Apple vs. doughnut

Now that we have an understanding of the bodily processes involved, let's have a look at the fate of our two different foods as they are digested and absorbed by the body.

The apple, which contains about 80 per cent water, 15 per cent carbohydrate (mainly fructose with some dietary fibre) and five per cent of protein or less, has been mostly digested by the time it gets through the small intestine. This process starts in the mouth, as we chew it into pieces and saliva starts to break down the carbohydrate. The stomach will help to break down the small amount of protein, but further carbohydrate breakdown is suspended due to the highly acidic environment. When the apple mush (now called chime) enters the small intestine, the environment is changed from acid to alkaline by the release of bicarbonate (very similar to good old baking soda) from the pancreas. The digestive enzymes here can now get to work on breaking down the carbohydrate into tiny pieces, which are absorbed by the villi (along with the protein) and transported to the liver. The liver will then remove any toxins (e.g. from pesticides), and complete the final part of digestion, releasing the nutrients contained in the apple.

The body has used up energy and nutrients to complete this process, but it is rewarded with an abundance of nutrients from the apple. Firstly, there are amino acids from the protein, which are sent to a pool from which individual cells can access in order to repair themselves and communicate with other cells. Next, we have energy released from the carbohydrate, which goes to fuel our muscles, nervous system and brain. Because most of the sugar is fructose, it is stored in the liver until it is needed, helping blood sugar to remain relatively steady.

The humble apple also contains an abundance of nutrients for the body. Just under the skin lies half of the Vitamin C content of the apple. It also contains calcium, phosphorus, iron, Vitamin A and a healthy dose of potassium. In addition to those, the skin contains a compound called quercetin, a powerful antioxidant that reduces cardiovascular risk. If that wasn't enough, the flavonoids and phytochemicals that it contains seem to help fight against cancer. Finally, the skin contains lots of fibre, which helps to improve bowel function and reduced cholesterol absorption.

The doughnut, however, is a different proposition altogether. It is loaded with saturated fats, trans fats and refined sugar and is largely devoid of any nutritional value, other than energy, which it has in abundance. Digestion of the carbohydrate component will start in the mouth and the stomach will assist in further breaking the doughnut down. When it reaches the small intestine, the simple sugars will be readily digested and then absorbed by the villi. The bile that is released by the gallbladder will help to break down the different fats, but the picture here gets a little complicated. Saturated and trans fats take a different pathway than the more healthy fats, such as monounsaturated or polyunsaturated, and the pathway dictates whether the fats are healthy or unhealthy. The healthier fats generally are absorbed via the liver, whereas saturated and trans fats pass through the villi and are converted into triglycerides (the main form of fat storage in the body) and distributed to the body via the lymphatic system. They are also coated in cholesterol (from the liver) and hence the fats in a doughnut will raise the bad (LDL) cholesterol and reduce the good (HDL) component.

Trans fats, however, surpass saturated fats in the damage that they do. They have been shown to wreak havoc with the body's ability to regulate cholesterol and massively increase your risk of heart disease. They also get into the membrane (outer lining or skin) of our cells, causing them to harden. This has a negative effect on the functioning of our cells and disturbs the delicate ecosystem referred to earlier. In addition to that, a 2006 study reported by the American Diabetes Association showed that replacing healthy fats with trans fats (so that trans fats accounted for eight per cent of energy intake) led to a very significant weight gain (around seven per cent) - despite the fact that the amount of energy was the same. The conclusion? Trans fats should be avoided at all costs.

The take-home message

Hopefully this article has demonstrated why nutritionists and dietitians are always harping on about eating natural foods. It is not just the energy content that is important, but the nutrients within the foods play a large role in our long-term health. Although there is room for everything in moderation, the vast majority of your food should be natural and healthy. As a general rule, if a food looks like it has never been alive at any point, then it's best avoided or at least put in the category of 'occasional food'.