Nutrition Without the BS (Part II — Fats)

You are reading the second post about what science knows about nutrition. Part I covers digestive tract and carbs. The second part is the story about the fat.

We eat three types of dietary fats: triglycerides, phospholipids, and sterols.

Triglycerides

Triglycerides are quantitatively the most important; they take up approximately 95% of our daily fat consumption. Vegetable oils such as sunflower oil, olive oil, or peanut oil entirely consist of triglycerides.

The fatty acids that constitute triglyceride molecule differ in the degree of unsaturation which refers to the number of double bonds in their chemical makeup.

• Unsaturated fatty acids have at least one double bond.
• Monounsaturated fatty acids have one double bond
• Polyunsaturated fatty acids have at least two double bonds.

Primary fatty acids in our diet are oleic, palmitic (the first two account for approximately for 2/3 of our fatty acid intake), stearic, linoleic and
linolenic.

The firmness of the fat, whether it’s liquid or solid, is determined by the types of fatty acids attached to the glycerol molecule. The shorter the fatty acids attached, the lower the melting point of the fat and low melting point means that the fat will be liquid at room temperature.

Similarly, the more unsaturated the fatty acids that are attached, the lower the melting point, triglycerides composed of unsaturated fatty acids are, therefore, liquid at room temperature. Vegetable oils (sunflower, corn oil, olive oil, peanut oil, canola oil, soy oil) all consist mostly of unsaturated fatty acids, and they’re liquid at room temperature, butter is solid at room temperature, that’s because it mostly contains saturated fatty acids.

Triglycerides containing mostly saturated fatty acids are chemically more stable, and they are less prone to oxidation.

Hydrogenation describes the process in which unsaturated fatty acids convert into saturated fatty acids. Hydrogenation makes the oil more stable and less prone to spoilage and converts fatty acids naturally present in foods into trans fatty acids. Trans fatty acids have a negative influence on health.

1. Phospholipids

Phospholipids a are an important component of all cell membranes and are, therefore, present in small quantities in almost all plant and animal foods. They are also added during food manufacturing as an emulsifier (emulsifiers help mix oil and water to create a stable emulsion).

2. Sterols

The main sterol present in animal foods is cholesterol. Plants sterols are called stanols. Most of the ingested plant sterols leave the body via the stools. They also inhibit cholesterol absorption

Fat Digestion

Digestion of dietary fat involves the breakdown of the triglyceride molecule into two fatty acid molecules and one monoglyceride molecule. The fatty acids are taken by the body’s cells while the monoglycerides end up in the liver.

The liver releases a particle called very-low-density-lipoprotein (VLDL), a tiny lipid droplet that is surrounded by a coat of protein. It contains triglycerides and cholesterol. VLDL is taken up by the adipose tissue or muscles as an energy source when most of its triglyceride content is removed. What remains is low-density-lipoprotein (LDL).

Low-density lipoprotein has been very strongly linked to heart disease. So, the low-density lipoprotein is formed from very low-density lipoprotein as most of the triglycerides are removed and the cholesterol remains part of the molecule. Low-density lipoprotein serves as a way to transport cholesterol from the liver to tissues that need it (cholesterol is a the part of the cell membrane).

Tissues take up LDL particles via LDL receptor. If that receptor is defective, the body is not able to properly take up LDL cholesterol, and it remains in the bloodstream, and you are at risk of premature heart disease. Other people have an LDL receptor that works extremely well and as a result, they very effectively remove that LDL from their bloodstream and therefore they hardly ever get heart disease.

Cholesterol has a number of functions in the body:

• It is needed to make bile acids, which in turn are required for efficient digestion of dietary fats.
• It is used to make steroid hormones
• It is a component of cell membranes

Cholesterol is not a nutrient because our cells can make cholesterol. Only approximately one-third of the cholesterol in our bodies comes from our diet. The rest is synthesized in the body. For this reason, we can subsist on a diet that is completely devoid of cholesterol.

Fat Tissue

Body fat is composed of adipocytes (or fat cells). Adipose tissue has a number of functions:

• Heat insulator
• Energy storage
• Endocrine function

The third function was found only about 20 years ago when leptin was discovered. It refers to the ability of adipose tissue to produce hormones. These hormones help the adipose tissue to communicate with the rest of the body in regulating food intake. What is interesting is that most of the problems that arise in people that have very little body fat are not because they don’t store any fat, but because the production of certain hormones is disturbed.

Fat Cells

Fat tissue is composed of fat cells (adipocytes). But, how does the fat get in adipocytes in the first place?

When fat is absorbed into the bloodstream, it is put into specialized particles, chylomicrons, and these chylomicrons deliver the fat throughout the body. Fat mainly goes towards two organs: the muscle and the fat tissue. These two organs are characterized by the presence of an enzyme called lipoprotein lipase. Lipoprotein lipase breaks down these triglycerides in chylomicrons allowing the fat to be taken up into tissues.

If you are physically active after a fatty meal, the lipoprotein lipase in the muscle will be very active, and most of the consumed fat will be sucked into muscles to be used as a fuel. If you are inactive, most of that fat will be stored because lipoprotein lipase in the fat tissue will be most active. That’s how fat gets stored in the adipocytes.

Fat cells constantly switch throughout the day, from being a net consumer of fat to being a net releaser of fat.

Total fat mass is determined by the number of fat cells and the size of the individual fat cells. The individual fat cells can increase, this is called hypertrophic obesity, but also can have an increase in the number of fat cells, this is called hyperplastic obesity. For most people, the number of fat cells remains constant throughout the adult life. That means moderate changes in fat cells mostly occur via changes in the size of individual fat cells, meaning the fat cells expand when you gain weight, and they shrink when you lose weight. The average life span of a fat cell is about ten years, after which the fat cell is renewed (this is called fat cell turnover). At the same time the content of fat cells changes much more quickly (at a rate that is about six times faster).

Brown Fat

Brown fat exists in some mammals. It is involved in a process called cold-induced thermogenesis. What it means is that the tissue can produce heat, so it allows the animal to keep itself warm, via the process called chemical uncoupling. Brown fat cells are different from regular fat cells. They contain more mitochondria and multiple smaller lipid droplets.

Brown fat is activated when animals are exposed to the cold resulting in the production of heat and allowing the animals to keep itself warm. It was believed that brown fat is not particularly important for adult humans, but this changed in 2009 when a number of papers that were published together in the New England Journal of Medicine showed that adult people do have brown fat.

This finding may imply that people could lose weight by stimulating the heat production in the brown fat cell. It all depends on how important brown fat turns out the be in humans, at this point we don’t have the answer yet, but the results we have so far are quite promising. It also seems that obese individuals have less brown fat.

Essential Fatty Acids

Mammals lack the ability to produce essential fatty acids, so it needs to be provided in food. Essential fatty acids are linoleic acid (Omega 6) and linolenic acid (Omega 3).

Eicosapentaenoic acid (EPA), may be derived inefficiently from linolenic acid and is the main fatty acid found in fish.
Docosahexaenoic acid (DHA), also an Omega 3 fatty acid, is synthesized in the body from alpha-linolenic acid and is found in fish. DHA is present in retina and brain.

Cardiovascular disease and atherosclerosis

Cardiovascular disease (CVD, also called heart disease) is a class of disease that involves the heart and the blood vessels.

The most common form of cardiovascular disease is coronary heart disease (CHD), in which the coronary arteries (the blood vessels providing blood to the heart muscle) become narrow and rigid, restricting blood flow to the heart. Coronary heart disease is also called ischemic heart disease.

Another common form of cardiovascular disease is the stroke, in which the arteries supplying blood to the brain become blocked and blood flow is impaired.

Cardiovascular disease is most often rooted in a process called atherosclerosis. Atherosclerosis describes the buildup of plaque in the walls of arteries, leading to narrowing of the arteries and gradual obstruction of blood flow. Other cardiovascular diseases include heart failure, diseases of the heart muscle (cardiomyopathy), heart valve problems, and arrhythmias.

Ischemic heart disease is the leading cause of death in high-income countries.

Blood Cholesterol

Cholesterol is transported in two major forms:
- LDL (Low-density lipoprotein) and
- HDL (High-density lipoprotein)

LDL carries about 2/3 of the cholesterol in the bloodstream, and HDL carries another 1/3. The function of HDL is opposite to that of LDL. Instead of delivering cholesterol to tissues, HDL picks up cholesterol from the cells and tissues and delivers it back to the liver.

There is solid evidence that LDL contributes to coronary heart disease. LDL is taken up by specialized cells called macrophages in the artery wall, and these macrophages become foam cells that accumulate and form the atherosclerotic plaques that are at the root of coronary heart disease.

HDL is used to be called “good cholesterol”, but some more recent data suggest a more complex picture:

• Epidemiological studies confirm that people that have high HDL cholesterol levels are at lower risk of developing heart disease.
• The genetic evidence is not very conclusive in the sense that people that have genetically high HDL levels do not necessarily have a lower chance of developing heart disease.
• Drugs that raise HDL cholesterol don’t lower the risk of heart disease; they increase it.

Risk Factors for CVD (Cardiovascular Disease) and CHD (Coronary Heart Disease)

• Age.
• Family history (majority of genes contributing to CHD function independently of conventional risk factors. How these genes impact CHD is unknown).
• Gender (males are at higher risk, but this gender difference is decreasing).
• Obesity / diabetes
• Hypertension
• High blood levels of cholesterol and triglycerides (to a lesser extent)

Dietary Lipids and Health

1. Saturated Fatty Acids

Early ecological studies have linked saturated fat to heart disease, and this was the basis for the recommendation to limit saturated fat intake. Subsequently, other studies showed no such effect. Lately, the recommendation to lower SFA intake has been called into question, but the American Heart Association still recommends lowering SFA.
Even if eating SFA does not elevate the risk of heart disease, replacing it with non-saturated fat would be helpful since unsaturated fat lowers LDL cholesterol.

2. Unsaturated fatty acids

Unsaturated fatty acids, with the exception of trans fatty acids, have repeatedly been shown to lower blood LDL levels. This effect is most pronounced for the polyunsaturated fatty acids (PUFA) belonging to the Omega 6 family, represented by linoleic acid (found in vegetable oils such as sunflower oil)

3. Omega 3 fatty acids

The group of n-3 PUFA (=omega 3 PUFA) includes linolenic acid, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Early observations showed that populations that eat a lot of oily fish have a low occurrence of CVD. This observation is the basis for a recommendation to eat fish at least twice a week.

Thousands of studies have been done, and these studies show a wide range of outcomes, from no effect to 40% reduction in heart disease. Still, most experts judge the overall evidence in favor of a protective effect of n-3 PUFA on heart disease, whereas effects on measures of heart dysfunction (fibrillation, arrhythmia) are uncertain.

4. Trans fatty acids

They are found on the labels under the name partially hydrogenated oil or vegetable shortening. Consumption of industrial trans fats has been linked to numerous adverse health consequences, including cancer, diabetes, and coronary heart disease. While the link to cancer and diabetes is weak, the evidence is abound showing that intake of industrial trans fat is associated with elevated risk for CHD.

Most food producers in the developed world have moved away from using these in their manufacturing process, and some countries have started to ban it.

5. Dietary Cholesterol

Dietary cholesterol contributes only about 1/3 of the cholesterol content in the body, so the blood cholesterol level is relatively insensitive to dietary cholesterol intake. This finding has prompted a change in the recommendation, and it is now considered that dietary cholesterol intake is not a concern.

Next up: Protein.