Fats

Functions of Dietary Fats

• Fats, along with other two main macronutrients–carbohydrates and proteins–are a source of energy.
• Fats are necessary for the absorption of the fat-soluble vitamins A, D, E and K.
• In your body, fats can be converted to glucose, amino acids, cholesterol, bile acids, hormones or to structural lipids (phospholipids, lecithin, mainly in the cell membranes) ^[13].

Fat Types

• By origin: animal and plant fats
• By the type of bonds in the fatty acids:
  • Saturated
  • Unsaturated
    • Monounsaturated
    • Polyunsaturated
    • Trans fats

Recommended Fat Intake per Day

According to the Institute of Medicine in the US, the Recommended Daily Allowance (RDA) for fat for adults is 130 grams per day ^[8].

Chart 1. Recommended Daily Allowance (RDA) for Fat
AGE	RDA (g/day)
0-6 months	60
7-12 months	95
Children >1 year of age and adults	130
Pregnant women	175
Breastfeeding women	210

Chart 1 source: Institute Of Medicine ^[8]

Calories in Fats

All types of fats, animal and vegetable, saturated and unsaturated, contain 8.7 Calories per gram of the “net metabolizable energy” ^[1].

How many calories are in the pound of the body fat?

1 pound (454 g) of the body fat tissue contains about 87 % of the actual fat, that is 395 grams, which, multiplied by 8.7 Calories (energy in 1 g fat) gives 3,437 or about 3,500 Calories per 1 pound of the body fat tissue. However, when you lose weight, you do not lose only the body fat but also some muscle protein. An obese person with a lot of body fat initially loses weight mainly by burning the fat tissue and only a small amount of the muscle tissue, so a deficit of 3,500 Calories results in a loss of about 1 pound of body fat, but later, when a lot of body fat has been burnt, a deficit of 3,500 Calories results in a loss of less than 1 pound of fat tissue and in a loss of greater amount of muscle tissue than initially, altogether in a loss of more than 1 pound of the body weight, because burning of muscle tissue requires less energy than burning of fat tissue (loss of 1 pound of muscle tissue requires only about 1,800 Calories) ^[5,6].

How do you calculate calories from fats?

A calorie from fat you consume is the same as a calorie from carbohydrates or proteins. This means, 1,000 Calories from fat will give you the same amount of energy or will result in the same weight gain as 1,000 calories from carbohydrates or proteins.

What is the recommended percent of calories from fats?

The Institute of Medicine in the U.S. recommends 20-35% of calories from fat for children 4 years or older and adults; this is the so called Acceptable Macronutrient Distribution Range (AMDR) for fat ^[8].

Picture 1. Example of fatty foods

Dietary Fat Sources

Fat Digestion

1. Emulsification. In the first two parts of the small intestine–in the duodenum and jejunum–fats are broken down into small particles and mixed with the bile salts into micelles, which makes them able to be digested ^[10].
2. Digestion. Triglycerides are partially broken down in the stomach with the help of the enzyme gastric lipase, but mainly in the duodenum with the help of the enzyme pancreatic lipase into free fatty acids and monoglycerides ^[10].

Fat Absorption

1. Fatty acids and monoglycerides are absorbed into the small intestinal lining cells, where they join together to form triglycerides ^[11].
2. From the intestinal cells, a small fraction of lipids, namely medium-chain fatty acids (MCFA), are absorbed directly into the venous blood in the portal vein, which carry them to the liver. Triglycerides are packed together with proteins into large lipid particles called chylomicrones, which are absorbed into the lymph, which is carried by the lymphatic vessels into the venous blood ^[11].

Fat Malabsorption

Causes ^[12]:

• Liver diseases (hepatitis, cirrhosis) with decreased production of bile or disorders that prevent the bile delivery to the small intestine (gallstones, gallbladder or bile duct cancer, abdominal adhesions, inborn defects of the bile duct)
• Small intestinal bacterial overgrowth (SIBO) that can result in inactivation (deconjugation) of bile salts
• Pancreatic disorders (pancreatitis, cancer) with decreased production of the enzyme lipase
• Intestinal disorders: celiac disease, Crohn’s disease, tropical sprue, AIDS, infections (tuberculosis, Whipple’s disease, giardia, cryptosporidiosis, microsporidiosis), intestinal lymphoma, amyloidosis, lymphangiectasia (congenital narrowing of lymphatic vessels), damage of the small intestine by chemotherapy or radiation, surgical removal of a large part of the small intestine (short bowel syndrome)

Symptoms ^[12]:

• Acute symptoms: loose, whitish, bulky, sticky, floating and foul smelling stools (steatorrhea).
• Chronic symptoms: steatorrhea, weight loss, night blindness (due to vitamin A deficiency), bone pain and fractures (due to osteoporosis caused by vitamin D deficiency), easy bleeding (due to vitamin K deficiency), numbness and tingling in limbs (due to a peripheral neuropathy caused by vitamin E deficiency).

Diagnosis

Fat malabsorption can be confirmed by finding excessive amount of fat in the stool (positive fecal fat test).

Fat Metabolism – The Fate of Fat After Absorption

After absorption, dietary fats appear in the blood as triglycerides, which are carried by the blood lipoproteins (LDL and HDL), and free fatty acids, which are carried by the blood proteins called albumins.

From the blood, fats enter the body cells, where they can be burned to release energy or are converted to other substances.

1. During rest or moderate physical activity, in healthy people who eat regularly, dietary fats are the primary source of energy for the skeletal muscles and heart, but not for the brain, which can use only glucose as a source of energy. When you consume more fat than you burn, excessive fat is stored in the body fat cells (adipocytes) under the skin and in the abdominal cavity (body fat stores).

2. During the prolonged hard exercise (marathon), low-carb diet (high-fat or ketogenic diet) or fasting for more than one day, when your body glucose stores (glycogen) are depleted, dietary and body fats can be used as a source of energy for the muscles and heart. In your body, some fats can be converted to glucose–in the process called gluconeogenesis–and some to ketones, both of which can be used as an energy source by the brain and other organs ^[11]. Increased amount of ketones in the blood is called ketosis, which can result in increased excretion of ketones in the urine–ketonuria; ketosis and ketonuria due to fasting, low-carb diet or exercise have not been proven harmful for health. Ketones give a fruity-sweet-smelling odor to the breath and urine, though.

3. In untreated diabetes mellitus type 1 and 2, the body may not be able to use glucose because of lack of insulin or resistance to insulin, which prevent glucose to enter the body cells, so the dietary and body fats start to break down into fatty acids and ketones to provide energy, but when ketones appear in the blood in excessive amounts they can cause a life-threatening ketoacidosis ^[41].

Fats and Satiety

Fats are less satiating than proteins but probably as satiating as carbohydrates ^[15]. Fats can make certain foods, for example, vanilla yogurt, more palatable and can thus stimulate calorie intake.

Fats and Blood Lipids

Results of studies about the effect of fat intake on blood lipid levels are shown in Chart 4.

Good and Bad Fats

Good Fats

We have found insufficient evidence to claim any type of fat by itself is good.

Bad Fats

The only fats that were consistently associated with increased risk of cardiovascular disease in many studies were trans fats ^[33]. At this point of research, we can not say any other fats by themselves, like saturated fats, are harmful for health ^[33].

Animal and Plant Fats

Animal Fats

We have found insufficient evidence to claim that animal fats are more harmful for health than plant fats or vice versa.

In various reviews of studies, high meat, red meat and beef intake was associated with increased risk of colon cancer ^[70,72], dietary fat, red meat and milk intake was associated with increased risk of prostate cancer ^[58], and processed meat, but not red meat, was associated with increased risk of coronary heart disease and diabetes 2 ^[71]. To our knowledge, the direct cause-effect relationship between animal fat and any disease has not been proven so far, though.

Plant Fats

• In one 2011 systematic review of studies, high intake of olive oil was associated with decreased risk of cancer, but it is not clear if the association was due to monounsaturated fats or other olive oil ingredients ^[73].
• In one study ^[74], but not in one systematic review ^[75], high palm oil intake was associated with increased risk of chronic heart disease.

Insufficient Evidence of Health Benefits

There is insufficient evidence about the association between high-fat diet and weight loss, the risk of cardiovascular disease or diabetes mellitus 2 in healthy individuals ^{[24,25,38,52,68]}, and fasting blood glucose, HbA1C, and total and LDL cholesterol levels in individuals with diabetes mellitus type 2 ^[28,37], and changed behavior in children with autism ^[17,18].

There is insufficient evidence about beneficial health effects of adding fats to carbohydrate meals in individuals with diabetes mellitus. In several studies, fatty foods, when consumed with or 30 minutes before carbohydrate meals slowed gastric emptying, that is delivery of carbohydrates to the small intestine, and lowered blood glucose spikes after meals in some ^[19,20], but not all, individuals with diabetes 1 ^[21] or diabetes 2 ^[22].

There is insufficient evidence about the association between high intake of omega-3 fatty acids from foods (fish, some vegetable oils) or supplements and reduced risk of cardiovascular disease.

Replacing saturated fats with carbohydrates or proteins seems to have no beneficial effect on cardiovascular disease ^[46,52].

Insufficient Evidence of Health Risks

There is insufficient evidence about the association between the total fat intake and the risk of obesity, coronary heart disease, diabetes mellitus 2 ^[29,32,33] and cancer ^[58,59]. According to several systematic reviews of studies, there is insufficient evidence about the beneficial effect of low-fat diet on high blood cholesterol levels in otherwise health individuals with acquired hypercholesterolemia ^[26], or on the risk of heart disease ^[27].

According to several ^{[30,33,43,46,50,52]}, but not all ^[47,57], systematic reviews of studies, there is insufficient evidence about the association between high intake of saturated fats (red meat, high-fat dairy) and the risk of cardiovascular disease (heart attack, stroke).

The DASH (Dietary Approaches to Stop Hypertension) eating plan, which includes diet low in saturated and trans fats and high in potassium, calcium and magnesium, was associated with decreased risk of high blood pressure, but it is not known how much, if anything, fat intake contributed to this effect ^[53].

Low-Fat Diet

A low-fat diet is a diet in which less than 20% of calories comes from fats ^[65]. For a person with an intake of 2,000 Calories per day, a low-fat diet would include less than 45 grams of fat per day. For a list of foods low in fats check Chart 3 above.

Possible Benefits

There is insufficient evidence of health benefits of low-fat diets for healthy individuals ^[68]. Low-fat diet, without calorie intake reduction, does not result in weight loss.

• In individuals with Crohn’s disease, especially after partial surgical removal of the small intestine ^[61], low-fat diet can reduce fatty diarrhea (steatorrhea). Low-fat diet does not seem to reduce the bowel inflammation in Crohn’s disease, though ^[62].
• Low-fat diet can also help prevent fatty diarrhea in active celiac disease, lymphoma, chronic pancreatitis, liver disease, blocked bile duct and other causes of fat malabsorption ^[65].
• In chylothorax–the presence of a fat-rich fluid, called chyle, in the space between the lung and lung membrane–, which usually occurs after a surgery involving chest organs, or spontaneously in infants, a low-fat diet can help prevent chyle leakage ^[63].

Possible Harms

• Very low-fat diet can:
  • affect the absorption of fat-soluble vitamins (A, D, E, K)
  • rarely, result in a deficiency of essential fatty acids with scaly rash and slow wound healing, mainly in infants fed with skim-milk formula, which is low in linoleic acid ^[54,60]
• During rapid weight loss, low-fat diet may increase the risk of gallstones ^[55,56].

Fat-Free Diet

A fat-free diet includes less than 2 grams of fat per day. One of the rare reasons for a short-term fat-free diet lasting for several days, may be chylothorax (see above).

Frequently Asked Questions

1. Does low-fat diet helps to lose weight quicker than high-fat diet?

It is the greater amount of calories you spend than those you get with food that cause weight loss; the percent of fat in the diet by itself probably does not influence the rate of weight loss. However, some people stick easier with low- and some with high-fat diets.

2. Are fats a good source of energy?

1 gram of fats can provide about 9 Calories, while 1 gram of carbohydrates and proteins can provide about 4 calories. This means fats are more calorie dense but this does not automatically mean they are a better source of energy than carbohydrates and proteins.

3. Are there any fat burning/fighting foods for women?

No.