We often associate a lower BMI with ideal weight and good overall health. In many instances, this is true. However, overall health does not continue to improve as BMI continues to decrease. Instead, BMI is associated with overall health in a U-shaped curve, where BMIs that are either too high or too low are associated with decreased longevity.
A quick overview of BMI
Your body mass index (BMI) is calculated by dividing your weight in kilograms by height in meters squared. Maintaining a healthy BMI is important in establishing long-term health.1 That’s why, here at InsideTracker, we use your BMI as part of our InnerAge calculation. InnerAge provides you with a personalized roadmap to wellness. BMI is not a perfect indicator of health since it does not assess body fat percentage. We recognize that people with high levels of muscle mass, like athletes and weight lifters, may have higher BMIs, without high body fat. However, BMI is the most research-supported weight metric associated with long-term health. Achieving an optimal BMI one component of reaching your optimal health, but an optimal BMI does not always mean a lower BMI.
Understanding the U-shaped curve
Research has shown that there is an optimal range of BMI values associated with the greatest longevity.1,2,3,4 BMI values above and below this zone are associated with decreased longevity. At InsideTracker, we calculate your optimal BMI range based on your personal characteristics. While it is largely understood that a high BMI can be associated with decreased longevity, it is less well known that a low BMI can pose similar risks. Researchers speculate that lower BMI can be associated with decreased longevity, since individuals suffering from other health challenges may be more vulnerable to becoming underweight and vice versa.1
Individuals with low BMIs are at risk of decreasing their lifespans by 1 to 9 years, while individuals with high BMIs are at risk of decreasing their lifespans by 9 to 13 years.2 Although high BMIs can decrease an individual’s lifespan more significantly than low BMIs, the risks associated with low BMIs should not be ignored. This is why you may see that your low BMI is negatively affecting your InnerAge. No, it’s not a mistake! A low BMI may be working to speed up your biological clock, but we’re here to help.
What to do if your BMI is on the low end
To increase your BMI in a healthy way, work on gaining muscle. Optimizing certain biomarkers is critical to gaining lean muscle. Testosterone is an anabolic hormone that builds muscle and improves strength. Low levels of testosterone can decrease lean muscle mass. SHBG (sex hormone binding globulin) binds to free testosterone, affecting its bioavailability. Maintaining an optimal SHBG level is important to ensure healthy testosterone levels.
Cortisol is a catabolic hormone that acts to break down muscle. Chronically elevated cortisol can decrease muscle mass. Keeping your stress (physical and psychological) under control can help you build muscle. Getting 7-8 hours of quality sleep per night can help you maintain optimal cortisol levels and maximize muscle tissue growth and repair.9
Although optimizing these biomarkers is essential to gaining muscle, there are also non-biomarker related recommendations to help you meet this goal.
How to gain muscle
Your diet and exercise routine are both keys to building muscle. Adequate protein intakes, ranging from 1.4 to 1.8 g/kg of body weight have been demonstrated as optimal for building lean muscle mass and maintaining muscle mass in highly active individuals. Your protein need is calculated by dividing your weight in pounds by 2.2 to get your weight in kilograms. Then, weight in kilograms is multiplied by 1.4 for minimum protein intake, and 1.8 for maximum intake. Your daily protein intake should fall in between these two numbers.5,6,7
Adding resistance training on 3 non-consecutive days per week can also help you gain muscle. General guidelines suggest performing 1 to 3 sets of 8 to 10 exercises with 8 to 12 repetitions to start. Exercises can be performed with free weight, body weight, or machines. Include one exercise for each major muscle group: back, shoulders, biceps, triceps, abdomen, quadriceps, and hamstrings.8
Maintaining lean body mass is also important for long-term health. Engaging in regular physical activity can help you maintain your weight. Try to develop a routine and stick to it! Muscle-strengthening exercises can help you improve and maintain muscle mass, which will support your strength, mobility, flexibility, and balance - all of which are important for engaging in activities of daily living as you age. Keeping up your activity regimen independently is key to maintaining your weight over time. Maintaining a normal body weight can also help decrease your resting heart rate. A lower resting heart rate has been associated with increased longevity.10
Following these recommendations will help you gain muscle and increase your body weight in a healthy way. Remember, when it comes to BMI, there’s a sweet spot. Lower is not always better. Find out if your BMI is affecting your biological age.
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 Jørgensen, Terese Sara Høj, et al. “The U-Shaped Association of Body Mass Index with Mortality: Influence of the Traits Height, Intelligence, and Education.” Obesity, vol. 24, no. 10, 2016, pp. 2240–2247., doi:10.1002/oby.21615.
 Fontaine, Kevin R., et al. “Years of Life Lost Due to Obesity.” Jama, vol. 289, no. 2, 2003, p. 187., doi:10.1001/jama.289.2.187.
 Song, X, et al. “Relationship between Body Mass Index and Mortality among Europeans.” European Journal of Clinical Nutrition, vol. 66, no. 2, 2011, pp. 156–165., doi:10.1038/ejcn.2011.145.
 Wang, Z., et al. “Exploring the Non-Linear Association between BMI and Mortality in Adults with and without Diabetes: the US National Health Interview Survey.” Diabetic Medicine, vol. 33, no. 12, 2016, pp. 1691–1699., doi:10.1111/dme.13111.
 Campbell, Bill, et al. “International Society of Sports Nutrition Position Stand: Protein and Exercise.” Journal of the International Society of Sports Nutrition, vol. 4, no. 1, 2007, p. 8., doi:10.1186/1550-2783-4-8.
 Thomas, D. Travis, et al. “Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance.” Journal of the Academy of Nutrition and Dietetics, vol. 116, no. 3, 2016, pp. 501–528., doi:10.1016/j.jand.2015.12.006. Wilson, Jacob, and Gabriel J Wilson. “Contemporary Issues in Protein Requirements and Consumption for Resistance Trained Athletes.” Journal of the International Society of Sports Nutrition, vol. 3, no. 1, 2006, p. 7., doi:10.1186/1550-2783-3-1-7.
 American College of Sports Medicine Information on Resistance Training for Health and Fitness
 Dattilo M, Antunes HK, Medeiros A, Mônico Neto M, Souza HS, Tufik S, de Mello MT. “Sleep and muscle recovery: endocrinological and molecular basis for a new and promising hypothesis.” Medical Hypothesis. 2011 Aug;77(2):220-2.
 Boudoulas, Konstantinos Dean, et al. “Heart Rate, Life Expectancy and the Cardiovascular System: Therapeutic Considerations.” Cardiology, vol. 132, no. 4, 2015, pp. 199–212., doi:10.1159/000435947.