Many people claim that fat people are fat because they are lazy, ‘Just do some exercise and you’ll lose weight’ they say. It is true, there is an association between body weight and amount of physical activity performed. This is all the proof we need right? Well no, because from an association we cannot prove cause and effect. While fat people might be fat because they exercise less, it is just as likely that obesity causes reduced force output and hinders physical activity.
Adipose Tissue Stores More Than Fat
Adipose tissue is a store of fat and other lipid soluble compounds. In particular the fat soluble vitamins are stored in adipose tissue. One theory suggests that the more fat you have the more fat soluble vitamins you store, and as a result plasma levels of those vitamins fall. Indeed, obesity is associated with low level of vitamin D, and one theory suggests this results from the sequestration of the vitamin in the excess adipose tissue.
The Dopamine Link
Vitamin D is not a vitamin at all. It is actually a steroid hormone that interacts with the vitamin D receptor in the nucleus of cells. Here it causes increased expression of proteins, one of which is tyrosine hydroxylase. Tyrosine hydroxylase is the rate limiting step in the formation of dopamine, an important catecholamine chemicals involved in motivation, movement and mood. Low levels of vitamin D may reduce tyrosine hydroxylase activity, thus lowering synthesis of dopamine.
The Iron Link
Obese individuals also have low levels of iron. The reason for this is not known because unlike vitamin D, iron would not be stored in the fat cells. Iron is best known as a cofactor in haemoglobin, where it is required to transport oxygen in the blood. However, it also acts as a cofactor in the tyrosine hydroxylase enzyme, and low iron status may therefore be associated not only with poor oxygen transport, but also with a reduced synthesis of dopamine.
Cause or Effect
Therefore mechanisms exist to explain why obesity might be the cause of the association between body weight and physical activity levels. If obese individuals have low levels of two important micronutrients required for dopamine synthesis, motivation, mood and muscle activity could be affected. This is very theoretical, but increasingly researchers are investigating such relationships because it is becoming clear that weight gain is not caused by laziness.