"How does diet affect flexibility?" is one of the most common questions I get asked, and it's also one to which there isn't a straight answer. There is little, if any, good evidence to suggest a relationship between the foods you eat and increased flexibility. However, unhealthy eating habits may contribute to decreased range of motion, and it's linked to how diet affects our cells.
A crucial part of our cells' ability to function correctly is their capacity to sense the mechanical properties of their environment. Alterations in the mechanical properties of the extracellular matrix or the cellular mechanosensing qualities of the surrounding tissues have been shown to play a part in several diseases (Jaalouk & Lammerding, 2009). So much so that a medical imaging technique called elastography is used to measure changes in tissue stiffness to diagnose conditions like liver fibrosis (Nahon et al., 2009).
In metabolic diseases like diabetes, which can often be prevented with a healthy diet and regular exercise, both extracellular matrix mechanics and biochemistry are adversely changed. Higher than normal blood sugar levels cause collagen glycation (the attachment of sugar molecules to proteins or fats) and cross-linking, which increase extracellular matrix stiffness.
Furthermore, hyperglycaemia will cause cells to produce and secrete different amounts of matrix proteins, alters matrix protein breakdown, and may change integrin-binding sites on existing proteins such as collagen; integrins are receptors on cell surfaces that allow for adhesions between cells and the ECM (Kemeny et al., 2011).
These findings from the cellular biomechanics literature may help shine a light on the underlying mechanisms that cause increased passive stiffness and decreased range of motion commonly seen in people with diabetes (Rao et al., 2006).
There are plenty of claims floating around the Internet that state consuming certain foods or drinking 3 litres of water a day will cause direct improvements in range of motion. However, very little research supports such assertions, and they remain highly dubious at best. That being said, taking on too little water and eating too much sugar to the point of clinical dehydration and hyperglycaemia respectively may negatively impact a person's flexibility.
Jaalouk, D. E. & Lammerding, J. (2009) Mechanotransduction Gone Awry. National Reviews Molecular Cell Biology volume 10, issue 1, pages 63-73.
Kemeny, S. F. et al. (2011) Glycated Collagen Alters Endothelial Cell Actin Alignment and Nitric Oxide Release in Response to Fluid Shear Stress. Journal of Biomechanics volume 44, issue 10, pages 1927-1935.
Nahon, P. A. et al. (2009) Liver Stiffness Measurement in Patients with Cirrhosis and Hepatocellular Carcinoma: A Case-Control Study. European Journal of Gastroenterology and Hepatology volume 21, issue 2, pages 214-219.
Rao, S. R. et al. (2006) Increased Passive Ankle Stiffness and Reduced Dorsiflexion Range of Motion in Individuals with Diabetes Mellitus. Foot and Ankle International volume 27, issue 8, pages 617-622.