Due to the increasing prevalence of obesity, interest in nutrition has increased. Every 5 years, the US Department of Agriculture (USDA) revises the nutrition guidelines; the last publication was in 2015. The “pyramid” from 2005 has been changed to a graphic similar to the “My Plate” diagram recommended for weight control (http://www.chosemyplate.gov) and not only reflects the known requirements and “conventional” scientific wisdom for nutrients, but also, to some extent, represents a realistic and simplified recommendation based on what the American public is willing to accept and follow (Figure 16-1). The ChooseMyPlate website also contains recommendations for pregnancy. The nutrition guidelines may unfortunately reflect pressure from the food industry, according to many nutrition advocates.

The overall body of evidence examined by the 2015 Dietary Guidelines Advisory Committee (DGAC) identified that a healthy dietary pattern is higher in vegetables, fruits, whole grains, low- or nonfat dairy, seafood, legumes, and nuts; moderate in alcohol (among nonpregnant adults); lower in red and processed meats; and low in sugar-sweetened foods and drinks and refined grains.¹

The practicing obstetrician/gynecologist has the responsibility to be well versed in nutrition. We are sometimes the only physicians women see on a regular basis. Also, women usually make the food decisions for the family, so educating them may result in benefits to men and children. It appears that once obese, an individual has a difficult time achieving a normal weight. Therefore, prevention of obesity is a crucial goal.

Another reason that we are important in battling this disease is that obesity affects every aspect of obstetrical/gynecological practice. The increase in morbidity and mortality in pregnancy,² infertility issues, gynecologic cancers (with poorer survival³), and complications of surgery in the obese woman impels us to contribute to the prevention and treatment of obesity. The cornerstones of prevention and treatment are proper nutrition, exercise, and education of both the patient and physician.

Energy Concepts and Metabolism

The Human “Machine”

As machines, we are obligated to obey the laws of thermodynamics. The first law of thermodynamics states that the total amount of energy in the universe remains constant; therefore, our bodies cannot independently produce energy but must obtain it by ingesting nutrients and converting the energy obtained into other forms. The second law of thermodynamics states that any energy transformation is always in the direction of increased entropy, or disorder, and cannot be recovered for later use. The goal of our metabolism is to preserve a steady state. In our bodies, we actually do not obtain equilibrium, the complete reversibility of all reactions, but a “near” state, called energy balance. The energy content of the body and its composition stay constant only at the expense of needing renewed supplies (intake of fuel) and disposal of waste products (energy dissipation). Complete equilibrium would equal death, but the steady state allows renewal by the input and output of substances as described The body takes in fuel (macronutrients) and converts the chemical energy of that fuel into other forms of energy, such as mechanical energy and heat. Energy is liberated primarily by the process of oxidation by reactions in the mitochondria of the cells. There may also be partial oxidation (anaerobic reactions, i.e., glycolysis) of fuel, which is later converted and may be completely oxidized or stored. Glycolysis proceeds without the mitochondria or oxygen. An example is glucose conversion to pyruvate or lactate.

While macronutrients are metabolized by different pathways, the common pathway converges at acetyl coenzyme A (acetyl-CoA). In the mitochondria, the Krebs cycle leads to oxidation of acetyl-CoA, production of “reducing equivalents” (e.g., nicotinamide adenine dinucleotide, NAD), CO₂, and a molecule of adenosine triphosphate (ATP).⁴ The energy produced is used for bodily functions: synthesis of structures and compounds, cellular pumping mechanisms used for muscle contraction, physical activity, and the aforementioned heat production. The chemical products of this oxidation are carbon dioxide, water, and nitrogen oxides (waste). Waste or “by-products” of our metabolism are excreted by the lungs (carbon dioxide) or kidneys (water and urea). We do not excrete other metabolic products, such as amino acids, ketones, or lactate, at any significant rate.²

There are a number of hormones active in metabolism.^4,5 This section discusses these hormones.

Pancreatic Hormones

Insulin

Insulin is essentially an anabolic hormone. Its secretion by the islets (β cells) is regulated primarily by blood glucose concentrations. After a meal, insulin secretion is potentiated by intestinal hormones, the incretins. Incretins are peptides secreted by the gut after a carbohydrate-containing meal and include glucagon-like peptide (GLP-1) and gastric inhibitory peptide (GIP). They account for the difference in insulin response when glucose is ingested orally versus infused intravenously. Insulin acts to regulate enzymatic actions (dephosphorylation) and affects long-term regulation of gene transcription. Insulin has the effect of deposition of glycogen in liver and muscle, synthesis of protein in skeletal muscle, and storage of fat in adipose cells.

Glucagon

Glucagon, secreted by α cells, exerts its major effect on the liver. Its secretion increases when blood glucose levels fall or a meal high in protein is ingested. Stored glycogen is converted to glucose as needed.

Adrenal Hormones

Epinephrine

Epinephrine (adrenaline) release is stimulated by a fall in blood glucose concentration. Stimuli for its release are stress, anxiety, exercise, and blood loss. It is produced in the adrenal medulla and responds to hypothalamic signals. Through a chain of signals, increasing epinephrine raises glucose and esterified fatty acid levels in the bloodstream from stored glycogen and fat breakdown.

Norepinephrine is a neurotransmitter that acts on the same cell receptors as epinephrine and responds to the same stimuli. It is present at high concentrations during strenuous exercise. While its basic presence is at the sympathetic nerve terminal and is reabsorbed by it, it can leak into the circulation and act as a hormone.

Cortisol

Cortisol is highly bound to plasma proteins, such that only 5% of the cortisol circulates as free, active hormone. It regulates gene expression and is mostly catabolic. It increases fat mobilization, increases skeletal muscle dissolution, and increases gluconeogenesis in the liver. Its release is stimulated by pituitary adrenocorticotropic hormone (corticotropin, ACTH).

Pituitary Hormones

Growth hormone is mainly anabolic. It increases the synthesis of cartilage and increases bone length. Secretion is increased by a fall in plasma glucose, stimulating gluconeogenesis and fat mobilization. It also acts to stimulate production of insulinlike growth factors 1 and 2 (IGF-1 and IGF-2, respectively), mediators of the effects of growth hormone.

The pituitary gland also produces thyroid-stimulating hormone (TSH), which drives the thyroid gland, and ACTH, which increases production of cortisol.

Thyroid Hormones

The thyroid hormones thyroxine (T₄) and triiodothyronine (T₃) have longer-term effects than other hormones involved in metabolism. They are primarily catabolic, stimulating energy expenditure and influencing the breakdown of muscle protein. Action is believed to be at the level of the mitochondria. Deficiency of these hormones decreases the metabolic rate but alone is not considered responsible for significant obesity. The third form of thyroid hormone, reverse T₃, is metabolically inactive but may be involved in energy conservation during stress or starvation, when its production increases.

Hormones Secreted by Adipose Tissue

Leptin

Leptin is a peptide hormone secreted by fat cells. The larger the adipocyte, the more leptin it produces. Leptin appears to cross the blood-brain barrier and exert its effect at the hypothalamus, decreasing appetite. The feedback mechanism that operates creates an increase in appetite when fat stored in adipocytes is low, therefore stimulating increased food intake. Of interest is that mutation in either the leptin gene or its receptor is associated with morbid obesity; however, few cases of obesity are associated with the gene mutation.

Adiponectin

Adiponectin is a peptide also secreted by fat cells. It protects against insulin resistance, usually by decreasing it. Larger adipocytes secrete less adiponectin; therefore, obese women produce less of this hormone.

There are other related hormones called “adipokines,” including some inflammatory cytokines, such as tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6).⁵

The basic essential nutrients in our diet are carbohydrates, protein, and fats (Figure 16-2). In addition, we require vitamins and minerals to catalyze metabolic processes.

Carbohydrates

Carbohydrates consist of two groups, simple and complex. Both groups supply energy for metabolism. Glucose is the simple carbohydrate to which basic nutrients are reduced for energy production. Exogenous simple carbohydrates, such as sucrose, a dimer of glucose and fructose, are rapidly absorbed and rapidly raise blood sugar. These two components have slightly different effects on stimulating insulin production but are considered to have a “high glycemic index” due to causing a rapid rise in insulin. Simple carbohydrates are consumed in large amounts by the American public; the estimate ranges from 22% to 25% of our total calorie intake.

Simple carbohydrates are also found naturally in many foods, such as milk (lactose) and fruit. An important issue is the fact that simple carbohydrates, which occur naturally in foods such as fruit, come in a “package,” which often contains other vital nutrients and fiber.

Complex carbohydrates are exemplified by starches and “fiber.” Starches are polysaccharides, meaning they are many units of glucose joined together by acetyl linkages. They are considered to have a lower glycemic index than simple sugars; they raise blood sugar more slowly because more metabolic steps are required to break down the compounds before absorption. Complex carbohydrates occur naturally in grains and vegetables as well as in refined foods.

All carbohydrates provide 4 Calories of energy per gram. A Calorie equals 1 kilocalorie of energy released per gram when a substance is ignited in a Caloric bomb.

Any excess carbohydrate ingested will be stored in 1 of 2 ways: converted to glycogen and stored in the liver or stored in adipocytes as fat. Because the brain requires about 100 g of glucose per day, which is close to the amount that can be stored in the liver, the major residual storage form is fat, triacylglycerol (TAG). TAG is hydrophobic and coalesces into small droplets for ease of storage. The efficiency of energy storage of fat is 8 times higher per gram than carbohydrate or protein.

Protein

Protein, consisting of amino acids, is the nutrient that, while also providing energy, has primarily a structural and enzymatic function. There are 9 amino acids that cannot be produced by the body and therefore must be obtained through food. These, the “essential” amino acids, are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. All amino acids have the same basic structure, differing only in a side group. The simplest amino acid, glycine, has a hydrogen as its side group.

Animal sources of protein contain all of the essential amino acids, but many plant-based foods do not. An exception is soy. To obtain all of the essential amino acids, vegetarians need to consume a combination of foods that together will supply all of them.

The body’s protein is not generally used as a fuel beyond the amount ingested daily. Each gram of protein contains 4 kcal of energy.

Fats

Fats (lipids) are composed of chains of fatty acids. They provide energy that can be used fairly rapidly or stored. In fact, any energy remaining after basic metabolic functions are performed is stored as fat. The survival value of this is obvious unless the food consumer is already “overnourished.”

Included in the category of lipids are saturated/trans and unsaturated fats, the former being found in processed foods as well as naturally in most animal products.

Butyric acid is one of the fatty acids responsible for the flavor in butter. Of the approximately 40 naturally occurring fatty acids, those without carbon-carbon double bonds are classified as saturated, and those containing carbon-carbon double bonds are classified as unsaturated. Palmitic and stearic acids are the most common saturated fatty acids, and oleic and linoleic acids are the most common unsaturated fatty acids. Oleic acid is monounsaturated because it has only 1 carbon-carbon double bond. Linoleic, linolenic, and arachidonic acids are polyunsaturated because they have 2, 3, and 4 carbon-carbon double bonds, respectively. Examples of where saturated fats occur in nature are dairy, meat, and eggs.

Unsaturated fats are most commonly found in vegetable oils, nuts, and seeds. Polyunsaturated and monounsaturated fats are considered to be healthier. Saturated fats are believed to increase levels of low-density lipoprotein (LDL) cholesterol and contribute to the risk of coronary artery disease (CAD).⁸

Among the fatty acids, omega-3 and omega-6 appear to promote heart health. They are found mainly in fish, vegetable products, and nuts. They are both cis– isomers rather than in the trans– configuration. Trans fats are deleterious to heart health.^10,11

Each gram of fat produces 9 kcal of energy, that is, has a greater “calorie density” than carbohydrate or protein. This information is valuable in counseling about eating patterns. Fats are essential, however. Many vitamins are fat soluble and therefore are not available to our bodies unless we consume some fat. As mentioned, fat is our major form of energy storage.

Vitamins and minerals are naturally occurring chemicals that are vital for catalyzing metabolic reactions, incorporating into body structures, and functioning of many body systems. Some, such as vitamin D, promote absorption of other substances, such as the mineral calcium. For each vitamin, there is an official recommended daily allowance (RDA) (Table 16-1), more recently expressed as Dietary Reference Intake (DRI), a recommended minimum intake suiting the needs of most individuals. The RDA is actually that quantity per day needed to avoid “deficiency” diseases, for example, scurvy, rickets, beriberi, and pellagra. Most naturally occurring foods contain enough vitamins (in a balanced, varied diet) to meet the RDA for all vitamins except D.