Living with lipids: Reflections of a fat physiologist

This paper provides a summary of the British Nutrition Foundation Annual Lecture held at the Royal College of Physicians, London, on 17th November 2015. Over a 10-year period, the average person consumes approximately 300 kg of fat, but his or her body fat stores may remain close to 15 kg or so. To explain how the daily influx of dietary fat is managed by the body, this lecture focused on the special role of adipose tissue in orchestrating the fat mobilisation and storage pathways and considered what happens to fat metabolism when adipose tissue dysfunctions and how this can explain the metabolic changes that occur in obesity. Body fat stores are not static: they turn over. This was demonstrated in a study conducted in collaboration with Professor Peter Arner (Karolinska University Hospital, Stockholm) and Dr Kirsty Spalding (Karolinska Institute, Stockholm) using an approach that capitalised on carbon 14 entering the food chain (and consequently humans) in the 1960s due to atmospheric nuclear bomb testing at the time. In the mid-1960s, test ban treaties were introduced and subsequently concentrations of carbon 14 in the atmosphere declined but labelling with carbon 14 of the body tissue (including adipose tissue) of people who were alive during the bomb testing period continued. This meant that by measuring the radioactivity of fat samples from these people, it was possible to estimate the average age of the adipocytes (DNA) and the fat stored within them (triacylglycerol). Results indicated that the average age of the adipocytes was 10 years and the fat within these cells was 1–2 years old, meaning that within the life of each adipocyte its fat store turns over approximately five to six times (Spalding et al. 2008; Arner et al. 2011). On a day-to-day basis, the body must process the fat that is consumed in the diet, which on average is about 70 g, with each meal perhaps providing 20–30 g (sometimes more, sometimes less). The high amount of fat entering the bloodstream immediately following a meal poses a metabolic challenge and it needs to be quickly cleared and transported to other tissues. The main target for dietary fat is adipose tissue, which is present all around the body, predominately lying just underneath the skin but is also found in the abdominal cavities surrounding the abdominal organs. Adipose tissue mainly comprises adipocytes, which are essentially large droplets of fat surrounded by a thin layer of cytoplasm, richly supplied with capillaries and bound together by connective tissue. The fat that we eat is almost entirely in the form of triacylglycerol (triglyceride) and it is stored within adipocytes in this form. After eating, triacylglycerols, composed of three fatty acids that are esterified to one molecule of glycerol, are absorbed in the small intestine and transported through the bloodstream to capillaries in adipose tissue. Fat stores are replenished in adipocytes through the action of the enzyme lipoprotein lipase which, in the capillaries of adipose tissue, liberates the fatty acids so that they can be taken up by the adipocytes and re-esterified to make new triacylglycerol. The pathway is reversed when fat is mobilised for fuel: triacylglycerol stored in adipocytes is hydrolysed within the cell and the non-esterified fatty acids are then transported in the bloodstream to tissues such as the liver and the skeletal muscle, where they are used as a fuel for further metabolism or oxidation. Glycerol is transported to the liver where it may be used to make glucose. A lot of what we know about the fat mobilisation and fat storage pathways has come from the study of isolated adipocytes in vitro but this type of research does not capture the complexity of how adipose tissue is orchestrated in vivo. Adipose tissue is not just a collection of adipocytes: there is a dense network of capillaries, through which flows a complex mixture of hormones and substrates that regulate adipose tissue metabolism. And so to begin to understand the dynamics of adipose tissue, my research team at the University of Oxford designed a technique to study adipose tissue in vivo. This technique involves catheterising human participants to collect blood that has just emerged from the capillary beds of adipose tissue on the abdominal wall (‘tummy fat’), and also arterial blood that supplies this fat depot. The arterial blood serves as a comparison, which allows the changes that occur as blood passes through the subcutaneous abdominal adipose tissue to be examined. Blood flow rate is also measured (Frayn et al. 1993). Fat mobilisation following an overnight fast was studied in 13 healthy, normal weight volunteers using this technique. The overnight fast was predicted to activate this pathway as the body would be in need of fuel and this was indeed found to be the case, with non-esterified fatty acids being released at a high rate from the abdominal fat store. Volunteers were then provided with a high-carbohydrate, high-fat breakfast. Within an hour, the rate of non-esterified fatty release declined dramatically, and after 90–120 minutes, it was almost zero. This dramatic shift in metabolism occurred as tissues dependent on fatty acids in the fasted state had switched to using glucose entering the gut as part of the meal. The rate of fatty acid delivery from the abdominal fat store was found to increase as the volunteers gradually re-entered a fasted state (Coppack et al. 1990). The major regulator for this change in metabolism is the hormone insulin, released from the β-cells in the pancreas. Insulin concentrations are low during the fasted state. After consuming a meal, insulin concentrations rise very rapidly, primarily due to the carbohydrate component of the meal. As insulin concentrations rise, fatty acid delivery from adipose tissue is suppressed until the body re-enters the fasted state when insulin concentrations drift back towards baseline and fatty acid release from adipose tissue returns. This has been demonstrated in human studies using a ‘glucose clamp’, where insulin is infused into volunteers at high physiological levels alongside a constant infusion of glucose to ensure that blood glucose levels do not fall (i.e. the glucose is ‘clamped’). These clamping studies show that high insulin levels cause fatty acid release from adipose tissue to be switched off, indicating that when the body is in a fed state the rise in insulin concentrations suppresses fat mobilisation (Karpe et al. 2005). Studying adipose tissue metabolism in vivo has also shed light on the fat storage pathway. To measure the rate of extraction (i.e. storage) of triacylglycerol by the enzyme lipoprotein lipase, which breaks apart the fatty acids and glycerol in triacylglycerols so that they can be taken up by adipocytes, blood samples were taken from volunteers (using the catheterisation method as described above) about 3 hours after eating a fatty meal. To compare the presence of large fat molecules (i.e. triacylglycerols) in the samples, the light scattering properties of the arterial and adipose tissue blood plasma samples were measured. Results showed that arterial blood was more turbid than the blood that had passed through the adipose tissue, indicating that the large triacylglycerol molecules in the adipose tissue had been extracted by the enzyme lipoprotein lipase. This technique has been used to measure 24-hour triacylglycerol extraction in abdominal adipose tissue of nine healthy, lean men (Ruge et al. 2009). In this study, samples of the volunteers’ blood were taken at baseline following an overnight fast, after breakfast, lunch and dinner, and throughout the night. The meals consumed were of equal macronutrient composition and supplied the energy required by each individual volunteer. After the overnight fast, the rate of triacylglycerol extraction was found to be low. After breakfast, the rate increased and then declined in the interval between breakfast and lunch, but not to baseline. The same pattern occurred following lunch and dinner, meaning that there was an incremental increase in peak triacylglycerol extraction rates following each meal. The night sampling revealed that triacylglycerol extraction following the final meal steadily declined until it plateaued in the early hours of the morning. These data highlight that people spend the majority of every 24 hours in varying degrees of a fed state and do not spend much time in a true fasted state. This same technique has also been used to study the rate of the triacylglycerol extraction in skeletal muscle. Data obtained from resting muscle of the forearm demonstrate that the rate of extraction is much less than that of adipose tissue and not so dynamic. The two adipose metabolism pathways, fat mobilisation and fat storage, interact and occur at the same time, with one or other pathway predominating depending on whether the body is in a fasted or fed state. In the fasted state, there is a net flow of fatty acids out from the adipocytes into the capillaries and bloodstream. In the fed state, triacylglycerol is delivered to the capillaries and is hydrolysed by the enzyme lipoprotein lipase after which there is a flow of fatty acids into the adipocytes for esterification and storage. Therefore in any one state, there is a net flow of fatty acids, either in or out of the adipocytes. We have called this the ‘transcapillary flux’ of fatty acids (Frayn et al. 1994). When fatty acids are flowing into the adipocytes during fat storage, the ‘transcapillary flux’ of fatty acids is said to be positive, and when fatty acids are flowing out of the adipocytes into the capillaries during fat mobilisation, the ‘transcapillary flux’ of fatty acids is said to be negative. The ‘transcapillary flux’ of fatty acids measured in vivo over a 24-hour period is shown in Figure 1. Zero represents no flow of fatty acids between the adipocytes and the capillaries. After an overnight fast, the body is in a state of fat mobilisation and the ‘transcapillary flux’ of fatty acids is negative, meaning that there is a net efflux of fatty acids from the adipocytes. After consuming a meal, the outflow of fatty acids from the adipocytes is stopped and fat storage is activated. This means that ‘transcapillary flux’ of fatty acids is now positive. Fat storage increases incrementally after each meal, indicating that adipose tissue adapts to each successive meal by up-regulating the fat storage pathway. The ‘transcapillary flux’ of fatty acids does not become negative again until the early hours of the morning when the body switches back into a state of fat mobilisation. The total negative area under the curve demonstrates the amount of fat mobilised over a 24-hour period and the positive area under the curve demonstrates the amount of fat stored. During a state of energy balance, the total of the negative area under the curve and the positive area under the curve equals zero. This fat balance could easily be tipped if more or less food was consumed or more energy or less expended. These data demonstrate that healthy adipose tissue performs a very useful function in helping the body to manage the daily influx of dietary fat (Frayn 2002). The uptake of dietary fat by adipose tissue protects others tissues such as the liver, skeletal muscle and pancreatic β-cell from an excess flux of dietary fatty acids, or what some people have called ‘fat overflow’. The metabolism of fat by adipose tissue appears to be parallel to the metabolism of glucose by the liver. The liver stores excess carbohydrate that enters the circulation in the form of glucose as glycogen and, when fuel is required, glycogen is hydrolysed by enzymes within the hepatocytes to deliver glucose into the bloodstream for other tissues to use. Adipose tissue plays exactly the same role for dietary fat. In the fed state, fat is stored in adipose tissue, which is then hydrolysed by enzymes within the adipocytes during the fasted state to supply other tissues, such as skeletal muscle, with fatty acids for fuel. The body appears to be less able to cope with the influx of dietary fat when there is an excess of adipose tissue, and this might explain the metabolic problems that are associated with obesity. The rate of triacylglycerol extraction into adipose tissue in obese men (BMI 30–31) was measured using the techniques described previously. Figure 2 shows that triacylglycerol extraction into adipose tissue was much less efficient and not as dynamic as that seen in lean men (Fig. 1) (McQuaid et al. 2011). Calculations based on the ‘transcapillary flux’ of fatty acids in obese men indicate that less time is spent storing fat in adipose tissue (i.e. in a positive state of ‘transcapillary flux’) than in lean men. One way of looking at these data is to say that in obesity the adipocytes are full and cannot cope with the inflow of more fat. If fat cannot be deposited into adipose tissue, then it ends up in other tissues. Fat stored in non-adipose tissue (now often called ‘ectopic fat’) is strongly associated with adverse metabolic changes seen in obesity, such as insulin resistance, which may underlie increased risk of type 2 diabetes and cardiovascular disease. Thus, excessive adipose tissue is detrimental to health because of the impact this has on fat metabolism in the body. These effects on fat metabolism are also evident when there is a deficiency of adipose tissue. There is a genetic condition called lipodystrophy whereby people lack adipose tissue, either partially or totally, and they almost universally have a metabolic phenotype identical to people with too much adipose tissue: they become insulin resistant, nearly always develop type 2 diabetes and have high blood pressure (Garg 2011). This is because when there is not enough adipose tissue to safely remove dietary fat, other tissues are not protected from the excess flux of dietary fatty acids, which is identical to what happens in obesity. This lecture has highlighted that our fat stores are dynamic. They turn over on a year-to-year basis but also, more interestingly, on a minute-to-minute and hour-to-hour basis. It has also highlighted that adipose tissue plays a special role in helping the body to cope with the influx of dietary fat that occurs after every meal, just as the liver does for glucose, but when there is too much or too little adipose tissue there will be adverse consequences: ectopic fat deposition (fat deposited in other tissues), insulin resistance, type 2 diabetes and cardiovascular disease. As with all things dietary, adipose tissue is beneficial in moderation but dangerous to health in excess. I would like to thank my many colleagues, too numerous to mention individually, who have worked with me over the years, and our funding bodies, especially Wellcome Trust, British Heart Foundation, Biotechnology and Biological Sciences Research Council and Humane Research Trust. I owe special thanks to Dr Lucy Chambers of the British Nutrition Foundation, who helped me enormously with the task of putting this onto paper.

Topic: Adipose Tissue and Metabolism
Subfield: Physiology
Field: Medicine
Domain: Health Sciences