Les additifs du tabac

5. Do additives make tobacco more addictive?

• 5.1 How could additives affect addictiveness of tobacco products?
• 5.2 What does the evidence say about addictiveness of specific additives?
• 5.3 What does the evidence say about specific additives influence on nicotine addiction?

5.1 How could additives affect addictiveness of tobacco products?

In principle, there are a variety of ways in which additives could increase addictiveness.

They might:

• Enhance the nicotine content directly
• Increase the bioavailability of nicotine – so the body reacts more strongly
• Make it easier to inhale tobacco smoke
• Produce substances in the smoke which increase addictiveness of nicotine
• Alter the physical properties of tobacco smoke, such as the size of the smoke particles.

5.2 What does the evidence say about addictiveness of specific additives?

Some tobacco additives may act directly on the central nervous system. Their concentration in tobacco smoke is in general too low to have a significant effect. However, indirect additive effects cannot be ruled out.

No tobacco additives have been identified so far which are addictive by themselves. There is a possibility that some produce by-products which could be addictive. Sugars, polysaccharides and cellulose fibres, all of which occur naturally in tobacco, produce aldehydes when burned. One of these, acetaldehyde, is known from animal tests to be potentially addictive. Sugars, which are added in large quantities to tobacco products, increase the aldehyde levels in smoke.

In addition, experiments using denicotinised cigarettes show that a mixture of other components of cigarette smoke are probably involved in promoting tobacco cravings and reinforcing the desire to smoke. These components of smoke do not have pharmacological effects like those of nicotine, but they play a role in sustaining smoking.

5.3 What does the evidence say about specific additives influence on nicotine addiction?

Sugars

Sugars occur naturally in tobacco, but a large proportion of tobacco additives are sugars or their derivatives. Heating and burning produce many breakdown products of sugars including aldehydes. One of these, acetaldehyde, enhances the addictiveness of tobacco in experimental animals.

This is thought to be due to its action as an inhibitor of the enzyme monoamine oxidase (MAO), which in turn increases levels of one class of neurotransmitters. Smokers have reduced levels of MAO in the brain. However, there is no proof that acetaldehyde in smoke alters blood levels of the chemical significantly. Other aldehydes in smoke may be equally important, though. Further investigation of other aldehydes is needed before the role of sugars as indirect contributors to addiction can be confirmed.

Ammonia

Ammonia affects nicotine absorption by altering the acidity (pH) of smoke. Cigarette smoke is normally lightly acidic, and most of the nicotine occurs in a positively charged form which does not pass easily across cell membranes. The pH in the lungs is somewhat higher (indicating less acidity) which converts some nicotine to its uncharged form. Tobacco companies started using ammonia compounds to increase smoke pH values in the early 1970s.

However, experiments do not indicate that ammonia-laced tobacco produces significantly higher blood levels of nicotine. Absorption of nicotine in smokeless tobacco, which occurs through the cells lining the mouth, depends on the pH of the product. But it appears that for cigarettes the main sites of absorption in the lungs are largely unaffected by ammonia in inhaled smoke. The fluid in the lungs acts as a chemical “buffer”, maintaining a stable pH. Absorption of nicotine is in any case higher in the lungs than the mouth because of the larger surface area of lung tissue.