Newborn human infants are very vulnerable to hypothermia for several reasons.
They have higher ratio of body surface area to body volume.
Their heads are proportionally larger.
They have a small musculature.
They cannot shiver.
They are poorly insulated.
They can't move to warmer areas.
And their nervous systems are slow to respond to cold conditions.
Fortunately, they have Brown Adipose Tissue (BAT) which generates heat when they are cold.
Perhaps it's a good time to use the technical terms of Adipose Tissue instead of Fat cells as well as Lipids instead of Fats.
If you look at the White Adipose Tissue (WAT) on the right, you will see that it's largely a storage bag with a single lipid-filled space. On the other hand, the Brown Adipose Tissue (BAT) has many small lipid droplets, many mitochondria (the energy converters of the cell), and many capillaries. Hence its brown colour.
Interestingly, Brown Adipose Tissue is more closely related to Muscle Tissue than it is to White Adipose Tissue. So, calling it all fat, gives an entirely wrong impression.
Put simply, WAT stores lipids while BAT is thermogenic tissue. When activated, BAT cells take lipids and run them through the mitochondria to generate heat rather than synthesise ATP (the body's chemical energy storage molecule).
So that's how infants can generate heat without shivering and thus, avoid hypothermia.
The presence of BAT in adults has long been known, but its physical form and disposition have only been displayed relatively recently.
When Jan Nedergaard and his colleagues at the Wenner-Gren Institute in Stockholm ran a Positron Emission Tomography (PET) scan on an adult patient to look for tumor metastases they received a surprise. They were the first to display the disposition of brown adipose tissue deposits in an adult human.
The researchers were seeking tissue with increased glucose uptake, an indication of a tumour, by labelling fluoro-2-desoxy-glucose. This is essentially, mildly radioactive glucose (it releases photons). These can be detected by PET, however, because brown adipose tissue also actively takes up glucose, they were able to visualise it in situ for the very first time.
Don't be alarmed by the apparent complexity of the diagram opposite.
Let's start at the top where the stimulus arises: a meal, high energy reserves (WAT?) and a drop in body temperature combine to trigger activity in BAT.
You will see that they act on the VMN which is part of the Hypothalamus, an important control centre in the brain.
The VMN sends a chemical signal called Nor-adrenaline (NE) to the BAT cells. In response, they begin to break lipids (TG) into free fatty acids (FFA) which go into the mitochondrian (the oblong box). But instead of making ATP, the Uncoupling Protein (UCP1) causes the cells to cycle hydrogen around, which generates heat.
The heat produced is carried away to the rest of the body by the circulating blood.
In a recent review, the roles of adrenaline antagonists (ẞ-blockers) and here are the results:
KEY: O↑ = Increases obesity; O↓ = Reduces obesity.
Adrenaline agonists (mimics like amphetamines) O↓
Warm air conditioning O↑
Exposure to constant low temperatures O↓
Scientists were able to stimulate brown fat growth in mice, but human trials have not yet begun.
However, studies using mice exposed to 4°C for 8 weeks (an extreme regime!) have demonstrated the growth of atherosclerotic plaque in blood vessels perhaps caused by the activation of brown fat. It is recommended that safe acclimatisation should be used to understand the growth of brown fat in adult humans (5 to 10 °C) in future experiments.
Another approach is based on the discovery that β3-adrenoceptor agonists enhance glucose metabolic activity of brown adipose tissue in rodents.
In rare cases, brown fat continues to grow unchecked; this leads to a tumour known as a hibernoma.