Special Report: An Overlooked Risk Factor in Heart Disease

Q: What are triglycerides?

A: Dietary fat consists predominantly of triglycerides (TGs); Also called triacylglycerols (TAGs), triglycerides consist of a glycerol backbone to which three fatty acids are linked. Fatty acids differ in their chain length, number, position, and geometry (c/'s or trans) of their double bonds. These differences determine the properties of the fatty acids, such as their melting characteristics, solubility in water, and health properties. Fatty acids that have all the carbon atoms in the chain linked by single bonds are called saturated fatty acids (SFAs).

Unlike SFAs, unsaturated fatty acids contain one double bond (monounsaturated fatty acid [MUFAs]) or more double bonds (polyunsaturated fatty acid [PUFAs]) at specific positions in the carbon chain. The nature of. TGs, and dietary fat therefore, is determined by their fatty acid constituents. Most dietary fats consist of a mixture of SFAs, MUFAs, and PUFAs. Fats that are rich in SFAs tend to be solid at room temperature; fats rich in MUFAs and PUFAs tend to be liquid at room temperature.

Q: What is the purpose of triglycerides?

A: TGs are an important energy source at 9 kilocalories per gram, (kcal./g.), compared with 4 kcal./g. for carbohydrate and protein. This is particularly important for newborns and young children, who have high energy demands.

As TGs deliver the fatty acids in dietary fat, they are also important for the absorption of fat soluble vitamins (A, D, E, and K) and for providing essential fatty acids (omega-3 and omega-6). These fatty acids have a whole host of functions of their own, such as cell structure and signaling, growth, gene expression and production of eicosonoids. They make food more palatable by giving a creamy mouth-feel to food. They also act as a solvent to flavors.

Q: Does the body manufacture tyiglycerides, or are they found in foods?

A: The body is able to make fatty acids and TGs from excess dietary carbohydrate; however, it is not able to make the essential fatty acids (omega-3 and omega-6). That is why they are called essential. Different kinds of fatty acids and therefore TGs can be found in various foods; this is why, as with all nutrients, it is important to get a healthy balance of different foods.

Q: What is a normal blood triglyceride level?

A: Normal TG levels range between 0.5 and 2.0 millimoles per liter (mmol./L.), but they increase following a meal containing fat; thus, it is important to measure levels in the fasting state. Fasting guidelines are below 1.7 mmol./L.

Q: What role do triglycerides play in heart disease?

A: Plasma TGs are an independent risk factor for cardiovascular disease (CVD). A recent meta-analysis reported that each 1 mmol./L. increase in TGs increased the relative risk of CVD by 14 percent in men and 37 percent in women after adjustment for high-density lipoprotein-cholestrol (HDL-C) levels. However, it is argued that they should be considered more as a marker for risk rather than as an independent factor, thus keeping the focus on cholesterol.

Fasting values and postprandial (after-meal) values influence a number of mechanisms involved in the atherosclerotic processes. TGs circulate in the blood in specialized lipoproteins, collectively called TG-rich lipoproteins, particularly very-low-density lipoproteins (VLDLs) and chylomicrons). These TG-rich lipoproteins have a direct atherogenic effect. Furthermore, elevated TG levels may influence mechanisms associated with the development of CVD, including thrombogenesis (blood clotting), endothelial dysfunction (impairment of the vascular wall), and oxidative stress.

Q: How do triglycerides differ from cholesterol?

A: Both TGs and cholesterol circulate in the blood and originate in part from the diet (an exogenous source); they are also producedby the body (an endogenous source). They are both classified as lipids, but they have different structures and functions within the body.

Relatively small amounts of cholesterol are consumed in the diet (less than 1 percent dietary lipid) in comparison to the amount of TGs consumed (approximately 98 percent dietary lipid). Cholesterol is also produced by the liver and circulates in lipoproteins (VLDL, LDL, and HDL), which have complex and specialized functions.

Cholesterol that circulates in low-density lipoprotein (LDL) is considered 'bad' because it is carried to peripheral tissues and can be taken up into arterial walls. Cholesterol circulating in HDL is considered 'good' because it is carried away from peripheral tissues to the liver for removal. This is why the ratio of LDL-C and total cholesterol to HDL-C is so important in CVD risk.

Q: How do triglycerides interact with cholesterol?

A: The interaction of TGs and cholesterol is complex and involves the interplay of lipoproteins (chylomicrons, VLDL, LDL, and HDL) and the exchange of TG and cholesterol between these lipoproteins. The level of TGs in the blood and its composition also influences the, production of cholesterol by the liver. For example, a high level of saturated TG in the diet increases the delivery of saturated fatty acids to the liver and has subsequent effects on cholesterol production by the liver.

Q: How do genes affect triglyceride levels?

A: Numerous genetic factors influence TG levels by their effect on different receptors, enzymes, lipoproteins, and transport proteins involved in TG metabolism.

Q: Which foods raise triglyceride levels?

A: A diet with very high intakes of carbohydrates increases TG levels because there is an increased conversion of excess carbohydrate to TG in the liver.

Q: How do almonds lower triglyceride levels?

A: A study carried out by my group at King's College London {American Journal of Clinical Nutrition, 2008) found that almond nuts resulted in a lower level of TG concentrations compared to a meal rich in sunflower oil in the eight hours following the meal. As we know that the immediate changes that occur in TG concentrations following a high-fat meal influence the risk of CVD, this acute TG-lowering effect of almonds may have beneficial implications for CVD risk. However, it is also important to investigate whether almonds lower TG concentrations in the long term and therefore reduce fasting TG concentrations.

Q: Approximately how many almonds a day would we have to eat to lower triglyceride levels?

A: Almonds should be consumed as part of an overall healthy eating and lifestyle and one food should never be considered the 'cure-all' for any disease or condition. Individuals should look to certified health care professionals, like registered dietitians, to help them develop a diet appropriate for their medical condition. One-ounce, or 23 almonds, however, can be incorporated into a diet low in saturated fat and cholesterol to help maintain healthy cholesterol levels. This is evidence that almonds, when incorporated into a balanced diet, lower total and LDL-C concentrations. However, as mentioned previously, it is still uncertain whether almonds influence fasting TG concentrations.

Based on all the scientific evidence regarding nuts and heart disease, the Food and Drug Administration (F.D.A.) has approved a qualified health claim that states the following:

"Scientific evidence suggests but does not prove that eating 1.5 ounces per day of most nuts, such as almonds, as part of a diet low in saturated fat and cholesterol may reduce the risk of heart disease."

Q: Are almonds considered to be high in fat?

A: Almonds, like all nuts, are high in fat. However, the primary fat in almonds is monounsaturated, which is good fat. Evidence now shows that the type of fat (monounsaturated versus saturated) is more important for us in terms of CVD than the total amount of fat (except at excessive intakes). So most of us should be trying to ensure we consume the right type of fat, like that found in almonds, rather than worry too much about the quantity of fat. However, as a note of caution, some of the population does consume too much fat, so some of us need to think about the type of fat as well as the amount we are consuming.

Interestingly, studies undertaken by our group at King's College London have found that a large proportion of the fat in almonds may be unabsorbed and excreted (up to 40 percent). A one-ounce serving, or 23 almonds, has 13 grams of unsaturated fat and only 1 gram of saturated fat. The U.S. Dietary Guidelines recommend that the most of our fat intake be unsaturated.

Almonds offer other nutritional attributes besides fat. One ounce of almonds is an excellent source of vitamin E (7.4 milligrams [mg.] of alphatocopherol), magnesium (76 mg.), and manganese (0.6 mg.). It is also a good source of copper (0.3 mg.), phosphorus (137 mg.), and riboflavin (0.3 mg).

Q: Is fat in almonds dangerous?

A: The fat in almonds in unsaturated and is therefore considered a healthy fat. The main fatty acid is oleic acid, which is monounsaturated, also found in large amounts in olive oil. Large clinical studies have shown that replacement of saturated fat or carbohydrate with mono-unsaturated fat results in lower total and LDL-C levels. Many research and clinical studies have investigated the effects of almonds oh health and have demonstrated the almond's role in helping to maintain a healthy heart and healthy cholesterol levels. One such study is from Linda Loma University, where researchers studied the effect of almonds on cholesterol levels. In this study, 25 healthy subjects (14 men, 11 women) with a mean age 41 consumed three diets for four weeks with a two-week run-in diet: The diets were similar in the number of calories: a Step 1 diet (as a control); a low-almond diet (10 percent of the energy from almonds); and high-almond diet (20 percent of the energy from almonds). The low-almond diets lowered LDL-C levels ('bad' cholesterol) by 3 percent, whereas the high almond diet lowered LDL-C levels by 9 percent.

Unlike other nuts, almonds have been involved in two dose-response studies, which show that twice the effect with twice the dose; the Loma Linda study is one of these. Our research on TGs builds upon the body of evidence we already have on the benefits of including one ounce or 23 almonds, as part of a diet low in saturated fat and cholesterol.


An Interview with Sarah Berry

Sarah Berry, M.D., is Lecturer at King's College and divides her time between research and teaching in the Department of Nutrition and Dietetics. Her studies is focus on the broad theme of diet and cardiovascular risk, particularly dietary lipids (triglycerides, or triacylgly-cerols) and the effects and mechanisms by which they affect vascular function and cardiovascular health. She is also investigating the health effects of interesterified fats, which are used by the food industry as alternatives to trans fats).

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