The fatter the arm, the greater the overestimation. However, after correction for arm circumstance, there still remains a positive relationship between body mass index (BMI) and both systolic and diastolic blood pressure. There is a suggestion that BMI exerts its effects mainly on diastolic blood pressure with little independent effect on the systolic pressure, which is mainly related to age.
The mechanism by which obese people have a high blood pressure is uncertain. It is probable that obese people eat more sodium and less potassium and, therefore, may develop a rise in blood pressure due to dietary factors. 
It is probable also that high blood pressure is more closely correlated with central obesity than with BMI alone. Intravenous glucose tolerance tests have demonstrated that obese people are relatively resistant to insulin, and this concept is now the subject of a great deal of research.
It has been postulated that insulin resistance leads to a rise in intracellular sodium concentration and to renal retention of salt and water. Thus insulin itself may play some role in the aetiology of essential hypertension, particularly in obese individuals. Since obesity is itself associated with high blood lipids levels, glucose intolerance and high blood pressure, fat people are more prone to coronary heart disease.
It is possible, however, that after allowing for these factor. BMI alone may not itself be an independent cardiovascular risks factor. This is only a theoretical consideration as most obese people do have the other cardiovascular risk factors and thus have a high risk of death. When people lose weight, their blood pressures tend to fall at a rate of 1 mm Hg/kg reduction of body weight.
Hypertension and obesity relationship
