Just because a product contains no trans fat does not mean that it is healthy. There could well be other, equally or more harmful substances in it.

I am prompted to write this because today (5 January 2007) there was a letter in The Straits Times Online Forum in which a writer complained that the label on a brand of US-made peanut butter was misleading, because it did not state that it contained trans fat even though it is made with fully hydrogenated vegetable oil.

The complaint is, technically, not valid because if the peanut butter was made with fully hydrogenated vegetable oil, then it is true that it does not contain any trans fat.

Trans fats are found only in partially hydrogenated vegetable oils, not in fully hydrogenated vegetable oils. However, no trans fat does not automatically mean not harmful to health.

However, there are still good reasons for highlighting this issue because the product in question is not healthy, but many consumers may not understand this.

To understand this, we need to know what the process of hydrogenation is all about.

Hydrogenation is a process of adding hydrogen to oil, in order to turn liquid oils (mono or polyunsatured fats) into solids (saturated fats).

In liquid oils, the molecules are bent. They cannot be packed closely together and that is why the oil is soft, or liquid.

In partially hydrogenated oils, the oil molecules become more or less straight, but with a slight kink. They pack more closely together and the oil becomes semi-solid.

However, the oil molecules have been twisted “out of shape” as they are no longer shaped like the original, bent molecules. This makes them extremely harmful to health. And such twisted oil molecules are called trans fats.

In fully hydrogenated oils, the oil molecules have all been broken up at the point where they previously bent or had a kink. Now the molecules are all straight. They pack very closely together.

This oil is now hard and completely saturated. It no longer contains twisted, "out of shape" molecules. It now contains no trans fat.

However, it does not mean that the oil has suddenly become healthy again.

Fully saturated fat is very hard and not edible. It can be consumed only when it is mixed with liquid oils (in this case, oils naturally found in peanuts) to form a semi-solid grease.

Yes, it is true that the oil now has no trans fat.

But this does not cancel out the fact that it had gone through a highly unnatural hydrogenation process involving high heat, high pressure and the use of toxic substances, such as nickel, as catalysts.

To begin with, hydrogenated fats are typically manufactured from cheap, poor quality oils – including toxic oils like cottonseed which are not suitable for human consumption,

Moreover, such oils had already turned rancid – because they had been extracted from oil seeds using high heat.

High heat – as well as exposure to light and oxygen – makes oils rancid. This is why oils were traditionally stored in dark bottles, in cool places.

Modern commercial oils are sold in see-through plastic bottles because they had already turned rancid and gone through deodorisation and other processes to mask the rancidity.

Rancid oils are very toxic. They are as harmful as trans fats, or possibly even more harmful.

Even though they may contain no trans fat, such oils contain lots of free radicals, which can cause serious damage to body cells. Some scientists believe that, apart from trans fats, rancid oils are another major cause of heart disease, cancer and other degenerative diseases.

After the hydrogenation process, the resulting margarine / vegetable shortening is grey and smelly. It has to be bleached, deodorised, artificially coloured and artificially flavoured before humans would consume them.

Rats and cockroaches would still avoid this stuff.

Yet many health authorities, nutrionists and other health experts – including the Singapore Health Promotion Board – encourage consumers to take soft margarine simply on the basis that they contain lower levels of trans fat.

This is taking a very narrow and ill-informed viewpoint. Just because a product has low or no trans fat does not mean that it is healthy.

To know whether or not a product is healthy, we need to know what ingredients they are made from, how they are made, and so on.