QUESTIONS / ANSWERS

Fat Mass

Masse musculaire squelettiquee

Phase Angle (PhA) and Impedance Ratio (IR)

Osteoporosis / Osteopenia

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«I encountered an interpretation problem with the Biody Xpert which indicated to me that the person did not have enough body fat.»

Biody Xpert is a class IIa medical device. Literature suggests that it is recommended for men not to fall below 7% body fat and for women not to fall below 12%. This is why we have set a red indicator when these limits are crossed. That being said, we know that in certain sports disciplines, some athletes temporarily go below this threshold. In such cases, it is up to the practitioner, depending on the individual and the discipline, to adjust the reference value.

«I added more than 10 kg to the person just to modify the parameters, and the software results were great. However, in reality, if this person had 10 kg more, they would be overweight. What do you think?»

The measurement results depend on the impedances, weight, height, and sex. If you add weight to the person while using the same impedances, the result will necessarily be incorrect. It is impossible to provide an accurate interpretation in this case because if the person had 10 kg more, their impedances would inevitably be different.
Conclusion: The result you obtained is purely hypothetical and therefore not applicable.

«I’m confused by the body fat results for one of my patients. She’s in the orange zone with only 1.5 kg above the reference value, while other heavier patients with the same 1.5 kg of excess body fat are in the light green zone.»

This is logical. The reference values for body fat are calculated as a percentage of the person’s total weight because 1 kg of excess fat in a person weighing 63 kg cannot be compared to 1 kg in a person weighing 80 kg.
Let’s take a simple example to illustrate:
Your patient has 1.5 kg of excess body fat, which represents 2.38% excess body fat (excess body fat weight [1.5 kg] divided by total weight [63 kg]). Now let’s consider a person weighing 80 kg who also has 1.5 kg of excess body fat; this will represent only 1.87% excess body fat. Since the buffer zone (light green) between green and orange is 2%, in this case, the person weighing 63 kg will fall into the orange zone, while the person weighing 80 kg will be in the light green zone.

«I don’t quite understand the difference between raw body fat mass and body fat mass at constant hydration.»

These are two metrics, each with its own relevance.

Raw body fat mass refers to the weight in kilograms of all lipids (including phospholipids, which are essential lipids). This is a critical metric for longitudinally tracking the evolution of the three main compartments that make up a person’s weight:

Water
Fat-free dry mass (proteins and minerals)
Fat mass
The raw body fat percentage is also a crucial metric, but it can be misleading. When weight changes due to fluctuations in water content, the fat percentage will increase or decrease even though the actual fat mass in kilograms remains unchanged.
Example: For a person weighing 75 kg with 15 kg of fat mass, their fat percentage is calculated as (15/75*100) = 20%. If their hydration increases by 2 liters, their total weight will go from 75 to 77 kg, and their fat percentage will decrease to (15/77) = 19.48%. Conversely, if they lose 2 liters of water, their total weight will drop to 73 kg, and their fat percentage will increase to (15/73) = 19.48%.
This phenomenon is observed, to our knowledge, across all bioimpedance devices: if I drink water, my body fat percentage decreases, and if I lose water, my body fat percentage increases.

Body fat percentage at constant hydration: To avoid this misleading phenomenon, we recalculate the person’s weight based on an ideal hydration rate of 73.3% of fat-free mass.
Example: If this person is overhydrated, we recalculate their weight at the ideal hydration level—e.g., 75 kg in this case. Thus, regardless of variations in water content, if the fat mass in kilograms does not change, the body fat percentage will also remain constant.

«I am sending you the results of a patient from this morning. I did not dare to show him the results. Even though I know I shouldn’t focus solely on BMI, it’s unsettling when someone is -5 kg under in terms of fat mass. I’d appreciate your feedback on this measurement.»

Firstly, the information we provide about the quantity of fat mass, whether excess or deficit, does not mean that a deficit necessarily indicates a lack. A low fat mass is generally a good sign.

Secondly, the tolerance level for reference values increases with age. Your patient is a 77-year-old man weighing 70 kg. The fat mass level not to exceed for someone his age is 26%. If he were 20 years old, the limit would be 15%. His 5 kg of fat mass below the reference corresponds to -7% fat mass. His fat mass percentage is therefore 19%, which is excellent. If he were 20 years old, this would represent +4% fat mass, corresponding to 2.8 kg of excess fat mass.
Our approach:
We use the following method, which provides results close to those of Skinfold tables:

A percentage that gradually evolves from age 15 to 70:

Minimum not to fall below for men: 7%
Minimum not to fall below for women: 12%
Maximum not to exceed for men by age: At 15 years old: 15%. At 70 years old: 26% (fixed beyond 70 years).
Maximum not to exceed for women by age: At 15 years old: 20%. At 70 years old: 31% (fixed beyond 70 years).
The Skinfold tables seem excessive for women, which is why we capped the maximum at 31% at 70 years old.

Why does the maximum tolerance increase with age?

A slightly elevated fat mass does not pose significant risks. The real risks for diabetes, cardiovascular issues, etc., are associated with visceral fats. For younger individuals, the fat mass percentage is kept stricter (15%) because statistical studies show that elevated fat mass in youth is a significant predictor of long-term obesity risk.

This tolerance level gradually increases as the risk diminishes with age, provided the threshold is not exceeded. For older individuals, being slightly overweight is not a major concern; their comfort is prioritized over encouraging restrictive diets, which are often inappropriate and unsustainable. However, there is still a maximum threshold that should not be exceeded.

Observation:
Regardless of age, if the fat mass percentage is high, it is strongly recommended to measure the waist circumference or the waist-to-hip ratio. These are indicators of potential visceral fat presence since subcutaneous fat poses little to no risk, whereas, as mentioned earlier, visceral fats can lead to cardiovascular risks.

Regarding your client:
Based on the measurement results, for his age, weight, and height (77 years old, 170 cm, 70 kg), he is in perfect condition. His fat mass is low and within the green zone, which is excellent. However, despite having a low fat mass for his age, if his waist circumference exceeds 102 cm or if his risk index (height divided by waist circumference) is above 0.50, this should raise concern. Always consider taking these measurements when in doubt. They can be analyzed under the “Patient Risks” and “Metabolic Risks” indicators.

« Our patients would like to know their target weight. Could you add this function to the software?»

We do not provide this feature for several reasons:

Defining reference criteria is complex. A criterion based on the reference fat mass not to exceed could be provided, but this is not necessarily the most appropriate measure. Depending on the patient, the target may need to be slightly lower or higher. A criterion based on muscle mass is possible if the patient is lacking muscle. However, if they have a high muscle mass, it becomes much more delicate to define a target. A criterion based on hydration level is also possible, but in that case, you can simply refer to the “theoretical discrepancies” section to see the quantity of water excess or deficit and adjust the weight accordingly. However, be cautious, as other components (e.g., fat, muscle) may fluctuate, making this new target weight misleading.

All body components can change. Water, muscle mass, and fat mass can fluctuate in either direction. The target weight would depend on too many factors. For instance, if fat-free mass decreases while fat mass remains stable, the target might technically be reached, but it would not be a desirable outcome.

Regarding fat mass, the fat mass percentage is a snapshot at a given moment and represents a percentage of the total weight. If the total weight decreases, the fat percentage will increase.

Conclusion: Defining a fixed target weight is not feasible due to the variability of body composition factors. It’s better to assess progress through dynamic metrics rather than aiming for a static weight goal.

Let’s take the example of this person weighing 72.5 kg with 35% body fat, which equals 25.5 kg of fat mass. The reference value (target we want to reach) is 22%. The fat mass loss required to reach the target is calculated as (72.5 × 0.35) – (72.5 × 0.22), or 25.5 kg – 15.95 kg = 9.45 kg.

At this precise moment, they have 9.45 kg of excess fat. Now, let’s assume they lose these 9.45 kg of fat. What happens then? Their new weight would be 72.5 kg – 9.45 kg = 63.05 kg.

At this point, they should have reached the reference value for their age, which is 22%. Let’s verify: their reference fat mass at that point would be 63.05 kg × 0.22 = 13.87 kg.

We notice that their reference fat mass for 72.5 kg was 15.95 kg, but for 63.05 kg, it’s only 13.87 kg, showing a discrepancy of 2 kg.

This illustrates the three main reasons why it’s impossible to set an exact target weight. However, you can take a simpler approach: start by looking at the amount of fat mass to lose, found in the “Theoretical Discrepancies” section under “Fat Mass.” Adjust progressively based on new measurements and their evolution.

Feedback from the professional:
Thank you very much for this very precise explanation. I tried calculating it, but it is indeed impossible. I will base my approach on the theoretical discrepancies.

« Is muscle mass gain with water increase and little lean mass possible? I have a client who has gained kilograms of muscle mass, but little lean mass and a lot of water. »

Yes, of course, it’s possible. Indeed, that’s the whole point of lean mass, excluding fat, which allows us to determine whether the gain or loss of skeletal muscle mass is due to an increase or decrease in protein or water.

« I started a weightlifting challenge with clients, weightlifting at 11 a.m. with a protein meal and leucine right after. I would like to be able to quantify muscle gain well. Can you advise me on how to proceed? »

Muscle mass gain depends on two factors: water and protein fluctuations. If you want to perform a qualitative analysis, we advise you to cross-reference the data contained in

General overview: by comparing the evolution of total water and the evolution of lean mass excluding fat, it’s quick and allows you to see if the person is losing or gaining protein or water:
History of the Metabolic Protein Indicator (in kilograms) and History of the Total Water Indicator (taking the result in liters). To further refine the analysis, consult the history of Active Cellular Mass.
For even more precision: History of the water balance excluding fat. Indeed, muscles are composed of intra- and extracellular water, so it’s interesting to check if water fluctuations are due to intra-, extracellular, or both.

« Until what age are the reference measurements correct for the Biody Xpert? People aged 70 to 80 would be interested, but I don’t know if the results will be accurate… Thank you for your feedback. »

Apart from fat mass, there are no specific age-related tolerances. We use criteria for good physical fitness. It’s up to the professional to consider that, for example, if an 80-year-old person has slightly low muscle mass, it’s likely related to age. To illustrate, let’s take 3 examples:

An 80-year-old individual has muscle mass 10% higher than the reference, while another individual has muscle mass 10% below the reference. One could consider that the first individual, given their age, has retained a very good potential. As for the second individual, we will take into account their age and adjust their result, knowing that at their age, it will be difficult or even impossible to regain muscle mass. Consequently, we will focus on trying to maintain their remaining potential through nutrition and reasonable exercises.
The non-fat hydration rate is sometimes, but only sometimes, slightly higher in individuals aged 75 and above. We will take this into account and be slightly more tolerant by a point or 2 on the higher end of the tolerance range, but still with monitoring.
Until the age of 60, if skeletal muscle mass in the limbs is 10% below the reference, it’s really concerning and efforts should be made to regain muscle mass. However, if a 75-year-old person has skeletal muscle mass in the limbs of -10%, we will take a different approach.

The cutoffs for skeletal muscle mass and skeletal muscle mass in the limbs that we use are derived from publications, and to our knowledge, there are currently no serious publications on specific cutoffs by age group.

« Here are the two anonymized measurements of a patient with glioblastoma multiforme. I would like to understand the science behind the analyses, especially the phase angle, the frequency ratio… So, a few key scientific references would be appreciated. »

Indeed, your patient has a low phase angle and a high impedance ratio, which seems logical. I recommend reading these two publications: “Can phase angle determined by bioelectrical impedance analysis assess nutritional risk?” A comparison between healthy and hospitalized subjects Ursula G. Kyle a,b,c, Esther P. Soundar a,d, Laurence Genton b,e, Claude Pichard a,* Phase angle and impedance ratio: Two specular ways to analyze body composition. Emanuele Rinninella₁*; Marco Cintoni₁; Giovanni Addolorato₂; Silvia Triarico₃; Antonio Ruggiero₃; Alessia Perna₄; Gabriella Silvestri₄; Antonio Gasbarrini₂; Maria Cristina Mele₁

« I can no longer weigh my patient; he is too weak to get on the scale, so I enter his weight from the previous measurement. Does this have an impact on the phase angle and impedance ratio? »

The measurement results are accurate, but the interpretation of body compartments depends in part on weight. In this case, it is necessary to adjust the result since it takes weight into account.
On the other hand, the results of impedance and phase measurements are raw values that do not take into account weight or height…
So, we can track the progression of the disease in this type of patient just with the phase (PhA) and the IR (Z200/Z5), even if we cannot weigh them. That’s also one of the advantages of raw data.

« Thank you for the response. I’m not sure what IR corresponds to? »

The IR (impedance ratio) is obtained by dividing the impedance value at Z200 by the impedance value at Z5. A decrease in the Phase coupled with an increase in the IR is generally significant of a deterioration in health status. A high IR can reflect water retention, edema, but more commonly inflammation. A normal IR for a man is between 0.73 and 0.78. The first cutoff is at 0.80/0.81, and greater vigilance is required above 0.82/0.83. 0.73 is quite exceptional and generally corresponds to high-level athletes in very good physical condition. A normal IR for a woman is between 0.75 and 0.79. The first cutoff is at 0.81/0.82, and greater vigilance is required above 0.83/0.84. Observation:

Above 65 years for women and 70 for men, it tends to generally increase slightly progressively with age.
Very elderly individuals can go up to 0.90.

For the moment, we do not have an explanation to provide for the increase in IR from a certain age, but our measurement records on individuals aged 85 to 100 show that almost all measured individuals have low phase angles (below the cutoffs) and high IRs (well above the cutoffs).

« Can I rely on Biody Xpert results to detect signs of osteopenia and/or osteoporosis and monitor the evolution of bone mass in my patients? »

Regarding osteoporosis, Biody Xpert does not claim to offer the same level of certainty as dual-energy X-ray absorptiometry (DXA). In Bioimpedance, we do not have a 3D representation like DXA that allows us to determine bone density. We can estimate bone mineral content, but not bone density. Osteoporosis is often very localized. We provide a global result, not a result for a specific area. However, we do provide an indication of osteopenia risk:

In 80% of cases, our results are close to or very close to DXA, making it an excellent screening tool, quick and cost-effective.
In case of suspected osteopenia based on Biody Xpert results, it is advisable to validate the result with DXA, which is the Gold Standard in this field.
Another advantage of Biody Xpert is in patient monitoring in the case of osteopenia confirmed by DXA, as it allows for spacing out DXA checks.

« Here is one of my clients. His weight is 185 kg, his height is 191 cm, his body fat mass is 45%, and his hydration level is very low. Is this possible? »

You must have entered female instead of male. Your device is of a multi-algorithm type and takes into account the gender for both measurement interpretation and reference values. You can change it in Biody-Manager (Patient Manager, in front of the patient “click modify” (small pencil), then in personal information modify the gender). I simulated what this would look like with algorithms adapted to male. The result is now believable; the hydration level falls within the tolerance range. NB: For reference: In Biody Manager, in the top left corner, you have a figure representing a man or a woman.