Studies have shown that some widely held beliefs about certain foods may not be true after all. By Michael Lim
YOUR blood cholesterol is high and by lowering the intake of cholesterol in your diet, you will reduce your low density lipoprotein cholesterol (LDL-C) or “bad” cholesterol – is this truth or a fallacy? While you may not be wrong to think that loading yourself with LDL-C in your diet may lead to upward elevation of LDL-C in your blood, you may be surprised to know that the consensus in the latest 2013 American College of Cardiology/American Heart Association Guideline on Lifestyle Management to Reduce Cardiovascular Risk, is that there is as yet no definitive evidence that lowering dietary cholesterol can result in lowering of the blood LDL-C.
The reason for this is that it appears that the main source of cholesterol is from production by the liver and providing the liver with the appropriate ingredients such as saturated fat may have a larger impact on increase in LDL-C production.
What then is the right dietary advice that has been demonstrated to reduce LDL-C? A diet that is high in vegetables, fruits, low-fat dairy products, whole grains, poultry, fish, legumes, non-tropical vegetable oils and nuts; and low in sweets, sugar-sweetened beverages, and red meats appears to be the most healthy. Dietary plans which are of this nature, such as the DASH (Dietary Approaches to Stop Hypertension) diet and the American Heart Association diet, have been shown to lower LDL-C.
Low glycaemic index diets
There is a trend among food manufacturers to send their food products for measurement of glycaemic index. For the consumer, is there evidence that glycaemic index of a food has an impact on health? The glycaemic index is a ranking system to assess the degree of sugar increase when consuming equal portions of different carbohydrates.
Not surprisingly, foods which are high in fibre or complex sugars from non-starch vegetables will have a low glycaemic index. The fad for low-glycaemic diets rose from the concern that too much sugar will cause obesity which in turn may increase the incidence of diabetes. While this sounds perfectly rational, unfortunately, there is insufficient data to provide evidence that a low glycaemic diet will improve the health of your heart for non-diabetics.
There is a distinction between consuming excessive calories and consuming foods with a higher glycaemic index. A person who consumes low glycaemic index foods but has a high total calorie intake may be worse off than someone who consumes high glycaemic index foods but has a low total calorie intake. Hence, while excessive calorie consumption is detrimental for health, there is as yet no evidence that it is more worthwhile to pay a higher price for a food product with a lower glycaemic index if you are a non-diabetic.
A meta-analysis study (combining the data of previous trials) published in the Annals of Internal Medicine in March 2014 by researchers from Harvard and Cambridge universities raised a storm of controversy by suggesting that the current evidence did not clearly support guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of saturated fats.
For food lovers, this welcome news was short-lived as this publication was followed by widespread criticism of its methodology and inaccurate assessment of other trial data. The current consensus is that reducing the saturated fat content of the food to about 5 per cent of the total calorie intake is beneficial and will reduce LDL-C.
The US Food and Drug Administration (FDA) had issued a statement that partially hydrogenated vegetable oil (PHVO), the main source of trans fat in processed food will no longer be “generally recognised as safe”. However, recent data published in the Journal of the American Heart Association at end-August 2014 showed that not all trans fat is bad.
It appears that the bad effects of trans fat result mainly from excessive amounts of one sub-type of trans fat, namely trans linoleic (T-18:2) isomer sub-type. Current measures to reduce the trans fat in trans fat-rich foods including fried foods, margarine, processed meats, bakery products and biscuits have targeted mainly sub-types of trans fat which do not seem to have an adverse impact on heart health.
In the study, among the foods which use PHVO, only bakery products had significant amounts of the “bad” trans linoleic (T-18:2) isomer sub-type. Other than from PHVO, this “bad” trans fat can also be produced by vegetable deodorisation and high-temperature frying.
Before you reach for the salt shaker, think twice as there is strong evidence that reduction of sodium salt intake can reduce your blood pressure whether you have high blood pressure or not. The blood pressure lowering effect of reduced salt intake is more pronounced in those with high blood pressure. However, the decrease in blood pressure is modest (up to 7 mm Hg reduction).
For those who are considering switching some of the sodium salt to potassium salt, you may want to know there is no definitive evidence to show that a combination of decreasing sodium salt intake together with increasing intakes of other minerals, such as potassium or magnesium, has superior effects to reduction of sodium salt alone. There is also no evidence to show that increasing potassium intake lowers blood pressure.
Exercise and cholesterol
It is a common perception that exercise can help to burn the bad cholesterol and raise the good cholesterol levels. Studies that examined vigorous intensity aerobic exercise on an average of five months for an average of five days per week for about 40 minutes at each time, showed a modest reduction of LDL-C of 3-6 mg/dl, and no consistent effect on good cholesterol and triglycerides.
Hence, the impact of aerobic exercise on the cholesterol profile is minimal and for beneficial effects of cholesterol reduction to be seen in those with heart disease, significantly lower reduction in bad cholesterol is required. This often requires a combination of diet and medication. The aerobic exercise also has a modest beneficial effect of lowering blood pressure – up to 5 mm Hg reduction.
The largest impact on cholesterol level is diet. A healthy diet will not only reduce LDL-C but will also reduce blood pressure through sodium salt reduction. Cutting back on “bad” trans fat and total saturated fat will keep one in good heart health. Using a calorie counter to control the calorie intake and avoid obesity is also a useful lifestyle habit.
Aerobic exercise helps burn calories but has a modest effect on blood pressure and LDL-C. Hence, one has to balance between the beneficial effects of exercise with the risk of joint damage resulting from excessive exercise. As I always tell my patients – remember to exercise moderation in everything you do.