Recently, I was questioned twice about the lowered cut-off values for blood sugar and blood pressure in the recent guidelines. Why is it that unsafe blood sugar values are indicated at a fasting blood sugar of 100mg% instead of the cut-off of 126mg% used to define diabetes? Why have the recent American guidelines brought down the systolic blood pressure (BP) cut-off from 140 mmHg to 130mmHg? The answers to both questions are similar. Physiological variables like blood pressure, body mass index (which relates weight to height), blood sugar and blood cholesterol are part of the body’s normal form and function. However, they are associated with the risk of disease when their values are undesirably high. But how high is ‘high’? Traditionally, the minds of clinician as well as lay persons have been conditioned to think that the each of these variables has a cut-off value which demarcates “normal” from “abnormal”. Conveniently, these cut-offs have been used to divide all clinical conditions dichotomously: hypertension versus normal BP; high versus normal cholesterol; diabetes versus normal blood sugar; overweight versus normal weight. It is assumed that above any such cut-off there is certain risk of disease while below the value there is no risk at all.
Then, why do the cut-off values keep changing over time? Over the past 50 years, the value of undesirable BP has slid from 160/95 to 140/90 to 130/85 to 120/80 with even values above 110/70 being described as “sub-optimal”. Similarly, the threshold of fasting blood sugar for defining diabetes was brought down from 140mg% to 126mg%, with the 100-125 range demarcated as risk-prone ‘pre- diabetes’. Clearly, the human gene pool has not changed in the past 50 years to produce humans with a different physiology! The answer lies in population-based epidemiological research which has, time and again, demonstrated that the risk of a disease does not lie beyond an arbitrary horizontal threshold (the magical “cut-off”) but lies continuously distributed across a rising slope. So, 160 mm Hg poses higher risk of heart attack and stroke than 140 which in turn poses greater risk than 130 and so on till 110.
Similarly, even blood sugar values in the 100-125mg% range pose both a risk of future diabetes but also a higher risk of heart disease than values below 100. This progressive rise of risk represents the true picture of increasing harm associated with various physiological variables along their rise over a wide range, while the simpler clinical cut-offs help doctors to decide if treatment is needed. Even that decision making should not be guided by simple single risk factor cut-offs. The risk of a disease associated with any ‘risk factor’ is seldom determined by that risk factor alone, since multiple risk factors co-exist and combine to determine the actual risk of diseases like heart attack, stroke and chronic kidney disease in any individual. For example, the risk posed by BP for these diseases is considerably enhanced by the presence of diabetes.
Age, gender, smoking status, levels of BP and cholesterol (or cholesterol:HDL ratio) and presence of diabetes are among the risk factors that have be factored together while estimating the ‘absolute risk’. Treatment decisions are best dictated by that assessment. Lowering of clinical thresholds over time represents both a recognition of the progressive upslope of the risk association and accommodation of fresh evidence that reduction of a risk factor to a lower level reduces the risk of disease. It does not mean that drug therapy is required at mild or moderate risk levels. Quite often, diet and exercise are effective in lowering the risk, if earnestly attempted. Drug therapy is likely to be required only in persons at high risk, determined by very high levels of a single risk factor or, more commonly, by co-existence of several risk factors even at moderate levels.
What are the lessons for prevention? It is best to keep the physiological risk factors as low as possible in the risk range, for as many years of life as possible. Systolic blood pressure, for example, should be kept between 120 and 130 even if many adults will be unable to maintain it between 100 and 120. It is important to remember that a healthy diet and physical activity can simultaneously keep many risk factors low. BP, blood cholesterol and other lipids, blood sugar, body weight and abdominal obesity are all benefited by sensible dietary patterns and regular moderate physical activity. A drug, on the other hand, targets one risk factor only, its action being specific to that. So drugs are best reserved for treating persons at high risk while our living habits are best suited to keep us at the lower end of the risk scale. That is the best guideline of all!