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Why Low-Carb Diets Must Be High-Fat, Not High-Protein





Fat is the most valuable food known to Man

PROFESSOR JOHN YUDKIN

Introduction

We now know that we should eat a diet that is low in carbohydrates. But a plethora of books published in the last decade have been low-carb, high-protein, or low-carb, high-fat, or low-carb, high-'good'-fats, or all sorts of other mixtures. In other words, the real confusion lies in what we should replace the carbohydrates with: for example, should it be protein or fats? And if fats, what sort of fats? This article, I hope, will answer the question and put any doubts out of your mind. In a nutshell, carbs should be replaced with fats, and those fats should be mainly from animal sources.



Our bodies use carbs for only one purpose: to provide energy. When we cut down on carbs, the energy our bodies need has to come from somewhere else.



There are only two choices: Protein or fat.

ATP: our bodies' fuel

The fuel that our body cells use for energy is actually neither glucose nor fat, it is a chemical called adenosine triphosphate (ATP). A typical human cell may contain nearly one billion molecules of ATP at any one moment, and those may be used and re-supplied every three minutes.[i] This huge demand for ATP, and our evolutionary history, has resulted in our bodies' developing several different pathways for its manufacture.



Oxygen and mitochondria

Living organisms have two means to produce the energy they need to live. The first is fermentation, a primitive process that doesn't require the presence of oxygen. This is the way that anaerobic (meaning 'without oxygen') bacteria break down glucose to produce energy. Our body cells can use this method. The second — aerobic (meaning 'using oxygen') — method began after the Earth began to cool down and its atmosphere became rich in oxygen. After this event, a new type of cell — a eukaryotic cell — evolved to use it. Today all organisms more complex than bacteria use this property and all animal life requires oxygen to function. When we breathe in, our lungs are used to extract the oxygen in air and pass it to the bloodstream for transport through the body. And in our bodies, it is our body cells' mitochondria — little power plants that produce most of the energy our bodies need — that use this oxygen. The process is called 'respiration'. This process takes the basic fuel source and oxidises it to produce ATP. The numbers of mitochondria in each cell varies, but as much as half of the total cell volume can be mitochondria. The important point to note is that mitochondria are primarily designed to use fats.

Which source of base material is best?

The question now, in this era of dietary plenty, is: Which source is healthiest? There are three possible choices:

glucose, which comes mainly from carbohydrates, although protein can also be utilised as a glucose source by the body if necessary;

which comes mainly from carbohydrates, although protein can also be utilised as a glucose source by the body if necessary; Fats, both from the diet and from stored body fats;

both from the diet and from stored body fats; Ketones which are derived from the metabolism of fats

Not all cells in our bodies use the same fuel.



Cells that can employ fatty acids are those that contain many mitochondria: heart muscle cells, for example. These cells can make energy from fatty acids, glucose, and ketones, but given a choice, they much prefer to use fats.

Cells that cannot use fats must use glucose and/or ketones, and will shift to preferentially use ketones. These cells also contain mitochondria.

But we also have some cells that contain few or no mitochondria. Examples of cells with few mitochndria are white blood cells, testes and inner parts of the kidneys; and cells which contain no mitochondria are red blood cells, and the retina, lens and cornea in the eyes. These are entirely dependent on glucose and must still be sustained by glucose.

This means that when we limit carb intake, the same energy sources must be used, but a greater amount of energy must be derived from fatty acids and the ketones derived from fatty acids, and less energy from glucose.

Sources of glucose

To understand how a low carb diet works, we need to look at how we eat. This process is one of eating, digestion, hunger and eating again. During our evolution, we also must have experienced long periods when food was in short supply and we starved. This is a pattern our bodies are adapted to. And they have developed mechanisms to cope with a wide range of circumstances. Firstly, the human body must contain adequate levels of energy to sustain the essential body parts that rely on glucose. The brain and central nervous system may be a particular case as, although the brain represents only a small percentage of body weight, it uses between twenty and fifty percent of all the resting energy used by the body.[ii] Fortunately the brain can also use ketone bodies derived from fats. During fasting in humans, and when we are short of food, blood glucose levels are maintained by the breakdown of glycogen in liver and muscle and by the production of glucose primarily from the breakdown of muscle proteins in a process called gluconeogenesis, which literally means 'glucose new birth'.[iii]



But we don't want to use lean muscle tissue in this way: it weakens us. We want to get the glucose our bodies need from what we eat. Some of that will come from carbs, the rest from dietary proteins. Our bodies need a constant supply of protein to sustain a healthy structure. This requires a fairly minimal amount of protein: about 1 to 1.5 grams per kilogram of lean body weight per day is all that is necessary to preserve muscle mass.[iv] Any protein over and above this amount can be used as a source of glucose.



Dietary proteins are converted to glucose at about fifty-eight percent efficiency, so approximately 100g of protein can produce 58g of glucose via gluconeogenesis.[v] During prolonged fasting, glycerol released from the breakdown of triglycerides in body fat may account for nearly twenty percent of gluconeogenesis.[vi] Body fats are stored as triglycerides, molecules that contain three fatty acids combined with glycerol. The fatty acids are used directly as a fuel, with the glycerol stripped off. This is not wasted. As the glycerol is nearly ten percent of triglyceride by weight and two molecules of glycerol combine to form one molecule of glucose, this also supplies a source of glucose.



The case for getting energy from fat and ketones

When most people think of eating a low-carb diet, they tend to think of it as being a protein-based one. This is false. All traditional carnivorous diets, whether eaten by animals or humans, are more fat than protein with a ratio of about eighty percent of calories from fat and twenty percent of calories from protein. Similarly, the main fuel produced by a modern low-carb diet should also be fatty acids derived from dietary fat and body fat. We find in practice that free fatty acids are higher in the bloodstream on a low-carb diet compared with a conventional diet.[vii] [viii]



But fats also produce an important secondary fuel: 'ketone bodies'. Ketones were first discovered in the urine of diabetic patients in the mid-19th century; for almost fifty years thereafter, they were thought to be abnormal and undesirable by-products of incomplete fat oxidation. In the early 20th century, however, they were recognised as normal circulating metabolites produced by liver and readily utilised by body tissues. Ketones are an important substitute for glucose. During prolonged periods of starvation, fatty acids are made from the breakdown of stored triglycerides in body fat.[ix] On a low-carb diet, the fatty acids are derived from dietary fat, or body fat if the diet does not supply enough. Free fatty acids are converted to ketones by the liver. They then provide energy to all cells with mitochondria. Within a cell, ketones are used to generate ATP. And where glucose needs the intervention of bacteria, ketones can be used directly. Reduction of carbohydrate intake stimulates the synthesis of ketones from body fat.[x] This is one reason why reducing carbs is important. Another is that reducing carbohydrate and protein intake also leads to a lower insulin level in the blood. This, in turn, reduces the risks associated with insulin resistance and the Metabolic Syndrome.



Ketone formation and a shift to using more fatty acids also reduces the body's overall need for glucose. Even during high-energy demand from exercise, a low-carb diet has what are called 'glucoprotective' effects. What this all means is that ketosis arising from a low-carb diet is capable of accommodating a wide range of metabolic demands to sustain body functions and health while not using, and thus sparing, protein from lean muscle tissue. Ketones are also the preferred energy source for highly active tissues such as heart and muscle.[xi]



All this means that more glucose is available to the brain and other essential glucose-dependent tissues.

The case against getting energy from protein

We know, then, that dietary fats can produce all the energy the body needs, either directly as fatty acids or as ketone bodies. But, as there is still some debate about the health implications of using fats, why not play safe and eat more protein?



There is one simple reason: While the body can use protein as an energy source in an emergency, it is not at all healthy to use this method in the long term. All carbs are made up of just three elements: carbon, hydrogen and. oxygen. All fats are also made of the same three elements. Proteins, however, also contain nitrogen and other elements. When proteins are used to provide energy, these must be got rid of in some way. This is not only wasteful, it can put a strain on the body, particularly on the liver and kidneys.



Excess intake of nitrogen leads in a short space of time to hyperammonaemia, which is a build up of ammonia in the bloodstream. This is toxic to the brain. Many human cultures survive on a purely animal product diet, but only if it is high in fat.[xii] [xiii] A lean meat diet, on the other hand cannot be tolerated; it leads to nausea in as little as three days, symptoms of starvation and ketosis in a week to ten days, severe debilitation in twelve days and possibly death in just a few weeks. A high-fat diet, however, is completely healthy for a lifetime.



Perhaps one of the best documented studies is that of the Arctic explorer, Vilhjalmur Stefansson and a colleague.[xiv] They ate an animal meat diet for more than a year to see whether such a diet could be healthy. Everything was fine until they were asked to eat only lean meat. Dr McClelland, the lead scientist, wrote:

'At our request he began eating lean meat only, although he had previously noted, in the North, that very lean meat sometimes produced digestive disturbances. On the third day nausea and diarrhea developed. When fat meat was added to the diet, a full recovery was made in two days.'

This was a clinical study, but Stefansson had already lived for nearly twenty years on an all-meat diet with the Canadian Inuit. He and his team suffered no ill effects whatsoever.

Low-carb, high-fat diet and weight loss

There is just one other consideration: If you want to lose weight, the actual material you want to rid your body of is fat. But to do that you have to change your body from using glucose as a fuel to using fat ? including your own body fat. This is another reason not to use protein as a substitute for carbs, as protein is also converted to glucose.



If you think about it, Nature stores excess energy in our bodies as fat, not as protein. It makes much more sense, therefore, to use what we are designed by Nature to use. And that is fat.

So what levels of carbs, fats and proteins are required?

Clinical experience and studies into low-carb diets over the last century suggest that everybody has a threshold level of dietary carbohydrate intake where the changeover from glucose-burning to fat and ketone burning takes place. This varies between about sixty-five and 180 grams of carbs per day.[xv] If your carb intake is below this threshold, then your body fat will be broken down to generate ketones to supply your brain and other cells that would normally use glucose. In the early trials for the treatment of obesity, carb levels were very much reduced to supply only about ten percent of calories. This works out at around fifty or sixty grams of carb for a 2,000 calorie daily intake.



For diabetics, the level may need to be lower to counteract insulin resistance. Typical levels of carb intake for a type-2 diabetic are around fifty grams per day; the level should be lower still at about thirty grams a day for a type-1 diabetic.



A Polish doctor, Jan Kwasniewski, who has used a low-carb diet to treat patients with a wide range of medical conditions for over thirty years, recommends a ratio of one part carb to two parts protein to between three and four parts fat, by weight. I see no reason to disagree with this. What it means in practice is that on a 2,000 calorie per day diet, we should get:

Ten to fifteen percent of calories from carbs Twenty to thirty percent of calories from protein and Sixty to seventy percent of calories from fats.

Or put another way, as it is difficult to work out percentages in this way, fifty to seventy-five grams of carb and the rest from meat, fish, eggs, cheese, and their natural fats.

Potential for other diseases

The traditional Inuit (Eskimo) diet is a no-carb diet. It is notable that the Inuit diet described by Drs Vilhjalmur Stefansson and Hugh Sinclair in the 1950s is very similar in regard to percentages of fat/protein/carb intake to the experimental low-carb diets used in recent obesity studies.[xvi] The Inuit diet was comprised of seal, whale, salmon, and a very limited amount of berries and the partially digested contents of animals' stomachs. On this diet, blood cholesterol levels were very high as were free fatty acids, but ? and this in much more important ? triglycerides were low.[xvii] [xviii] It is interesting to note that the Inuit were of great interest to research scientists because they had practically none of the diseases we suffer, including obesity, coronary heart disease and diabetes mellitus.[xix] [xx]

References

[i]. Alberts B. Molecular Biology of the Cell, edn 4. New York: Garland Science; 2002: p 93.

[ii]. Cahill GF. Survival in starvation. Am J Clin Nutr 1998; 68:1-2.

[iii]. Exton JH. Gluconeogenesis. Metabolism 1972; 21:945-990.

[iv]. Volek JS, Sharman MJ, Love DM, et al. Body composition and hormonal responses to a carbohydrate-restricted diet. Metabolism 2002; 51:864?870

[v]. Krebs HA. The metabolic fate of amino acids. In Mammalian Protein Metabolism, vol 1, Munro HN, Allison JB, eds. New York: Academic Press, 1964:164

[vi]. Vazquez JA, Kazi U. Lipolysis and gluconeogenesis from glycerol during weight reduction with very low calorie diets. Metabolism 1994; 43:1293?1299.

[vii]. Phinney SD, Bistrian BR, Wolfe RR, Blackburn GL. The human metabolic response to chronic ketosis without caloric restriction: physical and biochemical adaptation. Metabolism 1983; 32:757?768.

[viii]. Bisshop PH, Arias AM, Ackermans MT, et al. The effects of carbohydrate variation in isocaloric diets on glycogenolysis and gluconeogenesis in healthy men. J Clin Endocrinol Metab 2000; 85:1963?1967.

[ix]. Cahill GF Jr. Starvation in man. N Engl J Med 1970; 19:668-675.

[x]. Klein S, Wolfe RR. Carbohydrate restriction regulates the adaptive response to fasting. Am J Physiol 1992; 262:E631?E636.

[xi]. Neely JR, Morgan HE. Relationship between carbohydrate and lipid metabolism and the energy balance of heart muscle. Annu Rev Physiol 1974; 36:413?459.

[xii]. Speth, John D. and Katherine A. Spielmann 1982 Energy source, protein metabolism, and hunter-gatherer subsistence strategies. Journal of Anthropological Archaeology 2:1-31.

[xiii]. Noli & Avery. Protein poisoning and Coastal Subsistence. J Archaeol Sci. 1988; 15:395-401

[xiv]. McClelland, et al. Clinical Calorimetry: XLV, XLVI, XLVII. Prolonged Meat Diets...... J Biol Chem 1930-31; 87:651, 87:669, 93:419

[xv]. Klein S, Wolfe RR. Op cit.

[xvi]. Stefansson V. The Fat of the Land. Macmillan Press, New York, 1957.; Sinclair HM: The diet of Canadian Indians and Eskimos. Proc Nutr Soc 1952, 12:69?82.

[xvii]. Bang HO, Dyerberg J, Nielsen AB: Plasma lipid and lipoprotein pattern in Greenlandic West-Coast Eskimos. Lancet 1971; I:1143?1146.

[xviii]. Feldman SA, Ho KJ, Lewis LA, et al. Lipid and cholesterol metabolism in Alaskan arctic Eskimos. Arch Pathol 1972; 94:42?58.

[xix]. Bjerregaard P, Dyerberg J: Mortality from ischaemic heart disease and cerebrovascular disease in Greenland. Int J Epidem 1988, 17:514?519.

[xx]. Sagild U, Littauer J, Jespersen CS, Andersen S: Epidemiological studies in Greenland 1962?1964. I. Diabetes mellitus in Eskimos. Acta Med Scand 1966, 179:29?39.

Last updated 11 June 2005

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