Nutrition fundamentals
Nutrition science divides what the body needs into water, macronutrients, micronutrients and the microbiota. It measures the macronutrients by the energy they supply, classifies foods by how far they are processed, divides vitamins by their solubility, and treats antioxidants as active substances that are not nutrients. The mainstream dietary recommendations converge on a more plant-based diet built from whole foods.
Variation in figures and definitions
Definitions and recommendations in nutrition vary, so a figure given in one source may be similar to, but different from, one found elsewhere. The quantities below, and many of the definitions, are for healthy individuals and do not account for physical activity or disease states, which change them a lot.
Reductionism and the complexity of food
Modern nutrition science emerged in 1926 with the discovery of the first vitamins. It began with a reductionist approach, simplifying things in order to understand them, as science, and especially medicine, often does. It focused first on discovering the micronutrients, by noticing the diseases their deficiencies caused, and then on the macronutrients. Those initial observations produced reasonable, simplified and practical theories, but the great complexity of the whole subject, of foods and of how they function to provide nutrition, is now recognised. Recent discoveries show that overall diet quality better explains the effect of diet on disease, and they show the importance of whole foods. It was once thought that an extract or a vitamin concentrate could fix this, but studies with whole foods have found that the whole foods work better.
The daily human diet typically includes about 100,000 chemicals, and whole foods contain a great many substances.
Nutrition is like a symphony, in which everything works together, and it cannot be fixed with one supplement for each problem. There is consensus that the quality and types of food deserve more attention than counting calories.
What the body needs
What a person needs to consume to stay healthy divides into water, macronutrients, micronutrients and the microbiota.
Water is the most important thing the body needs to stay alive, but it is not a nutrient. It is essential for life because the body is made of 60% water. Water serves as a transport mechanism for organic and inorganic substances, as the medium for intracellular metabolic reactions in a body that works like a very intricate chemistry lab, and for cooling. Humans need about two and a half litres a day, a figure that does not account for physical activity or states such as fever, and on average they excrete the same amount.
The macronutrients, carbohydrates, protein and fat, are the bulk of what is eaten, and the currency in which they are measured is the calorie. Some places use joules instead, and many scientific journals and books may use joules, while the calorie is the term most people know, so the conversion between the two can be useful. Per gram, carbohydrate supplies about four kilocalories, protein four, fat nine, which makes it the most calorie-dense nutrient, and fibre about two, because some fibre has been found to be digestible, especially with the help of bacteria in the intestines.
A portion of pasta contains 42 grams of carbohydrate, a portion of potato 27, of chickpeas 20 and of banana 27. Multiplied by four, each figure gives the calories the portion supplies, which can be set against a man's daily total of about 3,000.
Carbohydrates and the glycaemic index
Carbohydrates are made only of carbon, hydrogen and oxygen. They are the body's main source of energy, and the brain and exercising muscle prefer them by far over any other nutrient. They are either simple, the sugars such as monosaccharides and disaccharides, or complex, the starches and fibre such as oligosaccharides and polysaccharides. Their optimal sources, meaning what should ideally be eaten for optimal health, are whole cereals, vegetables, legumes and fruit. Their optimal quantity depends, but it could be between 55% and 75% of calories. The health benefits of eating whole grains include less constipation, a lower risk of heart disease and help with weight control.
Fibre is made of undigestible complex carbohydrates. Soluble fibre, which dissolves in water, helps lower glucose and cholesterol, and insoluble fibre helps food move through the digestive system. The best sources are whole grain foods, vegetables, fruits, legumes, nuts and seeds, at a minimum of 20 to 30 grams a day, but most people get only about 15 grams a day.
The glycaemic index measures how rapidly a carbohydrate raises blood glucose, on a scale from 1 to 100, where 100 is the rise caused by eating pure glucose. White bread has a glycaemic index of 75, banana 51 and soybean 16. High-glycaemic-index foods raise blood sugar rapidly and make insulin spike, which brings on a bout of hypoglycaemia and hunger and eventually leads to weight gain, fatty liver disease and insulin resistance.
Saturated fat raises postprandial insulin, the insulin after eating, more potently, and it too causes insulin resistance and fatty liver disease. It does so through lipotoxicity in the liver, pancreas, muscle and many other organs, which is one of the base pathophysiological mechanisms behind metabolic syndrome and diabetes.
Protein
Proteins are essential because they supply the amino acids from which the body builds structural, immune, transport and energy-source molecules. Unlike fat and carbohydrate, protein contains nitrogen, so a stated protein content is usually measured from the amount of nitrogen. Protein is not to be used as fuel and should be kept for structure, immunity and transport. There are 20 amino acids, eight or nine of them essential, because whether one of them is essential varies.
A complete protein contains all the essential amino acids and an incomplete one does not. An incomplete protein's limiting amino acid is the one it lacks, and another food can be eaten to compensate for it. The body keeps pools of amino acids and uses them as needed, so protein synthesis is fine as long as different food sources are eaten over a day.
The optimal quantity for adults is less than one gram per kilogram, or about 10% of total calories, which is about 46 grams a day for women and 56 for men. Newborns need 2.2 grams per kilogram, because they build a lot of structure in their first year, roughly doubling their weight, and they obtain it almost entirely from milk. World protein supply is practically double the requirement, and it is higher than needed overall even in disadvantaged regions, although those regions do have undernutrition problems. Overall intake is about 12% of total calories, when 10% is more than enough.
Excess protein is not used to grow more structure; because it is extra energy, it is either used as energy or deposited as fat. Many people think growing muscle needs a lot of protein, but muscle hypertrophy requires mechanical tension, micro-trauma and metabolic stress, which come together in training sessions, usually resistance or weight-bearing training. The tiredness or soreness the day after exercise is micro-trauma, and it stimulates the muscle to take up protein. People who do sports do not need a higher protein intake, even if their need may be a little greater; a higher intake is only for cases such as endurance athletes, those riding the Tour de France for example, and weightlifters who do bodybuilding, perhaps for teenage athletes, and not for most people. Higher consumption, especially of animal protein, is related to weight gain, higher cholesterol, and increased risk of cardiovascular disease, cancer, kidney stones and constipation, among other things.
The optimal sources of protein are cereals, legumes, nuts, seeds, fish, lean meats and nonfat dairy, and a portion of an animal source contains a good deal more protein than a portion of a plant source. The portion of meat that textbooks and articles usually mean is about the size of a deck of cards or the palm of a hand, and a chicken breast, many steaks and portions of fish are a good deal larger.
What matters is the whole package a food arrives in. Animal sources bring saturated fat, dietary cholesterol, trans fats, hormones, antibiotics, insulin-like growth factor 1 and lipophilic environmental contaminants, the persistent organic pollutants such as insecticides, pesticides and industrial chemicals including dioxins. Plant sources bring fibre, vitamins, minerals and phytochemicals, and they require only attention to variety and to eating an appropriate amount of food in a day.
Fats
Fats serve as energy stores and as an energy source when needed, as structure in all cell walls, as transport for lipophilic substances, and as protection for vital organs. Saturated fats have no double bonds, monounsaturated fats one and polyunsaturated fats more than one, which is where the names come from. Unsaturated fats are liquid at room temperature, as in vegetable oils and cold-water fish, and saturated fats are solid, as in animal fat, butter and coconut oil. The optimal sources are nuts, seeds and cold-water fish. The optimal quantity of fat varies, from 15% to 30% of calories, and saturated fat should be under 7%. A portion of walnuts, sunflower seeds or salmon contains between 12 and 18 grams of fat, a portion of beef 20 grams and of coconut milk 9 grams.
Many articles and social media posts promote coconut oil for its medium-chain fatty acids, but it is 91% saturated fat, the most saturated in a comparison of dietary fats, so it should not be used as a health food. Medium-chain fatty acids have been found to be usable by the body as energy, but coconut products contain very little of them and most of their fat is saturated, so there is little point in paying them much attention.
The chain length of a saturated fatty acid is thought to make no big difference to health, though this is uncertain; the main distinction found everywhere is between polyunsaturated, monounsaturated and saturated fat.
The omega-3 and omega-6 fatty acids are polyunsaturated and monounsaturated fats with good effects, especially the omega-3s, which reduce inflammation, blood triglycerides and cardiovascular disease risk. What matters is the ratio between them. Most people have about 10 to 25 parts omega-6 to one part omega-3, where the ratio should be about 1:1 to 4:1. The two compete, so an excess of omega-6 inhibits the omega-3s. Omega-6s are everywhere: in nuts and grains, and in vegetable oils, which are near-omnipresent in the diet and added on top of whatever the food already contains: corn, safflower, soybean, cottonseed, sesame and sunflower oil. The omega-3 sources are ground or milled flaxseed and flaxseed oil, canola and soybean oil, walnuts, soy foods, and cold-water fish.
Trans fatty acids are partially hydrogenated oils, very common in ultra-processed foods. They came into use a few decades ago for convenience in manufacture and sale: they are solid and so easy to transport, they are cheap, they do not go rancid as polyunsaturated and monounsaturated fats do, and they withstand repeated heating. There is no dietary need for them at all, so the ideal is to consume none, or less than 1% of total calories; about 2% is linked with a 25% increase in cardiovascular disease risk. Because they are heavily linked with cardiovascular disease, the World Health Organization has a ban on them starting in 2018 and is working with many countries to eliminate them, with a little more trouble in Africa and Asia.
The liver makes all the cholesterol the body needs, and the body has many mechanisms to reabsorb and recycle it. Cholesterol makes up cell membranes and is used to make hormones and vitamin D. Dietary cholesterol is in all animal foods, because it sits in animal cell membranes, so it is in the flesh and not only in the visible fat. No more than 200 milligrams a day should be consumed, because dietary cholesterol is associated with a higher risk of many cancers, cardiovascular disease and all-cause mortality; it has no calories but does have effects in the body. A portion of beef contains 74 milligrams, chicken breast and salmon are close to that, and an egg contains 213.
Alcohol
Alcohol behaves as fat in the body. It is very calorie-dense, at seven kilocalories per gram. For a man drinking moderately, one or two drinks a day, it makes up about 10% of total calories, and for someone with alcoholism up to 50%, and these are empty calories, with no nutrients attached.
Studies have associated moderate consumption with some benefits, which may come from the other components of alcoholic beverages, including some antioxidants. Red wine is the best-known and best-researched case, and it has more antioxidants than other beverages because it is made with the skin of the grape. The skins and peels of fruit, vegetables and pulses carry a lot of nutrients, which a whole grape also supplies.
Food processing and the NOVA classification
NOVA, a name and not an acronym, classifies foods into four groups by processing:
- Group 1, unprocessed and minimally processed foods: completely unaltered foods such as whole fruits, seeds, vegetables, a whole piece of meat or a raw egg, and foods that have had something done to them, such as squeezing, freezing, drying or pasteurising.
- Group 2, processed culinary ingredients: salt, sugar, honey, vegetable oils, butter.
- Group 3, foods altered a little more: salted, sugared or candied, cured, smoked or canned, together with cheeses, fresh bread from a bakery, and alcoholic drinks.
- Group 4, ultra-processed foods: foods in which the original food cannot be recognised at all, such as soda, snacks, ready-to-eat and instant foods, ice cream, chocolate, candies and margarines.
Group 4 foods are quite solidly linked with many diseases associated with weight gain and with cancer risk, and other studies have linked them with diabetes and cardiovascular disease. In one study, one group ate whole, unaltered foods, including calorie-dense ones such as meat, eggs, cheese and dairy, and another ate their ultra-processed equivalents; the whole-food group lost weight and the ultra-processed group gained it.
Vitamins and minerals
Micronutrients are the vitamins and minerals, called micro because they are needed in very small amounts, and they are essential to health nonetheless. Vitamins are organic compounds, because they contain carbon, with regulatory functions. They are not chemically related to one another and have different physiological roles, so they are a very heterogeneous group. Minerals form body structures from bone to teeth and hair and regulate chemical reactions, sometimes working together with vitamins. Vitamins are divided by how they behave chemically into water-soluble and fat-soluble.
The water-soluble vitamins, vitamin C and the eight B vitamins, are absorbed into the blood and cannot be retained for long, and any excess eaten is excreted in urine. They are easily damaged or lost in cooking and storage, so they are easier to find in fresh food. Although they are excreted, a huge amount of them could be toxic, a problem especially for people taking supplements, so patients should always be asked whether they take supplements and what is in them. Vitamin C is an antioxidant used in the synthesis of many substances, such as collagen and neurotransmitters, and it comes from vegetables and fruits, preferably fresh or at least not cooked. Humans are one of the few mammals that cannot synthesise it, along with monkeys, some birds and bats. The B-complex vitamins can be classified by function into energy-releasing vitamins, haematopoietic vitamins that help produce blood cells, and some with other functions, and they come from various plant and animal foods. Grains fortified with folic acid, eaten before and during pregnancy, can help reduce neural tube defects in the fetus.
The body stores vitamin B12 in the liver and other tissues, in amounts that could last about three to five years. Newborns have no such store, so it is important that the mother consumes it or takes a supplement. Vitamin B12 is made not by animals but by bacteria, which live in the soil, at the roots of some plants and at the end of animal intestines. Because it is made around the colon, humans cannot absorb the B12 made there, but it is abundant in faeces. Animals store it as humans do, so it is in their meat and a little in their milk. It is also present in fortified cereals and plant milks and in vitamin supplements, and it is the one nutrient that vegans will definitely need to supplement.
The fat-soluble vitamins, A, D, E and K, are stable to heat and are not lost in cooking and food processing. Any excess is stored in tissues and organs rather than lost in urine, and they can be toxic if taken in high amounts, especially in metabolically activated forms such as retinol. Eating normal amounts of foods containing them is not a problem; the problem could be high amounts taken as supplements. Vitamin A comes in an active form from animal foods and as beta-carotene, a precursor, from plants. Supplements that contain the active form can be toxic much more easily, so many supplements now present it as carotene. The fat-soluble vitamins serve the development and functioning of the eyes, skin, lungs, bones, teeth, the nervous and immune systems and blood components; they interact with one another and with minerals, and they come from plant and animal foods.
Vitamin D comes as D2 and D3. It is the only nutrient the body can make in its skin, on exposure to ultraviolet light, which means without sunscreen, although that is no recommendation to go without sunscreen all the time. About 15 to 20 minutes with the arms, face and neck exposed is said to be enough to produce it, but this depends on skin colour and latitude. Deficiency is more common in dark-skinned people living in northern or southern parts of the world, who may then need a supplement.
Antioxidants and phytochemicals
Antioxidants are biologically active substances that are not nutrients. They protect cells from free radicals, which attack DNA, cell and intracellular membranes and proteins, and cause or contribute to ageing and to the pathogenesis of many degenerative chronic diseases. Through these effects, antioxidants also stimulate the immune system, reduce inflammation, regulate hormones and have many specific anti-cancer functions. One of these is inducing apoptosis, the programmed suicide by which a damaged cell stops reproducing, which serves to stop a cancer or a precancerous lesion.
There are thousands of antioxidants: vitamins such as C, E and beta-carotene, minerals such as selenium and manganese, and the phytochemicals, which are starting to receive more attention. The phytochemicals include flavonoids, phenols, polyphenols and phytoestrogens, and there are tens of thousands of them. They are present especially in plant foods and are in the colours of foods: blueberries are blue because of a phytochemical, and the bitter flavour of broccoli or Brussels sprouts comes from phytochemicals too. They are powerful substances in plant foods, about 64 times more active there than in foods from animals. Phytochemicals supply no energy, but they are quite important.
One hundred grams of apple with the skin contains only 5.7 milligrams of vitamin C, yet it has the same activity as a 1,500-milligram vitamin C supplement, because the apple has hundreds of other substances that work together. Reductionist thinking extracts one component and takes it to be all the food has. The substances that work together in a whole food also have many other effects, and their anti-proliferative and anti-angiogenic effects are being studied as especially good for helping fight cancer.
Dietary recommendations
Studies keep finding the Mediterranean diet, the Dietary Approaches to Stop Hypertension (DASH) diet, Canada's dietary recommendation, Harvard's Healthy Eating Plate and the Planetary Health Diet to be beneficial. The recommendations and the study evidence point the same way: a more plant-based diet is better for health.
Their shared plate is about half vegetables with some fruit, a quarter whole grains and a quarter protein foods, with water as the drink of choice and nothing else. The protein foods could come from plant and animal sources, but they should come more from plant ones to improve health outcomes. Harvard's Healthy Eating Plate is the same, with a side note for healthy oils and with protein from nuts, beans, poultry and fish. It limits red meat and cheese and avoids bacon and processed meats, which the World Health Organization classes as a Group 1 carcinogen, as it does smoking. The Planetary Health Diet came out of the EAT–Lancet Commission, for people's health and planetary health. Besides the half plate of vegetables, it covers added sugars, salt and saturated plant oils, mostly plant-source protein with a little animal-source protein and dairy, a little starchy vegetable and a lot of whole grain. The foods that make it up are all whole foods. The plate describes how to eat across a day, not how every dish should look.
Population intake
In the United Kingdom and Germany, the shares of the macronutrients in what is eaten have stayed about the same, with rises and falls that can be related to policies and economic conditions, while the amount eaten has been going up. Intake has risen from 3,200 to 3,400 kilocalories a day in the United Kingdom and from 2,800 to 3,500 in Germany, against an average requirement of about 2,400 for women and 3,000 for men. The excess makes the problem of overweight and obesity easy to see. Translated into food, about 64% of what is eaten is meat, dairy and processed foods and only 36% plant-based foods. In the United Kingdom and Germany, carbohydrate makes up about 52% of calories and fat about 37%, above the range recommended for fat, and much of that fat is saturated fat, dietary cholesterol and trans fat. Intake has risen more steeply still in China and India, and fat consumption has been rising across regions since 1960.
Against the EAT–Lancet Commission's health boundaries, consumption of beef stands at 288% and of potatoes at 296%, egg consumption is very excessive, and everything else is under the boundary, with the plant sources notably under-consumed when it should be the other way round. These figures are given both as how the world eats and as North America's consumption; they do not strictly hold for Europe, though European consumption is close to them.