Rosemary Wilson, of Oamaru, asks :-

If I eat a really big meal does some of that instantly become body fat? Also I weighed myself after shifting 2-3 tonnes of hay bales one weekend and was gutted to find I had gained weight and people said it had converted to muscle. Is this so? If I do a lot of exercise I'm told I can gain weight as muscle is heavier than fat, but how fast does it happen?

Jeremy Krebs, an endocrinologist at the University of Otago's Wellington Campus, responded.

When we eat food it is digested and absorbed as the basic building blocks of each of the major food types or marconutrients. So as a general rule carbohydrate is broken down to glucose, protein is broken down to amino acids and fat is broken down to fatty acids and glycerol.

Once absorbed into the body those building blocks are reassembled and processed, largely by the liver and to some extent by muscle and fat tissue. Although it is possible to generate fat from glucose and amino acids, this is not an energy efficient process. So the body doesn't tend to do this. What it will do is to burn glucose as the preferred fuel for the energy needs of the body and store any fat as fat. So after any meal, the body will only use fat as a fuel once it runs out of glucose. The bigger the meal the less likely it is to have to use any of the fat consumed and therefore this gets stored in fat tissue until it is needed.

It is true that muscle is heavy. This is because muscle is associated with a lot of water, unlike fat tissue which is a very efficient way of storing a large amount of energy in a small space. Generally any changes in weight over the course of a day or even 2-3 days, are predominantly changes in body water rather than any large change in actual fat mass. So with a period of intense physical activity over a weekend you would not have had any major change in actual fat mass or muscle mass, but rather a change in total body water content. Don't give up though. The benefits of regular physical activity on fitness and well-being are as important as any changes in body weight.