Nutrition Basics: A Lifelong Guide to Energy and Macronutrients

Today, chronic diseases rather than infections are the leading causes of death. As cancer, diabetes, COPD and dementia become increasingly prominent, the lifelong importance of nutrition is clearer. Nutrition not only helps prevent these diseases but also provides powerful support during treatment. This has rightly brought sayings such as " food is medicine" and " you are what you eat" into focus.

The Complexity of Nutrition and the "Food Is Medicine" Approach

The "food is medicine" approach is increasingly studied for preventing and treating chronic diseases. Although conventional medicines have clearly defined molecular targets, diets contain many different components whose effects continue throughout life. Advising patients, particularly those with chronic illnesses, on what and how much to eat is therefore more complex than most health advice.

This comprehensive article summarizes contemporary nutritional concepts, focusing particularly on energy and macronutrients.

Historical Development

The Evolution of Nutritional Science

More than 2000 years ago, Greek philosophers and physicians recognized that food replaces inevitable losses of bodily material, that requirements vary across life stages and that food and air support "natural" body heat.

During the chemical revolution beginning in the late 18th century, scientists led by Antoine Lavoisier showed that body heat results from an oxygen-requiring combustion process, laying the foundations of modern metabolism (Figure 1).

In the first half of the 19th century, Gerardus Johannes Mulder and Jöns Jacob Berzelius identified and named protein, Michel Eugène Chevreul and others developed concepts involving fats and lipids, and many scientists classified sugars and starches as carbohydrates. We now know these are essential macronutrients that serve as metabolic fuels and have many other vital functions.

By the early 20th century, nutrients beyond macronutrients were recognized as necessary for optimal health. The 13 water- and fat-soluble vitamins recognized today were identified between 1912 and 1948. There are also 21 essential minerals, classified as macrominerals requiring 100 mg or more daily, such as calcium, and microminerals requiring less than 100 mg, such as iodine.

The Great Depression's Nutritional Impact and the Development of Recommended Dietary Allowances

The Great Depression caused widespread undernutrition, and the absence of nutrition-support programs was felt when the United States entered the Second World War. In 1940, the National Defense Advisory Commission asked the National Research Council, part of the National Academy of Sciences, to investigate undernutrition in the United States population.

In 1943, the National Research Council published the first Recommended Dietary Allowances (RDAs) for energy, protein and eight vitamins and minerals to prevent nutrient deficiencies. Many updates followed over the next forty years. The final update, published in 1989, included energy, protein and 25 vitamins and minerals.

RDAs were derived by estimating the average daily nutrient intake needed to meet the requirements of half of healthy people of a particular sex and life stage, termed the Estimated Average Requirement. The RDA was set 2 standard deviations above this average, aiming to meet most healthy individuals' nutrient needs (97.5%). When data are insufficient, an adequate intake level based on animal or observational studies is used.

The Development of Dietary Reference Intakes After 1994

In 1994, RDAs were reassessed and subsequently renewed as Dietary Reference Intakes. This broad set of evidence-based nutrient reference values includes RDAs, Tolerable Upper Intake Levels to avoid toxicity, and Acceptable Macronutrient Distribution Ranges. The ranges for each energy source represent intakes associated with reduced chronic disease risk while ensuring adequate essential nutrients.

Dietary Reference Intakes were published between 1997 and 2003, with updates for calcium and vitamin D in 2011, sodium and potassium in 2019, and energy in 2023. An expansion beginning with sodium in 2019 added the Chronic Disease Risk Reduction Intake, linking a nutrient to chronic disease risk. For people aged 14 and older, for example, reducing sodium to this level of 2300 mg (2.3 grams) daily or less is associated with lower hypertension and cardiovascular risk.

In the 1970s, federal attention shifted toward emerging chronic diseases. The Senate Select Committee on Nutrition and Human Needs published Dietary Goals for the United States in 1977. Recommendations later became the Dietary Guidelines for Americans, first published by the Departments of Agriculture and Health and Human Services in 1980 and now updated every 5 years. These represent a shift from the early 20th-century focus on nutrient adequacy to diet's effects on overall health and chronic disease risk.

Nutritional science continues to evolve, identifying underlying nutrient-sensing mechanisms and macronutrients' circadian and hedonic effects, and using artificial intelligence-based algorithms to predict individual food and dietary responses.

Nutrition Research and Policy Changes in Turkey

• 1920s–1950s: Early nutrition and health policies and campaigns began in the first years of the Republic, particularly programs addressing child health and nutrient deficiencies.
• 1962: The Department of Nutrition and Dietetics was established at Hacettepe University, beginning the training of nutrition professionals in Turkey.
• 1970s: The "Sağlık Ocağı" community health-center model was developed to expand basic healthcare in villages and rural areas.
• 1991: Breastfeeding Promotion and Baby-Friendly Health Facilities Program: Launched by the Ministry of Health to promote breastfeeding.
• 1994: Prevention of Iodine Deficiency Disorders and Salt Iodization Program: This program was launched to prevent iodine deficiency by promoting iodized salt use.
• 2004: Demir Gibi Türkiye Program: Launched to provide preventive iron supplementation for infants and children.
• 2005: Vitamin D Supplementation Program: Initiated to prevent vitamin D deficiency by providing free supplements to infants.
• 2010: Turkey Nutrition and Health Survey (TBSA): Regular surveys began to assess eating habits and health status. Turkey Healthy Nutrition and Active Life Program: Launched to combat obesity and encourage healthy eating. Nutrition-Friendly School Project: Jointly launched by the Ministries of Health and National Education to promote healthy-eating standards in school canteens.
• 2011: Climate Change Action Plan (2011-2023): The Ministry of Environment and Urbanization developed agricultural and environmental policies.
• 2015: Turkey Dietary Guidelines (TÜBER): Turkey-specific guidelines providing comprehensive information on eating habits.
• 2019: Turkey Healthy Nutrition and Active Life Program (2019-2023): an updated comprehensive program promoting healthy eating and combating obesity.
• 2021: Turkey Guidelines for Reducing Sugar Consumption/Use: Published by the Ministry of Health to reduce sugar consumption. Turkish Journal of Nutrition and Dietetics: Published as a scientific resource assessing nutrition policy and other health-related factors.

Energy and Macronutrients

Energy:

In the late 19th century, Max Rubner studied heat production and other energy losses in a dog maintained at constant weight in a calorimeter. Its food energy intake (17349 kcal) matched total energy losses (17406 kcal), confirming the first law of thermodynamics in a living organism. Wilbur Atwater soon extended the work to humans and derived metabolizable energy values of 4, 9 and 4 kcal/g for carbohydrates, fats and protein, respectively, adjusting for each macronutrient's chemical energy losses in feces and urine. These widely used Atwater values assume average "apparent" digestibility and energy losses in urinary end products of protein metabolism. They may not be exact for individual foods: almonds, for example, yield 4.6 kcal per gram in the human diet, substantially below the 6 kcal predicted from their macronutrient composition.

Rubner and Atwater showed that when intake balances energy losses through heat, feces, urine and skin, weight remains stable, reflecting energy balance. Current Dietary Reference Intakes use indirect calorimetry to measure total energy expenditure in people's natural environments over 1-2 weeks. Figure 2 presents a simple model of macronutrient conversion to body tissue and heat.

The Dietary Reference Intake for energy is the estimated average intake required to maintain energy balance according to age, sex, physical activity and life stage. Four activity levels—inactive, low active, active and very active—range from minimal independent living to vigorous activity. Estimates are further refined for pregnancy and lactation. Calorie intake can be estimated using an online calculator. Wearable sensors and mobile apps offer new opportunities to objectively assess dietary intake and activity at individual and population levels.

Protein

Proteins are synthesized from amino acids and are major structural and functional components of the body. Of the 20 amino acids in human proteins, nine are indispensable (essential); removing any from the diet causes physical symptoms and negative protein balance. Six are conditionally essential, meaning they cannot be synthesized sufficiently during rapid growth or severe metabolic stress. The remaining five are dispensable (nonessential) and can be synthesized to meet the body's needs.

In the 1930s, Rudolf Schoenheimer and colleagues showed that biomolecules such as proteins undergo continuous turnover and require dietary replenishment to maintain most bodily functions and health. Animal protein sources contain all nine essential amino acids, whereas plant proteins are generally deficient in one or more. Protein sources are evaluated using a digestible amino acid score, referenced to a preschool child's digestible essential amino acid requirements. Cow's milk, beef and eggs score highest, around 1; rice scores approximately 0.5. Soy scores about 0.9 and is a useful plant amino acid source. Vegetarians and vegans can maintain adequate protein intake with varied plant foods, including legumes, cereals, soy products, nuts and seeds.

Individual amino acid requirements are usually determined by isotope-tracing methods. Protein requirements are estimated through nitrogen balance, tracking fecal, urinary and skin nitrogen losses as intake changes. Inadequate total protein or any essential amino acid triggers metabolic adaptations that can impair infant brain development, immune competence and many other physiological and metabolic functions. Insufficient energy from fats and carbohydrates can also cause negative nitrogen balance. Conversely, excess amino acids during high protein intake can be deaminated to alpha-keto acids, which may be oxidized for energy or converted into glucose or fat.

Healthy young adults can maintain neutral nitrogen balance at 0.55 to 0.6 grams of protein per kilogram daily. The protein RDA is 0.8 grams per kilogram daily, incorporating a safety margin above this level for women and men older than 18. Infants and children require more to support healthy growth, and needs also rise in pregnancy and lactation. Higher intake may benefit subgroups including athletes, community-dwelling older adults and people with obesity dieting for weight loss.

Based on estimated urea production and small experimental studies, the tolerable upper limit for adults is 3.5 grams per kilogram daily—for example, 280 grams for an 80-kilogram adult. However, because of numerous adverse health effects, levels above 2 grams per kilogram daily should be avoided. Adult intake below approximately 0.4 to 0.5 grams per kilogram leads to muscle atrophy and functional impairment. The Acceptable Macronutrient Distribution Range for protein is expressed as a percentage of total energy and ranges from 10% to 35% for everyone older than 3.

Between 2015-2018, approximately 6% of Americans older than 1 had low protein intake, with greater prevalence in some subgroups, including those aged 71 and older and Hispanic and Black Americans. Increasing protein intake is not straightforward because protein foods usually contain other macro- and micronutrients. They are often consumed in mixed dishes high in saturated fat and sodium, such as meat sandwiches and kebabs. People eating only plant foods should pay particular attention to protein quality and to vitamins B12 and D, calcium, iron, zinc and iodine.

Fat

Body fat consists almost entirely of triglycerides, each with a glycerol backbone and three attached fatty acids. Saturated fatty acids have no double bonds, come from animal sources and are generally solid at room temperature. Unsaturated fatty acids contain double bonds and have two geometric isomers: cis, with the hydrogen atoms on the same side at the double bond, and trans, with them on opposite sides. Cis unsaturated fatty acids may have one double bond (monounsaturated) or multiple bonds (polyunsaturated), come from plant sources and are liquid at room temperature.

In the early 20th century, fat was considered a compact dietary energy source as triglycerides and a carrier of fat-soluble vitamins. In 1929, George and Mildred Burr discovered that linoleic and alpha-linolenic acids were necessary for growth and prevention of symptomatic deficiency in a mouse model. These n-6 and n-3 essential polyunsaturated fatty acids were later identified as precursors of numerous bioactive lipids and contribute to many functions.

Alongside linoleic acid, the n-6 polyunsaturated fatty acid arachidonic acid may become conditionally essential when its synthesis from linoleic acid through desaturation and chain elongation is limited. It is a precursor of eicosanoids such as prostaglandins, thromboxanes and leukotrienes, which have autocrine/paracrine and occasional endocrine functions and wide-ranging physiological effects.

The n-3 essential polyunsaturated fatty acid alpha-linolenic acid is a precursor of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These two n-3 fatty acids are important for inflammatory mediator synthesis, intracellular signaling, gene expression and cell membrane structure. Higher EPA and DHA levels have been associated with reduced cardiovascular risk, although the strength of these effects is still being studied. Among common vegetable oils, flaxseed, soybean and canola oils are good alpha-linolenic acid sources. Oily fish such as salmon and herring and fish oils are rich in EPA and DHA. Humans lack the capacity to convert alpha-linolenic acid into EPA and DHA.

Deficiency of either or both essential fatty acids is rare because they and their intermediates are abundant in Western diets and stored in body fat during lipolysis. The n-6 and n-3 fatty acids compete for the same desaturase enzymes regulating arachidonic acid, EPA and DHA synthesis. Adequate intakes of essential polyunsaturated fatty acids are set by life stage.

Monounsaturated fatty acids are components of cell membranes, particularly myelin in nervous tissue. Oleic acid is the principal monounsaturated fatty acid in foods and tissues. Olive, canola, peanut and sesame oils, as well as animal fats, contain it. Saturated fatty acids such as stearic and palmitic acids are nonessential and come mainly from animal-based foods such as full-fat dairy and fatty meats, and tropical oils such as coconut and palm kernel oil. These fatty acids raise total and low-density lipoprotein (LDL) cholesterol and increase cardiovascular risk.

Trans fatty acids are dietary unsaturated fatty acids obtained from ruminant meat and dairy or produced by industrial partial hydrogenation of liquid oils into semisolid or solid fats. They are associated with higher total and low-density lipoprotein cholesterol and increased cardiovascular risk. United States food companies were required to remove partially hydrogenated oils by 2018, substantially reducing industrial trans fats in the food supply.

Cholesterol

Although cholesterol is not required in the diet, it is important for cell membrane fluidity and is a precursor of steroid hormones, bile acids and vitamin D. Its recognized role in cardiovascular disease has changed over time, partly because its effects are difficult to isolate from those of other fats and carbohydrates. Reducing saturated fat and following a healthy diet containing fruit, vegetables, whole grains, low-fat or fat-free dairy, lean proteins, nuts, seeds and vegetable oils provides cholesterol intakes below levels thought to increase cardiovascular risk.

Except for red blood cells and central nervous system cells, all cells can oxidize long-chain fatty acids released from dietary triglycerides for energy. Adequate intakes for total fat have been set for infants aged 0-12 months, but no RDA or adequate intake is recommended for older people. The Acceptable Macronutrient Distribution Range for fat varies from 20% to 40% of energy across life stages.

Table 1: Values are average nutrient intakes from food and beverages consumed per person aged 20 and older in the pre-pandemic period, from 2017 to March 2020.

The Dietary Guidelines for Americans 2020-2025 recommend limiting saturated fat to less than 10% of daily calories from age 2. In the United States during 2015-2018, fewer than one quarter reported meeting this limit of less than 10%. Between 2017 and March 2020, adults aged 20 and older obtained an average of 12% of energy from saturated fat and 38% from total fat, exceeding the 35% upper Acceptable Macronutrient Distribution Range for this age group. Healthier patterns recommended by the Departments of Agriculture and Health and Human Services have less saturated fat (8%) and total fat (32%) and more n-6 and n-3 polyunsaturated fatty acids. Saturated fat should be replaced with polyunsaturated or monounsaturated fats, rather than refined carbohydrates and sugars. Steaming or boiling instead of frying, trimming visible meat fat, choosing low-fat dairy and lean meat, and reducing sweets and sugary snacks can also lower saturated fat intake.

Although limiting saturated fat has broad support, particular sources with differing chemical properties and potential atherogenic effects remain debated. Their cardiometabolic effects depend on interactions among nutrients and nonnutrient food components. For example, a high saturated fat intake from cheese has a smaller LDL cholesterol-raising effect than an equivalent amount from butter. The Dietary Guidelines for Americans 2025-2030 Advisory Committee is examining saturated fat sources and their relationship to cardiovascular risk.

Carbohydrates

The human diet contains two main digestible carbohydrate forms that supply cellular energy: sugars and starches. Sugars occur naturally in fruit, predominantly as sucrose. Lactose is a disaccharide in dairy products. Starches are polysaccharides produced by all green plants and commonly consumed in potatoes, rice, corn and wheat. Sugars added during processing include table sugar, dextrose, brown sugar, high-fructose corn syrup, cane syrup and honey. High sugar intake is associated with excessive energy intake, poor-quality diets, weight gain and obesity.

Table 2: Reference intakes for protein, fat, carbohydrates, fiber and water in the total diet. ND: not defined; RDA: Recommended Dietary Allowance; AMDR: Acceptable Macronutrient Distribution Range.

Under normal conditions, red blood cells and, to a lesser extent, neurons in the human brain rely exclusively on glucose for energy. Carbohydrate requirements across life stages, starting at age 1, are derived from average minimum brain glucose oxidation. The Acceptable Macronutrient Distribution Range is 45% to 65% of total energy for people aged 1 and older. The Dietary Guidelines for Americans 2020-2025 recommend avoiding added sugars before age 2 and limiting them to less than 10% of daily calories after age 2. Between 2013 and 2016, Americans aged 1 and older consumed an average of 266 kcal daily from added sugars, or 13% of dietary energy. More than 70% of children and adolescents aged 5 to 18 exceeded the 10% guideline, as did more than 50% of adults. Beverages with added sugar, excluding coffee and tea, contribute approximately one quarter of total sugar in the diets of Americans aged 2 and older.

Fiber comprises naturally occurring edible plant carbohydrates and indigestible components such as lignin. Functional fiber refers to isolated, extracted or synthetic nondigestible carbohydrates with demonstrated health benefits. Total fiber combines dietary and functional fiber. Higher dietary fiber intake is associated with regular bowel movements and better glycemic control. Randomized trials associate the highest intakes with lower body weight, serum cholesterol and systolic blood pressure; observational studies show 15% to 30% lower incidence of some common noncommunicable diseases and related deaths among those with high intakes.

Intestinal microorganisms anaerobically ferment dietary fiber in the colon to produce short-chain fatty acids, a beneficial metabolic process in conditions such as type 2 diabetes. Whole grains, fruit and vegetables are fiber-rich sources that also supply important micronutrients.

Life-stage guidelines for fiber from age 1 take its protective cardiovascular effects into account. Between 2017-2020, Americans aged 20 and older consumed an average of 17 grams daily—approximately half the recommended adequate intake and well below the optimum of 25 to 29 grams daily. Adequate intake is associated with reduced risk across many outcomes. Among Americans aged 1 and older, 94% do not meet their life-stage fiber adequate intake.

Early Dietary Guidelines: Carbohydrates

Early guidelines focused on total carbohydrate quantity and classification into simple and complex forms. This classification is now being reconsidered. Growing evidence suggests that foods with natural sugars have different metabolic effects from those with added sugars, which increase the risk of various noncommunicable diseases.

Carbohydrate quality relates to type, digestibility and fiber content. Both quantity and quality are now considered independent determinants of conditions such as type 2 diabetes.

From Macronutrients to Healthy Eating Patterns

Different energy requirements and varying needs for the three main macronutrients and many micronutrients across ten life-stage groups, alongside special populations such as pregnant or lactating people, add to the complexity of recommending healthy eating for patients with widely differing finances, preferences, cultural backgrounds and ethnic food traditions. Table 3 summarizes core energy and macronutrient recommendations. The Dietary Guidelines for Americans 2020-2025 provide an excellent source of expanded recommendations from birth through old age.

Patients with diet-related chronic diseases such as obesity, cardiovascular disease or type 2 diabetes may particularly benefit from these guidelines to reduce symptoms and comorbidities. Consultation with a registered dietitian or nutrition professional should be considered for complex nutritional needs.

Conclusion

The amount and pattern of food intake are major determinants of growth and lifelong health. Macronutrients supply energy and sustain these processes. Understanding their nuanced effects is important for effective care for all patients, particularly the growing number whose disease pathophysiology has a major nutritional component.