A macronutrient perspective on healthy eating

An excellent review of nutrition in health appeared in The New England Journal of Medicine recently. The concept of “food as medicine” is gaining traction and being studied as a means to address the growing burden of chronic diseases. Examples include the Mediterranean Diet which has been linked to a reduced risk of heart disease, stroke, type 2 diabetes, and certain cancers, as well as the DASH Diet which is designed to lower blood pressure. Dietary interventions complement traditional medical treatments, and can produce substantial health benefits.

The review focuses on macronutrients (Figure 1). A macronutrient is one of the three main categories of nutrients that the body needs in large amounts to function properly and maintain overall health:

• Protein: Essential for building and repairing tissues, making enzymes and hormones, and supporting immune function.
• Fat: Dense long-term storage of energy, helps absorb vitamins, and protects organs.
• Carbohydrates: The body's primary source of energy, fueling brain function and physical activity.

Proteins are polymers of amino acids that serve critical functional (i.e. enzymes) and structural roles in the cell. There are 20 amino acids, but 9 are deemed essential to humans because they cannot be synthesized by the body. The article distinguishes between complete and incomplete proteins:

• Complete Proteins: Contain all nine essential amino acids that the body cannot produce itself. Sources are primarily animal-based foods such as meat, poultry, fish, eggs, and dairy products. The downside is these animal products often contain unhealthy fats (see below).
• Incomplete proteins: Lack one or more essential amino acids. Sources are mostly plant-based foods like grains, legumes, nuts, and seeds, which tend to contain healthy fats.

Essential amino acids are crucial for protein synthesis and various physiological functions and provide the necessary building blocks for growth and repair of tissue. Thus, it is important to pair incomplete proteins with complete proteins. 

Fats in food are primarily triglycerides which consist of three fatty acids attached to a glycerol molecule via an ester bond. Each fatty acid is a polymeric chain of carbon atoms linked to one another typically by a single carbon-carbon bond. Unsaturated fatty acids possess one or more double carbon-carbon bonds, whereas saturated fats do not. The article categorizes fats based on their chemical structure and impact on health:

• Saturated Fat: Primarily found in animal products like fatty meats, full-fat dairy, and tropical oils (coconut, palm kernel). It is thought to raise LDL ("bad") cholesterol levels, increasing the risk of cardiovascular disease. The article suggests limiting saturated fat intake to less than 10% of daily calorie intake.
• Unsaturated Fat: Generally from plant matter and fish, and considered heart-healthy fats. They are further divided into:
• Monounsaturated Fat (possessing one carbon-carbon double bond): From Olive oil, canola oil, avocados, nuts.
• Polyunsaturated Fat: From vegetable oils (corn, soybean, sunflower), fatty fish, nuts, seeds.

Monounsaturated and polyunsaturated fats are considered to be roughly equally healthful.

Aside from their potentially harmful effects on cardiovascular health, certain fats can also provide essential polyunsaturated fatty acids not synthesized by the body. These are described in the article:

"In 1929, George and Mildred Burr discovered that two fatty acids, linoleic and α-linolenic, were required to facilitate growth and prevent symptomatic deficiency in a rodent model. These respective n−6 and n−3 essential polyunsaturated fatty acids were later found to be precursors to a wide range of bioactive lipids, contributing to multiple functions.

In addition to linoleic acid, arachidonic acid, an n−6 polyunsaturated fatty acid, can become conditionally essential when synthesis from linoleic acid through desaturation and chain elongation is limited. Arachidonic acid is the precursor to a number of eicosanoids, including the prostaglandins, thromboxanes, and leukotrienes, which participate in autocrine, paracrine, and occasional endocrine functions that have widespread physiological actions."

Carbohydrates are saccharides, which can consist of a single monosaccharide (simple sugar like glucose), or a polymer of monosaccharides linked together by glycosidic linkages (e.g. disaccharide, oligosaccharide, polysaccharide). Sugars tend to be mono- or disaccharides, whereas starches are polysaccharides.

The article focuses on two main types of digestible carbohydrates:

• Sugars: Simple carbohydrates that provide quick energy. Naturally occurring sugars are found in fruits (fructose), dairy products (lactose), and some vegetables. Added sugars are added to foods during processing or preparation, such as sucrose (table sugar), high-fructose corn syrup, honey, and concentrated fruit juices. Added sugars are associated with increased risk of weight gain, obesity, type 2 diabetes, and heart disease. The article recommends limiting added sugar intake to less than 10% of daily calorie intake.
• Starches: Complex carbohydrates that provide sustained energy. They are from grains (rice, wheat, corn), starchy vegetables (potatoes, peas), legumes (beans, lentils). Whole grains and starchy vegetables provide fiber, vitamins, and minerals, contributing to a healthy diet. Refined starches are processed grains stripped of fiber and nutrients, leading to rapid spikes in blood sugar and potentially contributing to chronic diseases.

Fiber is a non-digestible carbohydrate found in plant foods. It plays a crucial role in digestive health, blood sugar control, and cholesterol management. The article recommends adequate fiber intake based on age and gender.

In summary, the key recommendations for healthy eating are as follows:

1. Establish Calorie Requirements: Determine your individual calorie needs based on factors like age, sex, activity level, and life stage. Online calculators or consultation with a registered dietitian can help. Calorie intake should not exceed calorie expenditure so that weight remains stable and does not increase. 
2. Limit intake of: Added Sugars (less than 10% of daily calorie intake), Saturated Fat (less than 10% of daily calorie intake), and Processed Foods which are high in sodium, unhealthy fats, and added sugars (as little as possible).
3. Try to keep macronutrients in relative balance: Protein = 10-35% of total calorie intake, Fat = 20-40% of total calorie intake, and  Carbohydrates = 45-65% of total calorie intake.

Finally, meet daily fiber recommendations based on age and gender, and stay adequately hydrated throughout the day.

Figure 1. "Dietary Macronutrient Pathways for Preserving Energy Balance and Maintaining Weight and Body Composition in Healthy Adults. After digestion and absorption, the three main macronutrients — protein, carbohydrate, and fat — release their respective metabolically active substrates: amino acids, glucose, and free fatty acids or glycerol. These substrates, which for some pathways can be interconverted, have two main potential fates: replacement of lean tissue proteins, adipose tissue triglycerides, and glycogen catabolized during postingestive periods and provision of the necessary energy to fuel these processes and other biochemical reactions, including those producing the energy needed for physical activities. The energy-producing reactions consume oxygen and release carbon dioxide, water, and heat. Not all ingested macronutrient energy is available for metabolic processes. Average net digestive losses on mixed diets are 8%, 5%, and 2% for protein, fat, and carbohydrate, respectively; urea and other nitrogenous end-products of protein metabolism excreted in urine account for an additional 1.25 kcal per gram of protein. The classic Atwater values for protein, fat, and carbohydrate — 4, 9, and 4 kcal per gram — account for these fecal and urinary energy losses. Estimates of macronutrient intakes, shown in parentheses, are based on averages reported by women in the United States who were 20 years of age or older in the prepandemic period from 2017 until March 2020; the remaining values were derived from previous studies." (from Figure 2 of Heymsfield and Shapses, NEJM, 2024).