Nutrition 331: Nutrition for Health
Study Guide: Unit 12
In this unit, we examine the role of diet in coronary heart disease (CHD), particularly the dietary factors that influence blood LDL cholesterol, oxidized LDL, development of thrombi (blood clots), and hypertension. Cardiovascular disease is common in Canadians—over six busloads of people die every day from heart attack, heart disease, and stroke. Heart disease makes up over 10% of annual health care costs in Canada.
After completing this unit, you should be able to
- discuss the relationship between dietary factors and the blood level of LDL cholesterol, HDL cholesterol, and triglyceride.
- discuss the role of diet and lifestyle in the development of atherosclerosis and heart disease.
- discuss diet-related factors that affect the development of hypertension.
Section 1 Coronary Heart Disease
Review pages 175–187, “Dietary Fat, Cholesterol, and Health,” of Chapter 5: “The Lipids: Fats, Oils, Phospholipids, and Sterols.”
This section highlights the nutrition factors that lead to atherosclerosis and thrombosis, which cause coronary heart disease (CHD), heart attacks, and strokes. We will also discuss how diet can reduce or prevent LDL oxidation, prevent thrombi, and prevent heart arrhythmias.
Lowering Blood LDL Cholesterol
Note that there is a difference between dietary cholesterol and cholesterol in the blood. The great majority of cholesterol in the blood (total cholesterol) is contained in low-density lipoprotein cholesterol (LDL cholesterol) and high-density lipoprotein cholesterol (HDL cholesterol). The most reliable indicator of risk of CHD is the ratio of total cholesterol divided by HDL cholesterol. Therefore, the goal of risk reduction is to lower LDL cholesterol while raising HDL cholesterol. An elevated level of LDL cholesterol is a major risk factor for CHD. LDL cholesterol is more responsive than HDL cholesterol to dietary change.
A diet consistently high in saturated and trans fat will tend to increase LDL cholesterol, whereas a diet lower in these fats will lower LDL cholesterol.
Since the early 1990s, Canadian health professionals have used a guideline of less than 10% of calories from saturated fat to encourage lower blood LDL levels. Some health professionals recommend an even lower level of saturated fats (7% or less), especially for people with high LDL levels.
As pointed out in the textbook, trans fats are the worst type of fat (pp. 186–187). These fats not only raise the LDL cholesterol but also lower the HDL cholesterol level (in contrast to saturated fats, which actually raise HLD cholesterol) (Van Horn et al., 2008). Major sources of trans fatty acids are shortenings, most peanut butters, fried fast food, cookies, doughnuts, french fries, snack chips, and crackers. Hard margarine has a high content of trans-fatty acid; soft margarine contains much less; non-hydrogenated varieties contain none. Legislative steps have been taken to remove trans fats from the food supply. In New York City, for example, trans fats are banned in restaurants. The food industry has been reformulating foods in recent years and this has resulted in a steady reduction in the quantity of trans fat in foods. These fats will be removed from food in Canada in 2018.
It has been known for many years that PUFAs rich in omega‑6 fats lower LDL cholesterol. Two grams of polyunsaturated fat negates the effect of one gram of saturated fat. Monounsaturated fats are neutral in their effect on LDL cholesterol.
There has been much debate in recent years about the best substitute for saturated fats that may be removed from the diet. Some increase in polyunsaturated fat is generally encouraged. Many experts have encouraged the use of low fat/high carbohydrate diets. This type of diet certainly lowers LDL cholesterol; however, it often also leads to a fall in HDL cholesterol and a rise in blood triglyceride (an elevated triglyceride level is a well-established risk factor for CHD). As such, this combination of changes may well “cancel out,” leading to no change in risk of CHD. However, it seems that different types of carbohydrates have different effects. The harmful effects on blood lipids are most closely associated with sugar and refined cereals. Using these as a substitute for saturated fat does little to prevent CHD. But fibre-rich sources of carbohydrates, such as whole grains, have actions that help prevent CHD.
Unsaturated fats (both polyunsaturated and monounsaturated) have an advantage over carbohydrates in that they cause neither a fall in HDL cholesterol nor a rise in triglyceride.
The body can produce 75% or more of its own cholesterol, so dietary cholesterol has a fairly small effect on the blood cholesterol level. Foods high in saturated fats tend to be high in dietary cholesterol, so decreasing saturated fat also decreases dietary cholesterol.
Clinical and experimental studies demonstrate that viscous fibre also helps lower LDL cholesterol. Recall that viscous fibre tends to bind with the cholesterol secreted in bile juices, preventing cholesterol resorption in the large intestine (see Fig. 4.5, p. 124). Fibre may also help lower LDL cholesterol through bacterial fermentation of the fibre. The end product of this bacterial fermentation is a small fatty acid that slows cholesterol production when it is absorbed and sent to the liver.
The textbook mentions the role of sterol esters in lowering blood cholesterol levels (p. 71). There are some brands of margarine available in Canada that contain sterol esters, but they are quite expensive.
Table 11.3 of the textbook (p. 481) summarizes the possible effects of dietary changes in reducing LDL cholesterol; there is considerable individual variation in the response to dietary changes. When we add up these changes, we see that LDL cholesterol can potentially be reduced by about 23%, which could cut the risk of CHD by 30–40%.
Diet and the Prevention of CHD
Read pages 474–484, “Cardiovascular Diseases” of Chapter 11: “Diet and Health.”
The facts outlined above had become well established by the early 1970s. The conclusion was reached at that time that, in order to prevent CHD, it was critically important to lower the blood cholesterol level and that the best way to do that was to lower the intake of saturated fat (and to a lesser extent cholesterol). At the same time, dietary fat was seen as the single biggest problem in the diet. Based on this view, the standard diet advice given to the populations of the USA and Canada was to lower the intake of dietary fat, especially saturated fat. This advice dominated diet recommendations for four decades, from roughly 1972 to 2012.
But in recent years, major problems with this dietary advice have become very apparent. If saturated fat is indeed a major cause of CHD, then cohort studies should demonstrate that people who have a high intake of these fats also have a high risk of CHD. But cohort studies have failed to show this association (Mozaffarian, 2016). For that reason, the recommendation to focus on a reduced intake of saturated fat is now considered a much lower priority. Likewise, several major guides to heart health have removed all mention of dietary cholesterol. The textbook (pp. 481–482) is still adhering to the older recommendations.
However, in order to improve the balance between LDL cholesterol and HDL cholesterol, it is still recommended that much of the saturated fat in the diet is replaced with polyunsaturated fat. For reasons explained above, nothing useful is achieved by replacing saturated fats with refined carbohydrates, such as sugar and white bread. By contrast, unrefined carbohydrates, such as whole grain cereals, are very much recommended. Eating generous amounts of fruit and vegetables is also strongly associated with a reduced risk of CHD.
In the last few years, important new findings have emerged that directly link sugar-sweetened beverages with risk of CHD (Fung et al., 2009).
The textbook (pp. 181–184) discusses the value of omega‑3 fats from fish in the prevention of CHD. One of the major ways omega‑3 fats prevent CHD is by preventing arrhythmias (patterns of abnormal heartbeats). By preventing or controlling arrhythmias, sudden heart attacks may decrease.
Alpha linolenic acid (ALA) is an omega‑3 fat that is found in some oils, namely flaxseed (the richest source), soybean, and canola (a poorer source). Walnuts are another good source. The protection afforded against CHD by omega‑3 fats from fish appears to extend to ALA. Several epidemiological studies have reported an inverse relationship between intake of ALA and risk of CHD (Van Horn et al., 2008). However, findings lack consistency. Overall, the evidence that omega‑3 fat from plant sources prevents CHD is considerably weaker than is the case for omega‑3 fat from fish.
The textbook (pp. 482–483) mentions that alcohol may help prevent CHD, mostly by raising the HDL cholesterol, although some of the benefit comes from reducing the risk of thrombosis. Our best evidence is that alcohol consumption in moderation (one or two drinks per day) may reduce risk of CHD. Contrary to all the reports extolling the benefits of red wine, there is little evidence that one type of alcoholic beverage is superior to another (Temple, 2016). As the text notes, these benefits are limited to middle-aged and older adults, not young adults. The subject of alcohol and health is examined further in Unit 14.
The textbook describes the inflammatory process to tissue damage in arteries (p. 474) that speeds up the oxidization of LDL. Oxidation of LDL increases the rate of development of plaques that can clog arteries, impeding blood flow to the heart. Some nutrients and phytochemicals slow this reaction and so help slow the growth of atherosclerotic plaques. Antioxidants are one category of diet-related compounds that can slow the oxidation of LDL cholesterol. Major antioxidants obtained from the diet include vitamin C, vitamin E, beta-carotene, and other carotenoids. Supplements of these nutrients appear to have no value in the prevention of CHD. The textbook discusses this with respect to vitamin E (p. 301).
We can now summarize the key features of a diet that minimized risk of CHD:
- Eat a generous amount of whole grain cereals, fruit, and vegetables. Nuts and legumes (e.g., soy beans) are also heart healthy.
- Eat 2 meals of fish per week, preferably fatty fish.
- Reduce the intake of meat (especially processed meat). The fat content is of lesser importance. The above foods are far preferable to meat.
- Reduce the intake of sugar and refined cereals.
Some studies suggest that olive oil (high in monounsaturated fats) helps prevent heart disease, but the benefit may be specific to extra-virgin olive oil.
Other Important Factors Related to CHD
We have focused primarily on nutritional contributions to CHD. Other major risk factors include
- blood pressure. Hypertension doubles the risk of CHD.
- smoking. Smoking also doubles the risk of CHD.
- exercise. A sedentary lifestyle is associated with CHD, partly because exercise increases the HDL (good) cholesterol and lowers blood pressure. People who engage in vigorous aerobic exercise on a regular basis may cut their risk of CHD in half. Moderate exercise, such as walking at a brisk pace, is also beneficial.
- obesity. As weight rises, so does the risk of CHD. This relationship is partly explained by the fact that obese people tend to have a cluster of other risk factors, such as high blood pressure, high blood cholesterol, and poor glucose tolerance. The risk is greater with excess fat in the abdominal area, characterized by a large waist circumference. This cluster of risk factors is known as the metabolic syndrome (see textbook p. 480).
- diabetes. The presence of diabetes greatly increases the risk of CHD. CHD is actually the major cause of death in diabetics.
Adopting a healthy lifestyle can dramatically cut one’s risk of CHD. This fact was revealed by the Nurses’ Health Study, a cohort study of 80,000 American nurses. The investigators identified the three per cent of nurses with the healthiest lifestyle based on 10 behaviours. These included smoking, exercise, weight, and seven dietary variables. Nurses who chose to follow this lifestyle pattern had six times less risk of CHD than the average nurse (Stampfer et al., 2000).
Another highly informative example is the case of Poland. Poland went through a severe economic and political crisis during the 1980s and into the 1990s. One of the results of this was a sharp decrease in availability of animal-based food products. From 1990–2002, mortality rates from CHD dropped by 40% (Zatonski & Willett, 2005). Other contributing factors were a decrease in smoking and an increase in intake of fruit and vegetables.
Section 2 Hypertension
Read pages 484–488, “Nutrition and Hypertension” of Chapter 11: “Diet and Health.”
Read pages 328–331 of Chapter 8: “Water and Minerals.”
Salt, Sodium, and Hypertension
Hypertension (high blood pressure) is the dominant risk factor for strokes, so we will assume that anything that lowers risk for hypertension will lower risk for strokes.
The textbook (p. 486, under the heading “The Role of Risk Factors”) describes atherosclerosis, obesity, and insulin resistance as risk factors for hypertension. All of these risk factors can be prevented; they are all affected by diet. Diet is an important factor contributing to hypertension.
By looking at clues from different types of research, there are strong grounds for believing that hypertension is influenced by an excessive intake of salt. Three types of research support this conclusion:
- Population studies: Populations with higher salt intakes have higher average blood pressure. A decline in stroke rates was observed with the introduction of refrigeration in the 1950s. Refrigeration reduced reliance on salt as a food preserver.
- Clinical trials: When subjects in controlled studies are fed high salt diets for several weeks, blood pressure increases. Conversely, lowering the salt content of the diet lowers blood pressure.
- Animal studies: In a certain strain of rats, hypertension can be induced with a high salt diet.
Investigators have continued their research by asking the question, “What would be the effect of a reduction in salt intake on the entire population?” A cut in salt intake of about 2.5 grams per day (about 1000mg of sodium) lowers systolic blood pressure (the higher of the two blood pressure readings) by about 3mm in those with hypertension and by about half as much in those with normal blood pressure (He et al., 2013). It is predicted that such a change by a whole Western population would reduce the incidence of stroke by around 22% and CHD by around 16%. This reduction is comparable to the decline that could be achieved by treating all people who have hypertension with drugs.
A conservative estimate of the Canadian cost of name-brand antihypertensive drugs is $600 million per year. Taxpayers also pay for visits to health professionals to assess blood pressure and monitor effectiveness and side effects of blood pressure medications. In the long term, encouraging Canadians to lower their salt and sodium intakes could be less costly than aggressive screening for, diagnosing, and treating hypertension. Food processors also need to be encouraged to lower the amount of sodium consumed via processed foods.
The second most important issue to address to lower risks of developing hypertension or to treat it is weight management. Maintaining weight in the normal range is especially important in blood pressure control. Weight loss (for those who need it) is valuable to help lower elevated blood pressure: when inches come off the waistline, millimetres come off the blood pressure.
Potassium is a nutrient that plays a similar role to sodium but in the opposite direction. Evidence from several types of research, including randomized controlled studies, indicates that a raised potassium intake leads to a lower blood pressure (Aburto et al., 2013; Mente et al., 2014). As with sodium, this effect is stronger in persons with hypertension.
Alcohol consumption is also linked to hypertension. Numerous studies have demonstrated that people with a relatively high intake of alcohol (more than four drinks a day) are at an increased risk of hypertension and stroke. However, a moderate intake (one or two drinks a day) tends to lower blood pressure (Temple, 2016).
Finally, exercise affects blood pressure. Research studies have demonstrated that exercise helps prevent blood pressure from rising. The factors affecting blood pressure are additive. Taken together, a healthy lifestyle is both a powerful preventive mechanism and a potent therapy for hypertension.
Caffeine is often attributed as a cause of hypertension, but research indicates mixed results. Caffeine contributes to only small increases in blood pressure, if any (Rakic et al., 1999; Sudano et al., 2005).
Diet and the Prevention of Cardiovascular Disease
In recent years, the primary approach to the prevention of cardiovascular disease has been to stress dietary patterns rather than individual substances found in the diet. Many components of the diet act in conjunction to affect cardiovascular health. It is extremely challenging to disentangle the roles of the vast numbers of substances contained in these foods. Therefore, the best advice is the simple advice, namely to eat a healthy diet as described in this unit and later units. At the same time, we should not ignore the importance of a small number of substances that are particularly important, namely trans fats and replacing saturated fat with polyunsaturated fat.
This approach also applies to the control of blood pressure. A diet that has attracted a great deal of attention is the Dietary Approaches to Stop Hypertension (DASH) (textbook pp. 486–487). The diet includes a relatively high content of fruit and vegetables; moderate amounts of low-fat dairy products, fats, and oils; and has a relatively low content of meat and sweets. This diet is of proven effectiveness for lowering the blood pressure. At the same time, there needs to be an emphasis on a much reduced intake of salt.