To get the energy to keep our body running we need to eat.
Eating breaks down our food into carbohydrates called simple sugars. These can then be used to make an energy molecule called ATP which we either use immediately or store. Glycogen is the energy store in the liver and fat is the energy store in the body. By far the most common simple sugars in our diet are glucose and fructose. Sucrose, what we usually call sugar, has to be broken down to these substances as well.
We have a number of “control mechanisms” in place to stop us from eating more than we should. The hypothalamus in the brain is the appetite control centre. There are also substances in our body that tell us when to stop eating. Insulin and leptin tell us when we’ve had enough carbohydrates and proteins. CCK tells us when we’ve had enough fats. An enzyme called PFK-1 controls the conversion of glucose to ATP. This is so we don’t end up with too much ATP to convert to energy stores. Too much ATP in our system means that PFK-1 stops making it.
The problem with these control substances is that they work on glucose not fructose. This means that when we eat sugary food, our control mechanisms don’t come fully into play. We just keep eating because we don’t know we’re full and we convert the extra energy into circulating fatty acids in the blood stream and then to fat for storage.
Here’s an edited example from “Sweet Poison” by David Gillespie, to illustrate this point. The author talks in “calories” instead of “kilojoules” but you’ll get the idea:
A glass of whole milk contains 8g of fat, 8g of protein and 11g of carbohydrate in the form of lactose. Lactose is broken down into glucose. The hypothalamus accurately counts calories and controls the consumption of milk by monitoring the carbohydrates (using insulin and leptin), fat and protein (using CCK). The hypothalamus “sees” every single one of the 146 calories in the glass of milk. All the hormonal responses are triggered and I feel appropriately full for having consumed those calories. A similar quantity of apple juice contains no fat, no protein, 14g of fructose, 6g of glucose and 4g of sucrose (24g of sugars in total). The 4g of sucrose is broken down into an extra 2g of glucose and 2g of fructose. So, the drink effectively contains 16g of fructose and 8g of glucose. This is a total of 96 calories but the hypothalamus only “sees” 32 from the glucose. The 64 calories provided by the fructose goes undetected. To feel as full as I would after consuming the same quantity of milk, I would need to drink five times as much apple juice. If I actually did that I would create 35g of circulating fatty acids ready for storage as fat. Not too many people would drink that much apple juice but they frequently consume large amounts of soft drink. If you “drink fructose” with a meal, the fructose gives our body permission to consume many more calories without feeling full while converting them to fat.
Moral of this whole story: Avoid all foods with added sugar if you want to control your weight.