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Nutrition and Metabolism


What is fat?

Fat is one of the macronutrients in human diets.  Many people automatically associate fat with butter, lard or the greasy substances attached to animal meats.  In nutrition, plant oils, which are liquid at room temperature, are fats too. Fat represents the most complicated group of nutrients. There are many types of fats. Some of them are good and some are bad for your health. No natural food is made up of only one single type of fat.

What are functions of fat in human body?

Fat functions as energy fuel.  One gram of fat provides 9 calories, the highest of all nutrients. Excess energy from diets will be converted to and stored as fat in your body for later use. Fat also serves as building material for cell membranes and precursor for signaling molecules. Two particular types of fats, omega-3 and omega-6 also play important roles in the regulation of gene expression (turn on or turn off genes).

What are the major types of fat?

There are three major types of fats in human diets: saturated, unsaturated and trans fats. Since saturated fats have a great potency to increase cholesterol level in human blood, they are often called the “bad fats”. Unsaturated fats, including MUFA (monounsaturated fatty acids) and PUFA (polyunsaturated fatty acids), have been associated with beneficial effects in reducing the risks for cardiovascular diseases in human population. Therefore, they are often considered as “good fats”.  Trans fats are rich in artificially hydrogenated oils.  They cannot be properly broken down as energy fuel in human body yet have the greatest potency on cholesterol and fat tissue build-up.  Therefore, trans fats are the “worst” dietary fat. More details about the types of the fats and their effects on human health are described in Fat Metabolism 101. You will be surprised to know that not all unsaturated fats are the same. For example, omega-3 and omega-6 are both PUFAs but are associated with opposite effects on the risks for chronic diseases (Table 1).

Table 1. Common fatty acids in human diet.

Types Common name Structure Source
Saturated fats Lauric C12 : 0 Coconut oil, palm kernel oil
Myristic acid C14 : 0 Milk, coconut oil
Palmitic acid C16 : 0 Palm oil, milk, butter, cheese, cocoa butter, animal meat
Stearic acid C18 : 0 Palm oil, milk, butter, cheese, cocoa butter, animal meat
Unsaturated fats MUFA Palmitoleic acid C16 : 1 Marine animal oil
Oleic acid C18 : 1 Olive oil, canola, most dietary fat
omega-6 PUFA Linoleic acid (LA) C18 : 2 Corn, soybean, sunflower seeds and peanut oils
Arachidonic acid (AA) C20 : 4 Peanut oil. Small amount in meat, dairy products and eggs
omega-3 PUFA α-Linolenic acid (ALA) C18 : 3 Flaxseeds oil, olive oil, rapeseeds oil (Canola)
Eicosapentaenoic acid (EPA) C20 : 5 Fish oil, marine algae
Docosahexaenoic acid (DHA) C22 : 6 Fish oil, marine algae

How much fat do you need from your diet?

The Institute of Medicine of the National Academies recommends an Acceptable Macronutrient Distribution Range (AMDR) for fat intake between 20 to 35% of the total dietary calories for adults age 19 and older.  The AMDRs for fat are the same for both men and women but are higher for younger ages (Table 2)

Table 2. AMDR for fat for different age groups by the Institute of Medicine of the National Academies.

Age AMDR (% of total energy intake)
1 to 3 years 30-40%
4 to18 years 25-35%
19 years and older 20-35%

How does your genetic makeup determine the amount of fat you should eat?

Fat metabolism is controlled by many genes.  Many other metabolic pathways are also regulated by certain types of fats. Therefore, it is not surprising to see genetic variations of many genes, including APOE, APOA5, ADIPOQ, PPARs and LPL, determine how your body responses to the amount of fat you eat. Take the APOE gene as an example. The APOE4 variant has a higher rate of LDL (the bad cholesterol) formation so carriers are prone to high LDL cholesterol level.  In contrast, the APOE2 variant has a lower rate of LDL formation and the carriers normally have lower LDL cholesterol level. Due to the different in LDL formation rate, APOE4 carriers are better fitted with high-carb, low-fat diets while APOE2 carriers are better served by low-carb, high-fat diets. APOE4 variant is quite common in human population, ranging from 10 to 30% occurrence among ethnic groups. APOE2 variant is distributed from 2 to 13% depending on the ethnic groups.

Which foods are fat-rich?

Besides two forms of pure fats mainly used for cooking (animal fats and plant oils shown in Table 3), nuts and seeds, salad dressings, processed foods, chips and crackers, as well as dairy products are the most common fat-rich foods in modern life.  One ounce (14 halves) of English walnuts contains 18.5 grams of fat, an equivalent to 166 calories!

Table 3. Common fat-rich foods.

Animal fats and plant oil
Per 100 g (g) 100
Per Serving (g) 12.8
Serving Size 1 tbsp (12.8 g)
Canola oil
Per 100 g (g) 100
Per Serving (g) 14
Serving Size 1 tbsp (14 g)
Nuts and seeds
Walnuts, English
Per 100 g (g) 65.2
Per Serving (g) 18.5
Serving Size 1 oz (28 g)
Sunflower seeds, dry roasted
Per 100 g (g) 49.8
Per Serving (g) 14.1
Serving Size 1 oz (28 g)
Salad dressing
Ranch, commercial, regular
Per 100 g (g) 51.4
Per Serving (g) 7.7
Serving Size 1 tbsp (15 g)
Processed Foods
Bacon, pan fried
Per 100 g (g) 40.3
Per Serving (g) 6.4
Serving Size 2 slices (16 g)
Pizza, Cheese topping
Per 100 g (g) 12.3
Per Serving (g) 24.4
Serving Size 8 oz (199g)
Junk food
Potato chips
Per 100 g (g) 38.4
Per Serving (g) 10.9
Serving Size 1 oz (28 g)
French fries
Per 100 g (g) 16.1
Per Serving (g) 27.2
Serving Size 1 medium (169 g)
Dairy products
Cheese, cheddar
Per 100 g (g) 33
Per Serving (g) 9.3
Serving Size 1 slice (28 g)
Whole milk
Per 100 g (g) 3.25
Per Serving (g) 7.93
Serving Size 1 cup (244 g)

How to improve diets with healthy fats?

First you are recommended to use the GB Food Calorie and Nutrition Calculator to get the most accurate estimate of your current fat intake. Depending on your genetic background and your health status, you may want to maintain, increase, or decrease your fat intake.

In general, you want to increase the “good” fats and decrease the “bad” fats no matter how much fats you are getting from your diets.

First, avoid trans fats (the “worst” fats) as much as possible since trans fats are accumulative and the harmful effects are long lasting.  Trans fats are normally found in deep fried junk food such as fried potato chips and certain artificial fat substitutes such as margarine.  In the United States, trans fat less than 0.5% is listed as zero per serving in the Nutrition Facts label. You need to read the ingredients section to see if hydrogenated or partially hydrogenated oils, which are the main sources of trans fats, are used as ingredient when you are doing grocery shopping. If they are, you should avoid the food.

Secondly, limit the amount of saturated fats (“bad” fats).  Animal fats normally contain high level of saturated fats as well as high level of cholesterols.  

Be aware that not all plant oils are “good”.  Plant oils from two particular sources, coconuts oil and palm oil, are also mainly saturated fats and should be limited. Certain plant oils, such as corn oil, peanut oil and sunflower oil are imbalanced at the omega-3 and omega-6 ratio. These oils contain high levels of omega-6 but no omega-3. Long term consumption of these oils may increase your risks for chronic heart diseases or inflammations.

To get sufficient fat intake without risk too much saturated fat, you can switch to olive oil or canola oil as cooking oil. Both of these two oils have a high percentage of unsaturated fatty acids (the “good” fats) and a balanced omega-3 and omega-6 ratio
You can also get “good” fats from deep sea fish such as salmons and sea food such as oysters. These foods provide high levels of omega-3 as well as high quality proteins.

To reduce fat intake, you should limit the fat-rich foods as shown in Table 3. You should also choose fat free dairy products, or select lean meats, or trim off as much fat as possible when consume animal-based foods. Salad dressings are very high in fat. You need to pay attention to the amount of salad dressings when making salad to avoid over eating fats.

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