Body Mass Index

Nutrition_orange

What we eat, digest and absorb impacts our body mass index, which in turn can impact our hormonal system, our neurotransmitters, our levels of inflammation, and therefore our mental health.Rachubińska, K., Mińko, A., Rotter, I., Sołek-Pastuszka, J., Ustianowski, P., Skonieczna-Żydecka, K. and Grochans, E., 2024. The association between obesity, chronic inflammation, metabolic disorders and mood disorders among patients up to 12 months after hospitalization for SARS-CoV-2. Diagnostics. Panda, S.S., Nayak, A., Shah, S. and Aich, P., 2023. A systematic review on the association between obesity and mood disorders and the role of gut microbiota. Metabolites.

Your body mass index (BMI) has an impact on your mental health. Not just because of how you might feel about yourself and your body image, but also it has a knock-on biochemical effect. Chen, S., Zhang, H., Gao, M., Machado, D.B., Jin, H., Scherer, N., Sun, W., Sha, F., Smythe, T., Ford, T.J. and Kuper, H., 2024. Dose-dependent association between body mass index and mental health and changes over time. JAMA Psychiatry. Himmerich, H. and Mirzaei, K., 2024. Body image, nutrition, and mental health. Nutrients.

You can calculate your BMI using a tool provided by the National Institute of Health here. BMI under 18.5 is considered low; between 18.5 and 24.9 is considered normal; 25 to 29.9 is considered overweight; above 30 is considered obese.

Being overweight or obese results from an imbalance between food intake and energy expenditure which leads to an excessive accumulation of fat.

Being overweight or obese has been shown to negatively impact brain health, and people with high fat stores tend to be at greater risk of mental health issues.Rachubińska, K. et al., 2024. The association between obesity, chronic inflammation, metabolic disorders and mood disorders among patients up to 12 months after hospitalization for SARS-CoV-2. Diagnostics.

High BMI correlates with:

Reduced cognitive function

  • Being overweight or obese correlates with less brain mass and lower brain activity, which impacts cognitive health and doubles the risk of Alzheimer’s Morys, F., Potvin, O., Zeighami, Y., Vogel, J., Lamontagne-Caron, R., Duchesne, S. and Dagher, A., 2023. Obesity-associated neurodegeneration pattern mimics Alzheimer’s disease in an observational cohort study. Journal of Alzheimer’s Disease.

Depression

  • Being overweight or obese doubles the risk of depression Chen, D. et al., 2024. Obesity and risk of depressive disorder in children and adolescents: a meta-analysis of observational studies. Child: Care, Health and Development.

Inflammation

  • Fat cells produce inflammatory chemicals and store toxic chemicals in the body which can further increase inflammation which can lead to mental health issues Gómez-Ambrosi, J., 2023. Adipose tissue inflammation. Cells.

Hormonal imbalance

  • A 5% reduction in weight can normalize your hormone levels Gottfried, S. (2013). The Hormone Cure. New York, NY: Scribner, p. 226.
  • High body fat can impact stress hormones
    • Abdominal fat, and generally higher total body fat, correlates with increased levels of the stress hormone cortisol which is released by fat cells
    • Obesity dysregulates the HSD enzyme in fat tissue, which leads to chronically high cortisol levels in fat cells
    • Equally, fat tissues, the liver and the brain show the highest levels of HSD activity, which may explain why chronic stress and cortisol overexposure are so detrimental for obesity (adipose fat tissue), diabetes (the liver) and memory (the brain)
    • High cortisol levels correlate with anxiety and depressionSeckl, J.R., 2024. 11β-hydroxysteroid dehydrogenase and the brain: not (yet) lost in translation. Journal of Internal Medicine.
  • High body fat can impact sex hormones
    • In women, high fat stores, especially abdominal, can increase estrogen and testosterone. Obesity can result in anovulation (lack of ovulation) due to high male hormones, which results in reduced progesterone Pasquali, R., 2006. Obesity and androgens: facts and perspectives. Fertility and Sterility.
    • In men, high fat stores can reduce testosterone and increase estrogenOkobi, O.E., Khoury, P., De la Vega, R.J., Figueroa, R.S., Desai, D., Mangiliman, B.D.A., Vera Colon, O.L., Urruela-Barrios, R.J., Abdussalam, A.K., Diaz-Miret, M. and Hernandez Borges, S., 2024. Impact of weight loss on testosterone levels: a review of BMI and testosterone. Cureus.
  • High body fat can impact the metabolic hormone insulin
    • High body fat can cause insulin resistance, which in turn can cause blood sugar imbalances
    • Obesity is closely associated with increased risk of metabolic diseases such as insulin resistance. Insulin resistance is a major predictor for the development of type 2 diabetes
    • Obesity induced insulin resistance is caused by a number of factors: high body fat causes the secretion of inflammatory markers called “cytokines”. Inflammation contributes to insulin resistanceDonath, M.Y. and Drucker, D.J., 2025. Obesity, diabetes, and inflammation: pathophysiology and clinical implications. Immunity.

Being underweight can also be disruptive to mental health.Chen, S. et al., 2024. Dose-dependent association between body mass index and mental health and changes over time. JAMA Psychiatry.

Low BMI can:

Affect your hormones

  • Many of our key hormones are stored in body fat, and made from fat. Lowering fat stores can negatively impact our hormonal balance
  • Women who suffer from anorexia nervosa or bulimia, or high performance athletes can suffer from imbalanced hormones due to low BMI. Anorexia affects many hormonal systems in the body:
    • Causes an increase in growth hormones in an attempt to maintain blood glucose by increasing fat breakdown
    • Causes increased cortisol
    • Correlated with lower insulin and glucose levels
    • Causes a decrease in estrogen and other sex hormones Fricke, C. and Voderholzer, U., 2023. Endocrinology of underweight and anorexia nervosa. Nutrients.

Lower your sex hormones

  • Such as estrogen (especially estradiol) and progesterone
  • When a woman’s body fat is lower than 21% of total body mass, the hormonal control centres in her brain keep her from making enough estrogen to ovulate or to build up her uterine wall Gottfried, S. (2013). The Hormone Cure. New York, NY: Scribner, p. 185.
  • This can result in the dysregulation of menstrual periods as seen in patients with anorexia nervosaHoskens, L., Sevenants, L., Van Insberghe, M. and Casteels, K., 2026. Therapeutic approaches for endocrine dysregulation in anorexia nervosa in adolescents. Frontiers in Pediatrics. Golden, N.H. and Carlson, J.L., 2022. Update on the medical management of eating disorders in adolescents. Journal of Adolescent Health.
  • The lowering of sex hormones can have a knock-on effect of lowering key neurotransmitters such as mood, sleep and appetite regulating serotonin (linked with low estrogen) and calming GABA (linked with low progesterone)

Affect your stress hormones

  • Low BMI is associated with high cortisol in adults and adolescents
  • This is consistent with a mechanism designed to maintain a normal blood glucose, when available energy stores are low Misra, M. and Klibanski, A. (2014). Endocrine consequences of anorexia nervosa. [online] The Lancet Diabetes & Endocrinology, 2 (7), pp. 581–592. Available at: https://www.ncbi.nlm.nih.gov/pubmed/24731664 [accessed 11 Aug. 2026].

Cause nutritional deficiencies

  • If low BMI is the result of either not getting enough, or not getting the right macro and micro nutrients, this could be detrimental to your mental health
  • High quality and sufficient quantity of macro and micro nutrients are necessary for neurotransmitter and hormone production
  • Healthy fats are essential for mental health, and 60% of our brains is made of fats, so a deficit of healthy fats can have a negative impact on mental health Parletta, N., Milte, C. and Meyer, B. (2013). Nutritional modulation of cognitive function and mental health. [online] The Journal of Nutritional Biochemistry, 24 (5), pp. 725–743. Available at: https://www.ncbi.nlm.nih.gov/pubmed/23517914 [accessed 11 Aug. 2026].