There is a general consensus among all clinical specialties that the fat content of the average diet should be lowered to decrease the risk of cardiovascular morbidity and mortality. Low-fat diets are food where 30% or less of the calories come from fat. Multiple correlational studies have related a country’s cardiovascular mortality to the food consumption of its population.
A general rule is that if a provides 100 calories and it has 3 grams or less of fat, then it is a low-fat food. Common examples include vegetables, fruits, whole grain cereals, egg whites, chicken and turkey breast without skin, beans, lentils, peas, seafood, and low-fat dairy, among others.
Fats are essential to us, but we need to consume them in a limited amount. The main four types of dietary fats include polyunsaturated, monounsaturated, trans, and saturated fats. These four varieties of fats differ in their physical and chemical structures. The saturated and the trans fats are considered solid at room temperature, whereas the mono and polyunsaturated fats are liquid at room temperature. Regardless of their physical and chemical properties, these all different forms of fat provide nine calories for every gram consumed, which is much higher than the amount of energy supplied per gram of carbohydrates or proteins. The saturated and the trans fats raise the low-density lipoproteins (LDL) and are considered unhealthy, whereas the monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) which lower LDL are considered beneficial.
Current National Cholesterol Education Program (NCEP) guidelines for adults based on ATP III (Adult Treatment Panel III) recommends reducing intake of saturated fats to less than 7 % of the total calories and cholesterol to less than 200 mg/day. Guidelines also recommend that polyunsaturated fat constitute up to 10% of total calories, and monounsaturated fats constitute up to 20% of total calories.
There is abundant literature to suggest that a decrease or modification of serum cholesterol is a possible way to prevent atherosclerosis. Decreasing the amount of fat intake is an effective means of lowering the serum cholesterol concentration. Hence, a low-fat diet has been widely advocated by clinicians for reducing the cardiovascular-related morbidity and mortality of their patients.
There have been multiple issues of concern and controversies around the concept of a low-fat diet. The biggest concern with the promotion of the low-fat diet has been that manufacturing companies are touting products labeled as low-fat products, where they are replacing the fat with large amounts of refined carbohydrates, which increase the risk of metabolic disorders and hypertriglyceridemia. Studies are also reporting that diets rich in carbohydrates, and low in unsaturated fat, can also negatively impact lipoprotein risk factors, and increase the cardiovascular risks. There is also a proposed theory that the refined carbohydrates decrease the cardioprotective action of HDL by altering its metabolic functions. There has undoubtedly been a focus on replacing the carbohydrates for fats, but the specificities of the replaced carbohydrates remain poorly defined. These concerns have led to the development of alternative dietary approaches.
Studies have also raised concern over the potential of lowering HDL cholesterol, raising triglycerides, and cause unfavorable postprandial lipemic changes. So much so that the “2013 AHA/ACC Guideline on Lifestyle Management to Reduce Cardiovascular Risk” did not state any recommendations for dietary cholesterol and indicated a lack of sufficient evidence to show that lowering of dietary cholesterol reduces LDL-C (low-density lipoprotein cholesterol) or not.
The 2015 Dietary Guidelines Advisory Committee did not endorse limiting dietary cholesterol to less than 300 mg/dL as presented in their prior editions. The Committee, with a focus on dietary patterns rather than on the macronutrients.
Association with Cardiovascular Disease
There has been a direct relationship between dietary fat intake and cardiovascular disease (CVD). Besides, dietary cholesterol has been a focus of considerable attention due to a direct connection between diet and blood cholesterol levels and the subsequent risk for coronary artery disease. The level of LDL particles is the best predictor of cardiovascular risk. Studies have concluded that saturated fatty acids raised blood cholesterol levels, whereas PUFA's reduced serum cholesterol levels and MUFA's were neutral. Studies have also found myristic and palmitic acid to have cholesterol elevating effects, whereas stearic acid did not affect the levels. Trans fatty acids are similar to saturated fatty acids in raising cholesterol, as well. The level of saturated fats, trans-fatty acids should be low, and the levels of polyunsaturated fatty acids should be high. The results from the Nurses' Health Study, in which the women who consumed diets low in saturated and trans fatty acids and relatively high in unhydrogenated mono- and polyunsaturated fatty acids had the least risk for cardiovascular outcomes.
Recent studies have reported that in men, the reduction of total fat and saturated fatty acids from 36% and 12% of energy to 27% and 8% of energy, respectively, resulted in a substantial decline in the total and LDL cholesterol levels.. Over the years, although there has been some decline in the percentage of fat intake, there has been a paradoxical increase in the total amount of fat intake, suggesting that the total energy intake has increased. A large part of the U.S. population still consume diets that contain more total and saturated fatty acids than recommended in dietary guidelines, which is an area of concern.
Association with Cancer
The association between dietary fat and the risk of cancer development has had consistent support through multiple studies. There is epidemiologic evidence demonstrating associations between dietary fat intake and breast, prostate, colon, and even lung cancers in humans.
Of those cancers, dietary fat intake has been the most extensively linked with breast cancer. Various mechanisms have been suggested, including conversion of essential fatty acids to short-lived hormone-like lipids, the production of reactive oxygen species which carry the potential to induce changes in the genomic DNA changes, leading to alterations in gene expression. Other potential mechanisms include modifications in the hypothalamus-pituitary axis leading to alterations in hormone levels, the effect on enzyme functions affecting the estrogen, changes in the structure and functioning of the cells as well as changes in immune function. Studies have also suggested a positive effect of polyunsaturated fatty acid, especially the omega -3 fatty acids to have a protective effect against the development of the cancers and high animal fat to have the strongest positive correlation for the development of these cancers.
One potential mechanism for the relationship between fat intake and prostate cancer include altered levels of sex hormones. Studies have shown that mortality data from colorectal cancer correlated with the consumption of animal fat. Potential mechanisms for a diet and colon cancer link are primarily related to bile acid secretion or intestinal metabolism. Populations that consume foods containing olive oil or oils derived from marine animals and fish have a significantly lower likelihood of developing colon cancer, suggesting again that fat quality is much more important than the type of dietary fat.
Association with Obesity
Obesity is a chronic disease that is associated with a plethora of comorbidities like diabetes mellitus, dyslipidemia, hypertension fatty liver, obstructive sleep apnea, to name a few. It has multiple external and internal influences. Among the many environmental impacts, dietary fat intake is thought to have the strongest association. Energy imbalances result from excessive nutritional intakes along with low levels of physical activity. If we use BMI as the criterion to define obesity, more than one-third of adults in the United States gets categorized as overweight or obese. The rate at which obesity is increasing in this country and throughout the world is alarming. The relationship between diet composition and body weight has been studied in various forms of epidemiological studies, including ecologic, cross-sectional, and prospective studies. Most of the cross-sectional studies show that obese patients have a higher intake of energy from fat as compared to people of healthy BMI. Fat being an energy-dense food contributes to excess calorie ingestion as compared to other foods. There has also been a hypothesis that obese subjects have difficulty oxidizing fat and maybe under oxidizing it as compared to their leaner counterparts. Recent data suggest, for a reduction in absolute amounts of fat consumed as well as a decline in the percentage of total dietary intake at the population level, a concomitant decrease in body weight has not occurred.
As a health care team taking care of patients, we must work together and in unison to provide the best dietary care practices. Recent recommendations have focused more on advocating proper dietary patterns rather than laying emphasis on macronutrients. Commensurate with the above approach is the focus on a lifestyle approach, also called the Therapeutic Lifestyle Changes (TLC). It is the lifestyle component of the third report of the NCEP Adult Treatment Panel (ATP) III guidelines and is currently advocated by various health organizations, including the American Diabetes Association, American Heart Association, and The Obesity Society. It is currently a recommended therapeutic strategy for metabolic and cardiovascular health benefits.
The providers must develop tools to educate their team members, including nursing and allied health personnel, about this concept. This approach will help in forming a comprehensive team approach to propagate this strategy among the patient population. The team members need to comprehend the benefits of such a diet, which are not limited to improving the risk of developing metabolic syndrome, diabetes, hypertension, hyperlipidemia, obesity, and cardiovascular risk factors.
Patients should receive advice to follow a dietary pattern with emphasizes increased intake of vegetables, fruits, whole grains, low-fat dairy, poultry, fish, legumes, nontropical vegetables and oils, and limits consumption of sweets, sugar-sweetened beverages, and red meats. It also emphasizes the DASH dietary pattern along with lower sodium intake. Adults should also engage in aerobic physical activity to reduce LDL cholesterol and non -HDL cholesterol to counter the obesity epidemic and its various co-morbidities. The team of primary care providers, nurse practitioners, dietitians, and internists must be aware of the perils of high-fat content in the diet and form a multidisciplinary approach to manage the patient. A shared decision-making process with the patient is imperative to initiate this dietary change.
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