I’ve always had a fascination with brown fat. I spent many of my years in the UK caring for lambs in the first 48 hours of life, so any aspect of how to keep them alive caught my attention.
Up until last week, my knowledge of brown fat in lambs was limited to the fundamentals. It’s a kind of fat that lambs are born with. It keeps a lamb warm in the first few hours of life, and then it disappears. After that, the lamb relies solely on food to keep itself alive. I also knew that the nutrition of the ewe during pregnancy impacts lamb survival in those first 24 hours, so it more than likely impacted brown fat levels. And I also knew that genetics has an impact on lamb survival and thus, probably also had an impact on brown fat levels.
But that wasn't enough of an understanding for me. So, I went down a very long, winding rabbit hole to discover, ‘What is brown fat, and how does it work?’ and I’m here to tell you what I discovered.
I’ll start with a brief science lesson to familiarise you with some terminology. Then we’ll get into the course of events that occur during pregnancy and in those first 48 hours after a lamb is born, along with the genetic and nutritional impacts at each stage. Please stick with me through the science bit; it will make sense later on with some real-life context.
Energy production and how brown fat is different
The mitochondria are the powerhouse of the cell (how many of you got high school flashbacks from that?!). Within mitochondria, in almost all cells, there is a cycle that converts energy from food into ATP, the usable cellular energy that powers all cell activities.
In this cycle, protons are pumped from one side of the mitochondrial membrane to the other, creating a concentration gradient. Normally, protons flow back through ATP synthase to make ATP. However, in brown adipose tissue (BAT or brown fat), a special protein called UCP1 allows protons to bypass this process, releasing energy as heat.
Think of it like a dam, with the water levels trying to balance themselves out. In normal cells, you have excess water going through a turbine (ATP synthase), which is powering a generator; in BAT, you have a spillway (UCP1) releasing all of that energy immediately.
BAT is packed full of mitochondria (which is actually what gives it its brown colour), up to 10-20 times more than normal white fat tissue. This, combined with UCP1, makes it a key heat source for newborn lambs who need immediate heat.
Now that you have an understanding of what makes brown fat special, I’ll go through the stages of pregnancy and how the brown fat cells develop, highlighting the genetic and nutritional implications at each stage.
Brown fat development in lambs throughout pregnancy
Days 30-60
Certain stem cells migrate to specific regions of the fetus - the kidneys, heart, lungs and spine - and then commit to developing adipose (fat) tissue in those regions.
Genetic factors influencing brown fat: Genetic variation affects how many stem cells are allocated to each fat depot location. There are multiple genes controlling this process, more than we have time for today. There are Hox genes, TBX genes, BMP4 and a host of others. But what matters is that genetic differences at this stage can result in a 30-40% variation in stem cell allocation between lambs. And no amount of good feeding later will compensate for poor genetic allocation here.
Impact of ewe nutrition on brown fat development: There are no known nutritional implications for brown fat development during this period.
Days 60-80
Once all of the stem cells have finished committing to their locations, they must decide what type of fat to become. This decision is controlled by a protein called PRDM16. PRDM16 activates brown fat genes (as opposed to white) within each cell, so the more PRDM16, the more brown fat.
Genetic factors influencing brown fat: Lambs with higher PRDM16 convert more precursor cells to brown fat. Breeds like Icelandic sheep have genetic variants that enhance PRDM16 function, resulting in more brown fat to tolerate the cold conditions. Consequently, tropical breeds produce less functional PRDM16 as it has not been an evolutionary requirement.
Impact of ewe nutrition on brown fat development: There are no known nutritional implications for brown fat development during this period.
Days 80-120
Preadipocytes (immature brown fat cells) now multiply to increase cell numbers. A single preadipocyte can become 1000-10,000 cells!
Genetic factors influencing brown fat: Several genetic factors affect the rate at which cells multiply. Certain proteins, like growth factor receptors, show genetic variation in how efficiently cells respond to signals telling them to multiply. Other genes impact how quickly cells divide, while others impact how sensitive cells are to nutrient restriction, meaning poor maternal nutrition can affect cell multiplication in some animals more than others.
Impact of ewe nutrition on brown fat development: Finally, ewe nutrition begins to impact brown fat at a significant level. Trials have shown that nutrient deficiency in the ewe between days 105 and 147 of gestation reduced adipose proliferation by 15-37%.
Days 120-140
Preadipocytes must finally mature into functional BAT. This occurs when they begin to express UCP1 and build the mitochondria to house it, which we mentioned earlier. At this point, they stop proliferating, and the amount of BAT a lamb will be born with is determined.
Genetic factors influencing brown fat: There are gene variants that reduce UCP1 expression, and others that affect how efficiently cells complete this final step.
Impact of ewe nutrition on brown fat development: Because UCP1 is a protein, maternal protein deficiency in late pregnancy can limit its synthesis, leading to less UCP1 per cell. Other nutritional deficiencies also limit the synthesis of this protein. Maternal stress, resulting in elevated cortisol, has also been found to reduce UCP1 expression. Ewe nutrition is particularly important at this stage, because poor nutrition can waste all of the earlier developmental work of laying down the fat.
Loss of brown fat
Various biochemical and environmental triggers start to shut down UCP1 function in a lamb between six and fourteen hours of life. This includes things like changing oxygen levels in the lamb's bloodstream, temperature drops, cortisol spikes and varied glucose levels. Remaining UCP1 continues to work over the next twelve to twenty-four hours, but gradually degrades until there is none left.
However, UCP1 only continues to work if it has fatty acids available, which come from colostrum. So, lambs who do not receive colostrum are at a double disadvantage. They lack energy from a lack of food, AND they are also not able to utilise their remaining brown fat reserves efficiently.
Eventually, the remaining UCP1 disappears, and the animal loses the ability to express the gene entirely. BAT loses its high levels of mitochondria and, by day six, the former BAT looks like white fat tissue.
Ruminants lose the ability to produce brown fat forever at this stage. This is because BAT is quite an inefficient way of heating the body. Sheep are cleverer than that and have evolved to have an internal radiator in the form of a rumen.
What about twin lambs and brown fat?
Twins are impacted throughout the whole process due to restricted placental nutrition. They have fewer initial stem cells, reduced BAT from poor proliferation, and often have less UCP1 and mitochondria per cell.
As scanning percentages increase and twin lambs become more common, it is important to consider how you can reduce the twin-born impact on lamb survival by prioritising both survivability genetics and ewe nutrition in late pregnancy.
And there we have it, the bottom of the rabbit hole, ‘What is brown fat, and how does it work?’.
I hope that, by gaining a more in-depth understanding of brown fat in lambs, including the genetic and nutritional impacts on its development, you’ll have a better idea of what you’re doing when you’re feeding your ewes well (or not) during pregnancy, or when you’re selecting rams with good (or bad) survival breeding values.
Sometimes the most practical farming decisions are rooted in some pretty complex science, and I think that’s very cool. Once you understand what's actually happening at the cellular level, you can really optimise your management to work with these processes instead of against them. Good farming practices are just a matter of you giving these biological processes the best chance to work properly.
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In short: What you need to know about brown fat in lambs
- Brown fat forms before a lamb is born and fuels its first hours of life.
- Genetics play a crucial role in determining how much brown fat a lamb will develop
- Late-pregnancy ewe nutrition, including protein intake, directly affects the development of brown adipose tissue (BAT) in lambs.
- Colostrum intake enables lambs to use their remaining BAT stores efficiently.
- Twins require extra care to ensure they develop sufficient brown fat. Select rams with better breeding values for lamb survival, and prioritise the nutrition of twin-bearing ewes, to give twin lambs a better chance of survival.
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Join the conversation on The Hub
Take a moment to head over to the neXtgen Agri Hub to share your comments and questions. We’d love to hear your thoughts on brown fat.
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Further reading
- Breeding for higher lamb survival by Dr Mark Ferguson
- Casteilla, L., O. Champigny, F. Bouillaud, J. Robelin, and D. Ricquier. "Sequential Changes in the Expression of Mitochondrial Protein mRNA during the Development of Brown Adipose Tissue in Bovine and Ovine Species: Sudden Occurrence of Uncoupling Protein mRNA during Embryogenesis and Its Disappearance after Birth." Biochemical Journal 257, no. 3 (February 1, 1989): 665–71. https://doi.org/10.1042/bj2570665.
- Ojha, S., L. Robinson, M. Yazdani, S. M. Symonds, and M. E. Symonds. "Brown Adipose Tissue Genes in Pericardial Adipose Tissue of Newborn Sheep Are Downregulated by Maternal Nutrient Restriction in Late Gestation." Pediatric Research 74, no. 3 (2013): 246–51. https://doi.org/10.1038/pr.2013.107.
