scales are ESTIMATES, not direct measurements so, you shouldn’t expect a high degree of accuracy. They are just interesting to track the trend.
Bioimpedance scales like the Renpho also estimate the amount of protein in your body.
- Protein = fat-free mass — water — minerals — bone mass
Bioimpedance scales give you a handy dashboard to highlight areas that you may need to work on. They can also show you your trends and where you are improving. This positive feedback ensures people stick with the process for the longer term.
While not directly measured by the scale, the apps that come with bioimpedance scales typically allow you to track your waist-to-height ratio and other measurements.
We encourage people to track their waist weekly using Nutrient Optimiser and the Data-Driven Fasting app. Your waist to height ratio is one of the most powerful indicators of your metabolic health. Even more so than BMI, your waist to height ratio is an excellent indicator of your risk of dying of any cause. A waist-to-height ratio of around 0.5 means you are in good shape.
Keeping it simple
While some people love all the data, others find it overwhelming or too much hassle.
If you wanted to keep it simple, you could use your “skinny jeans”, wedding ring or old belt as an indicator. If you find you need to add new holes in your belt, you know you’re on the right track and vice versa. If you find your wedding ring is slipping off easily, you’re probably losing fat in all the right places.
A bioimpedance scale can be a handy way to track your metabolic markers at home. But the numbers you see are estimates that allow you to manage the long term trend and make adjustments to continue to move towards optimal.
- Data-Driven Fasting: How to Lose Weight and Reverse Type 2 Diabetes Without Tracking Your Food (The COMPLETE Guide)
- Why set and forget calorie and macro targets will fail you
- The satiety index (updated)
- Protein for weight loss: How much you need and why it works
- Nutrient Optimiser
- Data-Driven Fasting Challenge
- Nutritional Optimisation Masterclass