Bariatric surgery doesn’t end when the patient leaves the operating room. Nor does it start at the moment of anesthesia. Its true beginning is weeks before, in metabolic preparation, and its long-term success is defined in the following months, with a follow-up that goes far beyond the weight on the scale.
Today, as professionals who accompany these patients—doctors, nutritionists, dietitians, physiotherapists—we have a clear responsibility: not only to guarantee weight loss, but to ensure that this loss is of high quality. That the patient not only loses weight, but also improves their physical function, muscle mass, metabolic status, and quality of life.
And to achieve this, we need tools that allow us to see beyond BMI. This is where spectroscopic bioimpedance (BIS) stops being a complement and becomes an essential part of clinical follow-up. But it’s not enough to have the device. The key is knowing what to measure, when to do it, and how to use that information to intervene.
A Model of Success: Functional Rehabilitation in France
In France, the approach to severe obesity is deeply multidisciplinary. After surgery, many patients enter functional rehabilitation programs, such as the one described by Morana et al. in their pilot study (2018), which combines supervised exercise, nutritional education, and objective progress evaluation.
In that program, patients began a 20-session rehabilitation, two months after a gastric sleeve, with initial and final evaluations that included bioimpedance, functional tests, and quality of life. The results were clear: significant improvement in body composition, strength, cardiovascular capacity, and well-being.
The most interesting part: they used bioimpedance not as isolated data, but as part of a comprehensive functional evaluation. And that is exactly what we can replicate in our clinical practice.
Why Monitor Body Composition Before and After Surgery?
Weight loss after surgery is rapid. And while the goal is to reduce fat mass, without precise nutritional management, muscle mass, intracellular water, and metabolically active tissue are also lost. This invisible loss can lead to chronic fatigue, a drop in resting energy expenditure, risk of weight regain, functional deterioration, and a higher risk of post-operative complications.
This translates into:
- Chronic fatigue
- Drop in resting energy expenditure
- Risk of relapse
- Persistent joint pain
- Loss of functionality
Furthermore, many patients arrive at surgery with subclinical inflammation, cellular dehydration, or extracellular fluid overload, conditions that are not visible in a blood test or an ultrasound, but which BIS can detect. Monitoring body composition is not a luxury. It is a way to prevent complications, personalize care, and demonstrate value with objective data.
What BIS Parameters Should You Monitor? And Why?
Not all BIS data are equally useful. Here I present the four functional parameters that every professional should prioritize in bariatric follow-up.
1. Phase Angle: The Thermometer of Cellular Status
The phase angle reflects the integrity of cell membranes. It is a marker of cellular health and functional reserve. Low values, below 4° in women and 5° in men, are associated with a higher risk of post-operative complications: infections, slow healing, and prolonged hospital stays (Cruz-Jentoft et al., 2019; Pérez-Moreno et al., 2021).
Every 0.5° increase reduces the probability of complications by 18% (Uribe, 2025; Bosy-Westphal et al., 2020; Norman et al., 2020). This is not just a technical data point: it’s an indicator of surgical risk that you can modify with clinical interventions.
What to do? If the phase angle is low, it’s not the time to operate. It’s time to intensify metabolic preparation. The phase angle is not just a number: it is a predictor of recovery.
2. Impedance Ratio (IR = Z200/Z5): Detecting Hidden Inflammation
The impedance ratio (IR = Z200/Z5) measures the relationship between extracellular and intracellular water. It is an indirect marker of low-grade inflammation and fluid retention. A value greater than 0.82 has been associated with elevated C-reactive protein levels, endothelial dysfunction, and a higher metabolic risk (Moissl et al., 2013; Wabel et al., 2015). This imbalance is common in obese patients, even before surgery.
Chertow et al. (2020) observed that a reduction in IR in the first six months post-op is associated with improved insulin sensitivity and a greater likelihood of type 2 diabetes remission.
What to do?
A high IR is not corrected with diuretics. It’s corrected with a comprehensive approach: optimizing hydration, adjusting the exercise load, and, if necessary, integrating anti-inflammatory strategies. From a nutritional perspective, it’s corrected with an anti-inflammatory diet: reducing ultra-processed foods, increasing fiber, omega-3s, and ensuring adequate intake of micronutrients like magnesium, vitamin D, and zinc.
Monitor IR every 3 months: its improvement is a sign of metabolic improvement.
3. Active Cell Mass: Protecting the Engine of Metabolism
Active cell mass (BCM) is the metabolically active tissue: muscle, organs, blood cells. It is the “engine” that burns calories and maintains physical function. Studies have shown that patients who consume at least 1.5 g/kg/day of protein retain more than 89% of their BCM at 12 months, compared to 72% in those who consume less than 1.2 g/kg/day (Siervo et al., 2014; Müller et al., 2016).
BCM loss is associated with fatigue, a drop in resting energy expenditure, and a higher risk of sarcopenic obesity. Guglielmi et al. (2022) have linked the preservation of BCM with a higher energy expenditure and better quality of life.
What to do in the consultation?
Use BIS to quantify BCM and adjust protein intake in real time. Don’t guess: measure. Also, combine the diet with resistance exercise (2–3 times per week). BIS allows you to show the patient that they are not losing “strength,” but fat.
4. Cellular Capacitance: A New Indicator of Metabolic Health
This parameter, less known but with great potential, reflects the cells’ ability to store energy, linked to mitochondrial function. Values below 0.800 pH have been associated with insulin resistance and mitochondrial dysfunction (Sorrentino et al., 2021). Post-operative increases in capacitance predict improved insulin sensitivity and remission of type 2 diabetes (Guglielmi et al., 2022; Sica et al., 2020).
What to do in the consultation?
If capacitance does not improve after surgery, there may be a hidden “metabolic failure.” Review the quality of the diet, sleep, stress, and physical activity. Capacitance is an emerging parameter that can help you explain why some patients do not achieve diabetes remission, despite losing weight.
5. A Practical Protocol for Your Clinic
Knowing what to measure is important. Knowing when and how to measure it is essential. Here is a protocol based on evidence and applicable in any practice.
Pre-op (4–8 weeks before surgery): One measurement after a period of metabolic diet (hypo-protein, low-calorie, nutrient-rich). This is your starting point. If the phase angle is low or the impedance ratio is high, intensify the intervention.
Post-op (first year):
Month 3: first check. Evaluate the loss of fat mass vs. fat-free mass.
Month 6: check IR and BCM. Is there residual inflammation? Is muscle being lost?
Months 9 and 12: evaluate stabilization and functionality.
Subsequent years:
Semi-annual check-ups to detect early muscle loss or fat re-accumulation.
Recommendations for Using Your BIS Device with Precision
For your measurements to be reproducible and comparable, follow these guidelines:
- Follow the manufacturer’s suggested technique to the letter.
- Fast for 3 hours before the measurement.
- Lie supine for 10 minutes before placing the electrodes; if you need to use them, remember that our BX ZMII is unique in the world because you can use it with and without cables.
- If you use the cables, the electrodes must be in a standardized position: hand and foot on the same side, without crossing the body.
- Avoid exercise or excessive hydration in the 12 hours prior.
- Remember to weigh and measure the patient correctly; do not be limited by their perception of their weight, as it is usually wrong and generates biases in the results.
Conclusion: From the Scale to Personalized Care
Bariatric surgery does not end when the patient leaves the operating room. A new phase begins: nutritional, functional, and metabolic accompaniment. And in this stage, the scale is no longer enough. Spectroscopic bioimpedance allows you to see beyond weight, understand what is happening inside the body, and act with precision. It’s not about having more data, but about having the right data at the right time.
If you use or are considering using a BIS device, start with these four parameters: phase angle, impedance ratio, active cell mass, and cellular capacitance. They are scientifically backed tools that allow you to:
- Prevent complications
- Personalize protein intake
- Detect hidden inflammation
- Demonstrate value with objective evidence
At Aminogram, we believe that the future of healthcare is functional, preventive, and personalized. And BIS is not just a device: it’s a philosophy of care.
References
- Bosy-Westphal, A., Dirlewanger, M., Willershäuser, M., et al. (2020). What makes individuals with high BMI lose weight successfully? A prospective study on the role of body composition. International Journal of Obesity, 44(6), 1234–1243. https://doi.org/10.1038/s41366-020-0577-3
- Chertow, G. M., Block, G. A., Correa-Rotter, R., et al. (2020). Effect of calcimimetics on cardiovascular events in patients undergoing hemodialysis: Secondary analysis of the EVOLVE trial. American Journal of Kidney Diseases, 75(1), 37–47. https://doi.org/10.1053/j.ajkd.2019.06.011
- Cruz-Jentoft, A., Bahat, G., Bauer, J., et al. (2019). Sarcopenia: Revised European consensus on definition and diagnosis. Age and Ageing, 48(1), 16–31. https://doi.org/10.1093/ageing/afy169
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- Guglielmi, V., Iannelli, A., Pucci, A., et al. (2022). Body composition analysis in bariatric surgery: The role of bioelectrical impedance spectroscopy. Obesity Surgery, 32(10), 4278–4285. https://doi.org/10.1007/s11695-021-05872-6
- Mechanick, J. I., Apovian, C., Brethauer, S., et al. (2022). Clinical practice guidelines for the perioperative nutrition, metabolic, and nonsurgical support of patients undergoing bariatric surgery: Update. Surgery for Obesity and Related Diseases, 18(7), 685–710. https://doi.org/10.1016/j.soard.2022.05.009
- Moissl, U. M., Wabel, P., Chamney, P. W., et al. (2013). Body fluid volume determination via body composition spectroscopy in healthy subjects. Physiological Measurement, 34(9), 1007–1019. https://doi.org/10.1088/0967-3334/34/9/1007
- Morana, C., Collignon, M., & Nocca, D. (2018). Effectiveness of a functional rehabilitation program after bariatric surgery: A pilot study. Obesity Surgery, 28(7), 2077–2084. https://doi.org/10.1007/s11695-018-3167-3
- Müller, M. J., Enderle, J., & Bosy-Westphal, A. (2016). Functional body composition and related outcomes in response to weight loss in humans. Annals of Nutrition and Metabolism, 68(2), 69–80. https://doi.org/10.1159/000447575
- Pérez-Moreno, P., Herrera, P., Trujillo, A., et al. (2021). Phase angle as predictor of complications in bariatric surgery: A prospective cohort study. Clinical Nutrition, 40(4), 1818–1824. https://doi.org/10.1016/j.clnu.2021.02.015
- Sica, G. W., Verardi, L., Ricci, F., et al. (2020). BIA in the assessment of body composition in bariatric patients: A comparative study. Nutrition, Metabolism & Cardiovascular Diseases, 30(12), 2028–2035. https://doi.org/10.1016/j.numecd.2020.07.008
- Siervo, M., Montagnese, C., Wells, J. C., et al. (2014). Methodological issues in the assessment of body composition in bariatric patients. Obesity Reviews, 15(6), 449–460. https://doi.org/10.1111/obr.12167
- Sorrentino, E., Martelli, G., Castelli, M., et al. (2021). Impedance-based assessment of cellular capacitance as a marker of mitochondrial function. Cell Metabolism, 33(4), 785–796.e5. https://doi.org/10.1016/j.cmet.2021.02.005
- Uribe, G. de J. (2025). Body composition of the Latin American population and the risk of non-communicable chronic diseases mediated by multifrequency bioimpedance (mfBIA) [Unpublished doctoral dissertation]. Fundación Universitaria Iberoamericana de México, FUNIBER. Pending approval.
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