Nutrition

Nutrition is the study of the relationship between diet and states of health and disease. The vast majority of matter and energy required by an animal is obtained through the diet. Obviously, if an animal does not ingest any food or drink, it will not be able to meet its needs for matter or energy and will eventually die. Between the extremes of optimal health and death from starvation, there are an array of disease states that can be caused or alleviated by changes in diet. For example, a particular diet may meet an organism's requirements energy and protein, but lack a particular substance that the animal requires, such as Vitamin C. Without vitamin C, the organism will eventually suffer from some variety of sub-optimal health, such as scurvy. In other cases, excesses or imbalances in the diet can have have negative impacts on health, resulting in disease states such as obesity or osteoporosis. Finally, substances such as lead may be ingested by an organism have specific toxic effects.

The science of nutrition attempts to understand how and why specific aspects of diet have specific influences on health.

Overview

The human body comprises chemical compounds such as water, amino acids (proteins), fatty acids (lipids), nucleic acids (DNA/RNA), and carbohydrates (e.g. sugars). These compounds in turn consist of elements such as carbon, hydrogen, oxygen, nitrogen, and phosphorus, and may or may not contain minerals such as calcium, iron, and zinc. Minerals also ubiquitously occur in the form of salts and electrolytes. All of these chemical compounds and elements occur in various forms and combinations (e.g. hormones/vitamins, phospholipids, hydroxyapatite), both in the human body and in organisms (e.g. plants, animals) that humans eat.

The human body must necessarily comprise those elements that humans eat and absorb into the bloodstream. The digestive system, except in the unborn fetus, is the first step in helping to make the different chemical compounds and elements in food available for the trillions of cells of the body. In the digestive process of an average adult, about seven (7) litres of liquid, known as digestive juices, exit the internal body and enter the lumen of the digestive tract. The digestive juices help break chemical bonds between ingested compounds as well as modulate the conformation and/or energetic state of the compounds/elements. Yet many compounds/elements are absorbed into the bloodstream unchanged, though the digestive process helps to release them from the matrix of the foods where they occur. Any unabsorbed matter is eliminated in the feces. Only a minimal amount of digestive juice is eliminated this way; the intestines reabsorb most of it otherwise the body would rapidly dehydrate (hence the devastating effects of persistent diarrhea).

Study in this field must take into careful account the state of the body before ingestion and after digestion as well as the chemical content of both the food and the waste. The specific types of compounds and elements that are absorbed by the body can be determined by comparing the waste to the food. The effect that the absorbed matter has on the body can be determined by finding the difference between the pre-ingestion state and the post-digestion state. The effect may only be discernible after an extended period of time in which all food and ingestion must be exactly regulated and all waste must be analyzed. The number of variables (e.g. 'confounding factors') involved in this type of experimentation is very high. This makes scientifically valid nutritional study very time-consuming and expensive, which accounts for why a proper science of human nutrition is rather new.

In general, eating a variety of fresh, whole (unprocessed) foods has proven hormonally and metabolically favourable compared to eating a monotonous diet based on processed foods. In particular, fresh, whole foods provide higher amounts and a more favourable balance of essential and vital nutrients per unit of energy, resulting in better management of cell growth, maintenance, and mitosis (cell division) as well as of appetite and energy balance. A generally more regular eating pattern (e.g. eating medium-sized meals every 3 to 4 hours) has also proven more hormonally and metabolically favourable than infrequent, haphazard food intake.

History and recent developments

The role of nutrition in health was recognized by the ancient Greeks - and probably also much earlier - who advised that food should be one's medicine. Especially Eastern cultures have for more than two thousand years used certain foods (e.g. herbs) as remedy. Clearly these early insights were based on simple observations that nutrition - aside from the obvious need to eat sufficient energy - has a direct effect on the health of the organism (though the placebo effect may have overstated its significance in some cases).

In the 18th century, many sailors on long journeys died from scurvy; indeed, some naval ships lost more men through illness than through enemy action. The cause of this was not understood until it was discovered that adding fresh limes to the ship's supply of preserved food seemed to boost the sailors' resistance to the illness and resulted in fewer deaths. The existence of separate compounds/elements in food that are essential for survival had been discovered, and this discovery of vitamin C, which prevents scurvy, spurred the search for other so-called 'essential nutrients' (see next section). 'Vitamins' were first written about in 1912 by Sir Frederick Gowland Hopkins, who was knighted and received the Nobel Prize in 1929 for his achievements.

A hidden epidemic gradually emerged in the post World War II years, where non-communicable endemic illnesses began to flourish, such as heart disease, cancer, diabetes, and obesity. Although medical drugs are used large scale to manage this development, more and more people are becoming aware that the post-war modern lifestyle (e.g. poor choice of foods, less physical activity) may be the primary cause of such so-called lifestyle-related diseases. Despite this realization, however, lifestyle- and obesity-related diseases are becoming increasingly prevalent all around the world (see Nutrition, industry and food processing).

In the latter half of the 20th century, technological developments and a world-wide epidemic of lifestyle related diseases has spurred detailed research into the specific health effects of different foods. For example, in the years following the Second World War, the prevalence of heart disease in Western countries began escalating and came under the scrutiny of nutritional researchers around the world, especially in the USA. An increased intake of fat and cholesterol was thought to be a major cause, but later epidemiologic research has demonstrated that the prevalence of heart disease has continued to increase markedly despite a significant decrease in fat intake. Further, none of the performed randomized controlled trials assessing the role of dietary cholesterol or saturated fat in heart disease convincingly support the hypotheses that dietary cholesterol (unoxidized) and saturated fat (from fresh, whole foods) in and of themselves promote heart disease.

Later research has revealed the potential adverse effects on (heart) health of trans fatty acids, which were introduced to the human diet large scale with the new technology of lipid hydrogenation. Partially hydrogenated oils were added to many processed foods and helped make possible the production of solid margarine from cheap liquid vegetable oils, and helped increase the shelf-life of many foods. In the late 90’s the harmful effects of excess trans fatty acids in the diet came into the attention of the public and governments began restricting or banning its use.

While one ‘camp’ of researchers focused on the hypothetical benefits of reduced intake of fat and cholesterol, another ‘camp’ emphasized the possible benefits (initially for diabetics) of reducing intake of carbohydrates and thereby preventing excess blood sugar raise and stimulation of the fundamental, all-important hormone insulin. In the early 80's, researchers began to systematically measure the amount of sugar in subjects’ blood before and after consumption of foods containing a standard amount of digestible carbohydrate (e.g. 50 grams). The resulting comparable values of the blood sugar raising potential of different carbohydrate foods are ranked in the Glycemic Index. In general, the Glycemic Index demonstrates that eating processed, carbohydrate-dense foods (higher GI) results in a higher blood sugar raise and insulin release than does eating whole, unprocessed carbohydrate-containing foods (lower GI). In more recent years, researchers have focused more on the glycemic load which factors in the amount of carbohydrate eaten, whereas the glycemic index only compares the blood sugar raising potential of foods with a set amount of carbohydrate (i.e. eating large amounts of low GI foods can result in a higher blood sugar raise and insulin stimulation than eating small amounts of high GI foods).

The recent advances in molecular biology and gene technology, for example with the use of 'knock-out' animal models where specific genes are inactivated, have enabled a more informative study of how nutrition 'communicates' with our genes and thus with the chief metabolic hormones. Since the Human Genome Project was completed, the focus has shifted towards study of the proteome. The study of hormones and other cell-to-cell signalling molecules is receiving special attention.

The previous mechanistic view of food merely as fuel thus falls short. Foods and their diverse components are capable of affecting the control of metabolism directly via hormones and genes, to the effect that different foods can for example affect energy expenditure to varying degrees, also when energy intake is constant.

Nutrition and health

Ill health can be brought about by an imbalance of nutrients, producing either an excess or deficiency which in turn affects body functioning in a cumulative manner. In addition, as imbalance and thereby disease. Moreover, because most nutrients are, in some way or the other, involved in cell-to-cell signalling (e.g. as building block or part of a hormone), deficiency or excess of various nutrients affects hormonal function also indirectly. Thus, because they largely regulate the expression of genes, hormones represent a link between nutrition and how our genes are expressed, i.e. our phenotype. The strength and nature of this link are continually under investigation, but observations especially in recent years have demonstrated a pivotal role for nutrition in hormonal activity and function and therefore in health.

Mineral and/or vitamin deficiency or excess may yield symptoms of diminishing health such as goitre, scurvy, osteoporosis, weak immune system, disorders of cell metabolism, certain forms of cancer, symptoms of premature aging, and poor psychological health (including eating disorders). The list goes on and on; for reference, see Modern Nutrition in Health and Disease by Shils et al.

As of 2005, twelve vitamins and about the same number of minerals are recognized as 'essential nutrients', meaning that they must be consumed and absorbed - or, in the case of vitamin D, alternatively synthesized via UVB radiation - to prevent deficiency symptoms and death. Certain vitamin-like substances found in foods, such as carnitine, have also been found essential to survival and health, but these are not strictly 'essential' to eat because the body can produce them from other compounds. Moreover, thousands of different phytochemicals have recently been discovered in food (particularly in fresh vegetables), which have many discovered and yet to be discovered properties including antioxidant activity (see below). Other essential nutrients include essential amino acids, choline and the essential fatty acids.

In addition to sufficient intake, an appropriate balance of essential fatty acids - omega-3 and omega-6 fatty acids - has been discovered to be crucial for maintaining health. Both of these unique 'omega' long-chain unsaturated fatty acids are substrates for a class of eicosanoids known as prostaglandins). Alpha-linolenic acid (LNA) serves as the building block for the less-inflammatory PGE3 series of prostaglandins, whereas linoleic acid (LA) (and specifically its product, arachidonic acid, AA) serves as the building block for either the PGE1 (anti-inflammatory) or the PGE2 (pro-inflammatory) series. (The omega-6 fatty acid LA is the building block for the omega-6 fatty acid AA, but AA can also be obtained directly in the diet). The conversions of AA into the respective prostaglandins (PGE1/PGE2) have importantly been discovered to be under hormonal control, as certain hormones such as insulin and glucagon regulate the function of the enzymes responsible for the conversions. Because different types and amounts of food eaten/absorbed affect insulin, glucagon and other hormones to varying degrees, not only the amount of omega-3 versus omega-6 eaten but also the general composition of the diet is now known to determine health implications in relation to essential fatty acids, inflammation (e.g. immune function) and mitosis (i.e. cell division).

Several lines of evidence indicate lifestyle-induced insulin malfunction, referred to as insulin resistance, as a decisive factor in many disease states. Researchers have for long assumed that overfatness/obesity causes insulin resistance, which in turn causes type 2 diabetes (virtually all obese and diabetic individuals have marked insulin resistance). More recent evidence has however demonstrated that insulin resistance may well be the cause of overfatness/obesity as well as type 2 diabetes and possibly other lifestyle-related diseases. For example, it has been demonstrated that appropriate exercise, more regular food intake and reducing glycemic load all can reverse insulin resistance in overfat individuals, which means that lifestyle rather than the fact of being fat promotes insulin resistance.

Nonetheless, overfatness can unfavourably alter hormonal and metabolic status seemingly especially via the hormone leptin, and a vicious cycle may occur in which insulin/leptin resistance and overfatness aggravate one another. There is debate to what extent different dietary factors, such as intake of processed carbohydrates, total protein-, fat-, and carbohydrate intake, intake of trans fatty acids, and low intake of micronutrients, contribute to develop insulin- and leptin resistance. Most importantly, insulin- and leptin resistance are both strongly linked to chronic inflammation, which in turn is strongly linked to a variety of adverse developments such as arterial microinjuries and clot formation (i.e. heart disease) and over-expressed cell division (i.e. cancer).

A persistent high intake of processed carbohydrate-dense foods (high glycemic load) resulting in repeatedly high release of insulin and leptin seems to readily make cells less responsive to both of these crucial hormones (perhaps especially in combination with high intake of saturated fat/trans fatty acids and sedentary living). This type of negative feedback is ubiquitous in any biological system, all of which depend on appropriate hormonal regulation for survival and proper function. However, certain cell types appear to more readily become resistant to the effects of certain hormones (e.g. insulin, leptin) than others; certain fat cells (e.g. abdominal subcutaneous fat) may respond well to the fat-storing signal of insulin while other cells fail to properly receive/transduce the signal meant to induce important cellular and systemic effects. Why certain cells develop resistance to certain signalling molecules remains unclear, but it seems plausible that hormone resistance serves to protect cells from excess circulating amounts of the given hormone. Analogous to the way modern man-made pollution may potentially overwhelm the environment's ability to maintain homeostasis, the recent explosive introduction of high Glycemic Index- and processed foods into the human diet may potentially overwhelm the body's ability to maintain homeostasis and health (as evidenced by, for example, the obesity epidemic).

Antioxidants are another recent discovery. As cellular metabolism/energy production requires oxygen, potentially damaging (e.g. mutation causing) compounds known as radical oxygen species or free radicals may form. For normal cellular maintenance, growth, and division, these free radicals must be sufficiently neutralized by antioxidant compounds, such as certain vitamins (vitamin C, vitamin E, vitamin K and the aforementioned phytochemicals as well as other compounds, some of which the body itself produces. Different antioxidants are now known to function in a cooperative network, e.g. vitamin C can reactivate free radical-containing glutathione or vitamin E by accepting the free radical itself, and so on.

It is now also known that the human digestion system contains a population of a range of bacteria which are essential to digestion, and which are also affected by the food we eat. The role and significance of the intestinal bacterial flora is under investigation.

It is now commonly accepted amongst the medical profession that a diet high in fresh fruit and vegetables but low in animal fat helps to prevent heart disease and cancer.

Nutrition and longevity

Lifespan is somehow related to the amount of food energy consumed: this was first systematically investigated in the seminal study by Weidruch, et al. (1986). A pursuit of this principle of caloric restriction followed, involving research into longevity of those who reduced their food energy intake while attempting to optimize their micronutrient intake. Perhaps not surprisingly, some people found that cutting down on food reduced their quality of life so considerably as to negate any possible advantages of lengthening their lives. However, a small set of individuals persists in the lifestyle, going so far as to monitor blood lipid levels and glucose response every few months. See Calorie Restriction Society.

Underlying this research was the hypothesis that oxidative damage was the agent which accelerated aging, and that aging was retarded when the amount of carbohydrates (and thereby insulin release) was reduced through dietary restriction.

However, recent research has produced increased longevity in animals (and shows promise for increased human longevity) through the use of insulin uptake retardation. This was done through altering an animal’s metabolism to allow it to consume similar food-energy levels to other animals, but without building up fatty tissue. (Bluher et al, 2003)

This has set researchers off on a line of study which presumes that it is not low food energy consumption which increases longevity. Instead, longevity may depend on an efficient fat processing metabolism, and the consequent long term efficient functioning of our organs free from the encumbrance of accumulating fatty deposits. (Das et al, 2004) Thus, longevity may be related to maintained insulin sensitivity. However, several other factors including low body temperature seem to promote longevity also and it is unclear to what extent each of them contribute.

Nutrition, industry and food processing

Since the Industrial Revolution some two hundred years ago, the food processing industry has invented many technologies that both help keep foods fresh longer and alter the fresh state of food as they appear in nature. Cooling is the primary technology that can help maintain freshness, whereas many more technologies have been invented to allow foods to last longer without becoming spoiled. These latter technologies include pasteurisation, autoclavation, drying, salting, and separation of various components, and all appear to alter the original nutritional contents of food. Pasteurisation and autoclavation (heating techniques) have no doubt improved the safety of many common foods, preventing epidemics of bacterial infection. But some of the (new) food processing technologies undoubtedly have downfalls as well.

Modern separation techniques such as milling, centrifugation, and pressing have enabled upconcentration of particular components of food, yielding flour, oils, juices and so on, and even separate fatty acids, amino acids, vitamins, and minerals. Inevitably, such large scale upconcentration changes the nutritional content of food, saving certain nutrients while removing others. Heating techniques may also reduce food's content of many heat-labile nutrients such as certain vitamins and phytochemicals, and possibly other yet to be discovered substances. Because of reduced nutritional value, processed foods are often 'enriched' or 'fortified' with some of the most critical nutrients (usually certain vitamins) that were lost during processing. Nonetheless, processed foods tend to have an inferior nutritional profile than do whole, fresh foods, regarding content of both sugar and high GI starches, potassium/sodium, vitamins, fibre, and of intact, unoxidized (essential) fatty acids. In addition, processed foods often contain potentially harmful substances such as oxidized fats and trans fatty acids.

A dramatic example of the effect of food processing on a population's health is the history of epidemics of beri-beri in people subsisting on polished rice. Removing the outer layer of rice by polishing it removes with it the essential vitamin thiamin, causing beri-beri. Another example is the development of scurvy among infants in the late 1800's in the United States. It turned out that the vast majority of sufferers were being fed milk that had been heat-treated (as suggested by Pasteur) to control bacterial disease. Pasteurisation was effective against bacteria, but it destroyed the vitamin C.

As mentioned, lifestyle- and obesity-related diseases are becoming increasingly prevalent all around the world. There is little doubt that the increasingly widespread application of some modern food processing technologies has contributed to this development. The food processing industry is a major part of modern economy, and as such it is influential in political decisions (e.g. nutritional recommendations, agricultural subsidising). In any known profit-driven economy, health considerations are hardly a priority; effective production of cheap foods with a long shelf-life is more the trend. In general, whole, fresh foods have a relatively short shelf-life and are less profitable to produce and sell than are more processed foods. Thus the consumer is left with the choice between more expensive but nutritionally superior whole, fresh foods, and cheap, usually nutritionally inferior processed foods. Because processed foods are often cheaper, more convenient (in both purchasing, storage, and preparation), and more available, the consumption of nutritionally inferior foods has been increasing throughout the world along with many nutrition-related health complications.

Policy advice and guidance on nutrition

Most Governments provide guidance on good nutrition, and some also impose mandatory labelling requirements upon processed food manufacturers to assist consumers in complying with such guidance. Current dietary guidelines in the United States are presented in the concept of a food pyramid. Canadian guidelines are similar to those of the United States and are published in "Canada's Food Guide". Nutrition is good for your body.

Current issues and challenges

Challenging issues in modern nutrition include:

'Artificial' interventions in food production and supply:

Should genetic engineering be used in the production of food crops and animals?
Are the use of pesticides, and fertilizers damaging to the foods produced by use of these methods (see also organic farming)?
Are the use of antibiotics and hormones in animal farming ethical and/or safe?

Sociological issues:

How do we minimise the current disparity in food availability between first and third world populations (see famine and poverty)?
How can public advice agencies, policy making and food supply companies be coordinated to promote healthy eating and make wholesome foods more convenient and available?
Do we need nutritional supplements in the form of pills, powders, liquids, etc.?
How can the developed world promote good worldwide nutrition through minimising import tariffs and export subsidies on food transfers?

Research Issues:

How do different nutrients affect appetite and metabolism, and what are the molecular mechanisms?
What yet to be discovered important roles do vitamins, minerals, and other nutrients play in metabolism and health?
Are the current recommendations for intake of vitamins and minerals generally too low?
How and why do different cell types respond differently to chronically elevated circulating levels of insulin, leptin, and other hormones?
What does it take for insulin resistance to develop?
What other molecular mechanisms may explain the link between nutrition and lifestyle-related diseases?
What role does the intestinal bacterial flora play in digestion and health?
How essential to proper digestion are the enzymes contained in food itself, which are usually destroyed in cooking (see Living foods diet)?
What more can we discover through what has been called the phytochemical revolution?

References

Shils et al. Modern Nutrition in Health and Disease, 2005.
Bluher, Khan BP, Kahn CR, Extended longevity in mice lacking the insulin receptor in adipose tissue. Science 299(5606): 572-4, Jan 24, 2003.
The Times newspaper, January 31 2004 Could vitamins help delay the onset of Alzheimer’s? by Jerome Burne.
The Times newspaper February 28, 2004 Autism: I can see clearly now . . . by Simon Crompton
The Times newspaper March 10, 2004 Work up an Amish appetite by Anne-Celine Jaeger
Das M, Gabriely I, Barzilai N.Caloric restriction, body fat and aging in experimental models. Obes Rev. 2004 Feb;5(1):13-9.
William Eaton et al Coeliac disease and schizophrenia British Medical Journal, February 21, 2004.
Janssen I, Katzmarzyk PT, Ross R. Waist circumference and not body mass index explains obesity-related health risk. Am J Clin Nutr. 2004 Mar;79(3):379-84.
J Mei, SSC Yeung et al "High dietary phytoestrogen intake and bone mineral density in postmenopausal women."Journal of Clinical Endocrinology and Metabolism, 2001, Vol 86, Iss 11
Merritt JC "Metabolic syndrome: soybean foods and serum lipids."J Natl Med Assoc. 2004 Aug;96(8):1032-41.
Sobczak S, et al Lower high-density lipoprotein cholesterol and increased omega-6 polyunsaturated fatty acids in first-degree relatives of bipolar patients Psychol Med. 2004 Jan;34(1):103-12.
Walter C. Willett and Meir J. Stampfer,Rebuilding the Food Pyramid, Scientific American January 2003.
Weindruch R, et al. The retardation of aging in mice by dietary restriction: longevity, cancer, immunity and lifetime energy intake. (Journal of Nutrition, 116(4), pages 641-54.,April, 1986.)

External links

UN Standing Committee on Nutrition - In English, French and Portuguese
USDA Frequently asked questions
Texas University: A review of current nutrition academic articles
Nutrition and Nutritional Supplements Information
Department of Agriculture's Food and Nutrition Information Center.
Nutrition Dictionary
Nutrition advice for sports people from Nicholas Institute for Sports Medicine and Trauma
Agriculture & Agri-Food Canada
International Food Policy Research Institute
Food File Online Nutrition information for 1000s of food products
[1] Nutrition and diet information.

Disclaimer

Please remember that Wikipedia is offered for informational use only. The information is in most cases not reviewed by professionals. You are advised to contact your doctor for health-related decisions.