Nutrition: Macronutrient Intake, Imbalances, and Interventions

Issues of Concern

Excessive or deficient macronutrient intake is associated with adverse health outcomes in the general population and may originate from inadequate consumption of an individual macronutrient or an overall excessive or deficient energy intake.

Macronutrient Deficiencies

Sufficient protein intake is essential for health and well-being at all ages. In children, adequate protein intake is essential for growth and development.[13] In adults, adequate protein intake helps maintain lean body mass and prevent age-related loss of skeletal muscle mass.[3] Nevertheless, protein undernutrition is common in developed and developing countries. Worldwide estimates suggest that over 1 billion people have chronically insufficient protein intake, with up to 30% of the children in central Africa and South Asia and over 50% of the homebound elderly population in the United States being affected. [9]

The repercussions of protein malnutrition range from mild to life-threatening, depending on individual characteristics, the degree of deficiency, and the presence of exacerbating factors such as concurrent illness and insufficient energy intake. Consequences of protein deficiency can be unspecific and include stunting, anemia, intrauterine growth restriction, impaired nutrient absorption, cardiovascular dysfunction, muscle wasting, immunodeficiency, hypoalbuminemia, edema, loss of bone mass, skin atrophy, and impaired hormone production, particularly of growth and thyroid hormones and insulin.[9]

Classical forms of protein deficiency include marasmus, a protein-calorie deficiency, and kwashiorkor, a protein deficiency within an energy-sufficient diet. Marasmus is characterized by dry and wrinkled skin, extreme muscle wasting, loss of subcutaneous fat, and atrophy of internal organs with preserved histology. Kwashiorkor is characterized by severe edema that is more pronounced in the hands and feet, wasting, diarrhea, irritability, skin depigmentation, fatty liver, and organ dysfunction. While these conditions are usually described as distinct entities, there is significant clinical overlap, and many patients exhibit features of both conditions, termed marasmic-kwashiorkor.[6][14] Laboratory assessments can help distinguish between these conditions; kwashiorkor presents with more pronounced and severe decreases in transferrin, albumin, and total plasma proteins, resulting from more profound impairments in protein synthesis.[14]

Unlike protein deficiency, which is common in the general population, lipid and carbohydrate deficiencies are extremely rare. However, a low intake of lipids or carbohydrates can have important implications for health and disease.[15][16]

Strictly speaking, carbohydrates are not considered essential nutrients because the body can synthesize carbohydrates endogenously and use alternative energy sources. Moreover, the absence of dietary carbohydrates does not result in a characteristic deficiency. However, nutrient-dense sources of carbohydrates, such as whole grains, fruits, and vegetables, contain nutrients and bioactive compounds associated with many health benefits and are unavailable in other food sources.[3][15] For this reason, nutrient-dense forms of carbohydrates are included in dietary guidelines.

The human body can also endogenously synthesize various forms of lipids. However, in contrast to carbohydrates, lipids are an essential macronutrient. They must be sufficient in the diet to provide essential fatty acids and allow for the absorption of fat-soluble vitamins.[17] Essential fatty acids include alpha-linolenic acid (omega-3) and linoleic acid (omega-6).[18] Essential fatty acid deficiency is rare in individuals with a regular diet and most frequently presents in individuals with severe fat malabsorption or fat-free parenteral nutrition. Findings suggestive of essential fatty acid deficiency include dermatitis, alopecia, liver dysfunction due to mitochondrial dysfunction, and increased susceptibility to infections. [19]

Excess Macronutrient Intake

While macronutrients are not directly toxic, even when consumed in large amounts, chronic macronutrient overconsumption can be a cause for concern. Chronic excess energy intake from carbohydrates and fats has been associated with weight gain, obesity, type 2 diabetes, hypertension, and other adverse health outcomes associated with increased adiposity.[20][21] Interestingly, overfeeding on protein alone is not associated with increased adiposity and has been shown to improve body composition, especially in individuals who engage in resistance exercise.[22] For this reason, the main concern of excess macronutrient intake is excess calorie intake and the consequences of increased weight and adiposity. However, excess consumption of a single macronutrient in a calorie-appropriate diet implies that another macronutrient is being displaced to remain within calorie limits. If chronic, this practice can also result in nutrient deficiencies.

There has been considerable debate concerning the safety of high-protein diets, with particular attention to kidney function. Some authors have proposed that high-protein diets may lead to kidney damage and disease. This concern was initially proposed after scientists discovered that high-protein diets cause a compensatory increase in the glomerular filtration rate (GFR), originally thought to result from nephron loss.[23] However, this increased GFR is a normal adaptation to increase solute clearance, and more importantly, high-protein diets are not a risk factor for developing chronic kidney disease in otherwise healthy individuals.[23]

Recommended Intakes

Macronutrient requirements can vary widely between individuals depending on several factors such as age, body weight, physical activity levels, and associated medical conditions. In general, recommendations for macronutrient intake and distribution provide a great deal of flexibility. Provided that essential macronutrient and micronutrient needs are covered and appropriate calorie numbers are consumed, macronutrient distribution may be adapted to fit individual preferences and goals.

Adequate protein intake is key in preventing age-related loss of muscle strength and muscle mass (sarcopenia).[24][25] The current recommended daily allowance (RDA) of protein for healthy adults is 0.8 g/kg and represents the average daily intake required to meet the minimal protein requirements and maintain nitrogen balance in 97.5% of the population. In other words, the RDA is the minimal amount needed to prevent a deficiency in most people.[9][13] For this reason, the RDA has been heavily criticized in recent decades; more recent studies demonstrate methodological pitfalls within the nitrogen-balance method on which the RDA is based, and a growing amount of evidence reveals better outcomes when protein intake exceeds the RDA.[24] Healthcare professionals must understand that the RDA reflects a minimal rather than an optimal protein intake.

While an optimal lower limit for protein intake has not been established, some authors have reported significantly lower age-related decreases in skeletal muscle with a daily protein intake of 1.2 g/kg. [25] An upper tolerable limit for protein intake has yet to be established. However, studies have shown that healthy adults can tolerate a long-term daily intake of 2 g/kg or more.[9] As a percentage of daily calories, the acceptable macronutrient distribution range (AMDR) for protein has been set at 10% to 35% for adults.[26] Yet, caution must be taken when calculating protein requirements as a percentage of daily calories; dietary protein serves mainly as a source of amino acids rather than a source of energy, and percentage-based calculations can yield inadequate numbers in individuals with lower- or higher-than-average calorie requirements.[5]

For example, 10% of daily energy requirements would equate to 120 kcal or 30 g of protein for a person on a 1200-kilocalorie diet, resulting in an insufficient intake. For an athlete with a 4000-kilocalorie diet, where 35% of daily energy would equal 1400 kcal or 350 g of protein, this amount could be more than necessary for amino acid requirements and result in displacing calories otherwise allocated to better energy sources, such as lipids or carbohydrates.

Unlike protein, dietary recommendations for carbohydrates and lipids are more flexible.[17] Current guidelines recommend a fat intake between 20% to 35% of daily calories to ensure adequate levels of essential fatty acids and fat-soluble vitamins.[16][26] Additional recommendations include limiting saturated fats to <10% of daily calories to reduce cardiovascular risk.[6] For carbohydrates, the AMDR has been set at 45 % to 65% of daily calories, with a limit of 10% of daily calories coming from added sugars.[27][28]

Obesity and Weight Loss

With obesity rates on the rise, efforts have been made to characterize the role of macronutrient intake in promoting weight gain and facilitating weight loss. Historically, carbohydrates and fats have been theorized to be responsible for the rising prevalence of obesity, and low-carbohydrate and low-fat diets have been proposed as promising solutions.[29][30][31] However, neither carbohydrates nor fats are inherently fattening, and their restriction has not been proven differentially superior in promoting weight loss.[32] Additionally, it is important to distinguish between different sources of each macronutrient, as processed versions are related to weight gain and obesity, while unprocessed versions are not. [33]

Because obesity is a complex condition that stems from excess total energy intake rather than any individual macronutrient, focusing interventions on macronutrient restriction is unlikely to be effective. Moreover, studies have shown that public health interventions aiming to reduce sugar intake can result in a paradoxical increase in fat consumption.[31] When considering weight loss outcomes, low-fat and low-carbohydrate diets are equally effective, similar to other dietary patterns that result in caloric restriction without eliminating specific food groups.[29][32]

Special Populations

A multitude of conditions and situations can influence nutritional requirements. In the case of macronutrient intake, special consideration is warranted at certain life stages and for people with certain medical conditions. Such scenarios include childhood, pregnancy, athletes, and people with specific medical conditions like chronic kidney disease or liver disease.

Due to the metabolic demands of growth and development, children and adolescents have higher relative energy and protein requirements than adults.[25] Children between 1 and 3 years old have the largest protein requirement in grams per kilogram because this period has the fastest growth trajectory.[34] Generally, protein requirements are estimated to be 20% to 60% greater in children and adolescents than in healthy adults.[25] Older adults are thought to require larger amounts of protein due to age-related changes in protein metabolism, such as anabolic resistance, insulin resistance, and decreased insulin growth factor-1 (IGF-1) levels, which result in lower muscle protein synthesis and increased protein breakdown.[24]

Pregnancy and lactation greatly increase metabolic demands for energy and protein to cover the needs of the gravida while supporting fetal development.[34][35] These demands increase gradually each trimester and, if not met, may result in adverse maternal and fetal outcomes.

In the context of disease, protein intake is important in managing various conditions, such as chronic kidney disease (CKD) and chronic liver disease (CLD). In patients with CKD, protein intake must be carefully balanced to prevent malnutrition while delaying disease progression.[36] Protein restriction may help prevent or slow disease progression but risks worsening the sarcopenia associated with CKD, which is associated with increased mortality in this population. Protein restriction may be warranted in patients with a high risk of progression to end-stage kidney disease, while low-risk patients may benefit from a higher protein intake.[36] In the setting of liver disease, protein restriction was previously recommended to reduce ammonia levels and improve symptoms of hepatic encephalopathy in patients with CLD. However, studies have shown that protein restriction in patients with CLD compromises their nutritional status and results in worse outcomes than normal-protein diets. For this reason, current guidelines do not recommend protein restriction.[37]

While healthcare professionals need to know about these special situations and their influence on nutrient requirements, patients with special requirements should be referred to a specialist for optimal and timely nutritional management.

Food Quality and Nutrient Sources

Meeting nutrient requirements is essential at all stages of life. However, promoting a holistic perspective that ensures nutritional adequacy through a whole-food approach is imperative.[25] Studies have consistently shown that the effects of foodstuffs go beyond the sum of individual nutrients and depend on the food matrix, which refers to the interaction of the physical structure and composition of food, involving both nutrient and non-nutrient components.[25][38] Macronutrient requirements should be met through nutrient-dense whole foods to promote health and reduce disease risk.

Healthy sources of carbohydrates include legumes, whole grains, fruits, and vegetables.[3] Unlike processed grains, from which the germ and bran have been removed, whole grains are high in fiber and micronutrients and have been associated with decreased risk of cardiovascular disease, cancer, diabetes, and all-cause mortality.[3] Similarly, fruit and vegetable intake is inversely correlated with chronic noncommunicable diseases, including hypertension, cardiovascular disease, metabolic syndrome, and lung cancer.[3] While the reasons behind these health effects are not yet completely understood, it is likely related to the phytochemicals and bioactive compounds present in the food matrix of such foods.[3]

Dietary proteins can be found in both plant-based and animal-based food sources.[24] Plant-based sources of dietary proteins include legumes, soy products, grains, nuts, and seeds. Animal-based sources of protein include meat, dairy, fish, and eggs.[24][3] Generally, animal-based foods are labeled as complete proteins because they contain all the essential amino acids humans require. An essential amino acid cannot be synthesized endogenously and must be obtained through the diet. In contrast, plant-based foods tend to be labeled as incomplete proteins due to the frequent lack of one or more essential amino acids. However, it is important to note that protein and essential amino acid needs can be met through plant-based sources by combining various food sources with different amino acid profiles, which offsets the lack of an essential amino acid in a given food source.[24][25] Moreover, certain processing techniques can increase the digestibility of plant-based proteins to induce a more robust muscle protein synthesis response.[39] A recent clinical trial showed that a high plant-based protein diet supplemented with soy powder was equally effective in increasing muscle mass as an omnivorous high-protein diet supplemented with whey protein, which supports the idea that once a protein threshold is achieved, the qualitative amino acid difference between types of protein becomes of less importance.[40]

Dietary fats can be obtained from various sources and are classified as monounsaturated, polyunsaturated, saturated, and trans-unsaturated fats.[3] Foods usually contain a combination of different types of fats. Unsaturated fats can be found in fish, plant oils, nuts, and seeds. Saturated fats are more common in animal foodstuffs, and trans-unsaturated fats are found in processed vegetable oils. Unsaturated fats are associated with decreased cardiovascular risk and mortality, while trans-unsaturated and saturated fats are associated with adverse effects on health.[3] For this reason, care must be taken with dietary fat and animal protein sources to stay within the recommended saturated fat intake of less than 10% of daily calories.[6]

Clinical Significance

A healthy dietary pattern containing nutrient-dense food sources in adequate amounts is fundamental for health maintenance and disease prevention at all stages of life. Macronutrients are nutrients the body needs in large quantities to support energy needs and meet physiologic requirements.[3] Intake of each macronutrient must meet essential requirements while allowing for an adequate balance between protein, carbohydrates, and fats without exceeding calorie limits.

Per USDA recommendations, nutrient requirements should be met primarily through whole foods and beverages rather than supplements and include a variety of foods from different groups, including fruits, vegetables, legumes, whole grains, nuts, and seeds, while limiting the intake of added sugars and saturated fats.[28]

Deficient or excessive consumption of macronutrients may lead to adverse health effects and should be avoided. In particular, chronic excess calorie intake and weight gain should be avoided to reduce the risk of obesity and its associated conditions. Optimal protein intake should be ensured to minimize the risk of sarcopenia, especially among aging populations.

Nursing, Allied Health, and Interprofessional Team Interventions

Maintaining an adequate diet and macronutrient intake is key for maintaining health throughout the lifespan. Yet only a small portion of the population adheres to current dietary recommendations. Healthcare practitioners, nurses, dietitians, and other healthcare professionals should work together to identify patients with suboptimal dietary patterns and provide timely nutritional advice to prevent the development of adverse health outcomes associated with macronutrient deficiencies or excess intake.

Physicians, advanced practice providers, and nurses can identify and manage conditions associated with excess or deficient macronutrient intake during routine care and provide timely referrals to a dietitian. Dietitians can further assess patients' nutritional status, provide individualized dietary advice, and adjust as needed. Behavioral therapists can help patients improve their relationship with food, optimize dietary adherence, and identify barriers to behavioral change.

Healthcare providers should follow evidence-based nutrition guidelines and promote balanced and sustainable dietary patterns that fit individual needs and preferences. Patients should be educated on the importance of maintaining an adequate dietary pattern that includes sufficient protein, fats, and carbohydrates from nutrient-dense sources without exceeding calorie limits.