We need fat! It stores excess energy to keep us going when food is unavailable; it keeps us warm, provides a cushion to protect vital organs and secretes hormones and factors which regulate our appetite and help insulin to act efficiently. Unfortunately too much fat, especially in the wrong place, can lead to the secretion of substances that increase inflammation and prevent insulin action. This can lead to type 2 diabetes.
Not all fat is the same! The properties I describe above are mostly found in white fat located under the skin and in our abdomens. Brown fat does not store excess calories but is able to turn energy from food into heat. It has mostly been studied in rodents and young children and was thought to be almost absent in adult humans. Its main function is to maintain body temperature in cold conditions (of obvious benefit in newborns and infants). More recent imaging methods have demonstrated that brown fat deposits exist around the neck, along the backbone and in the chest space. Of interest, the amount of brown fat detected in people was found to be inversely correlated with BMI, i.e. slim people had more brown fat and vice versa.
Apart from low temperature, brown fat is also regulated by thyroid hormones and changes in its activity may contribute to some of the symptoms associated with thyroid dysfunction.
Scientists in Switzerland measured how well temperature was maintained following cold exposure in people with underactive thyroid before and after treatment with replacement thyroid hormone (levothyroxine). They found a marked improvement in the ability to keep warm in the cold once thyroid hormone levels were restored to normal.
In contrast, German scientists used imaging to evaluate brown fat in people with an overactive thyroid before and after anti-thyroid drug (ATD) treatment. Brown fat activity was higher but areas of white fat and skeletal muscle were lower before ATD when FT4 levels were high.
These studies provide one explanation for the heat and cold intolerance experienced by those with over- or underactive thyroids respectively but can brown fat levels shed any light on weight gain?
A third type of fat exists! This is known as Beige (predictably) or Brite (for brown in white). This fat starts as white fat but can be induced to mimic brown fat and turn energy into heat. Since white fat stores excess energy but brown fat burns it as heat, browning of fat has been suggested as a way of treating obesity.
Apart from sitting in a cold office, several factors have been shown to increase brown fat/Brite fat. These include smoking (which is not recommended given its wide-ranging negative impact on health, including thyroid eye disease) and various plant-derived products such as capsaicin (found in chilli peppers) and resveratrol (a component of red grapes). Antibiotics seem to have an adverse effect on browning of fat suggesting that the bacteria and other micro-organisms in the gut (known as the gut microbiome) may be implicated in the process. The role of the gut microbiome in thyroid disease, and how it is influenced by diet, stress and other life factors, will be covered in a separate article.
Does exercise lead to fat browning ? In mice and rats the answer is definitely yes; when rodents run on a wheel, cell markers of Brite tissue are increased in fat located under the skin (sub-cutaneous) but not in the fat around the abdominal organs (visceral). In humans, the results are less convincing, but this may be due to the focus on sub-cutaneous fat which is easier to sample. Further studies are needed, including visceral fat, to determine whether fat browning can be added to the list of the many benefits associated with exercise.