thumb|Idealized representation of a molecule of a typical [[triglyceride, the main type of fat. Note the three fatty acid chains attached to the central glycerol portion of the molecule.|alt=A space-filling model of an unsaturated triglyceride.]]
thumb|Composition of fats from various foods, as percentage of their total fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food. This helps to protect vital organs, until such time as the offending substances can be metabolized or removed from the body by such means as excretion, urination, accidental or intentional bloodletting, sebum excretion, and hair growth.
Adipose tissue
thumb|The [[obese mouse (left) has large stores of adipose tissue. For comparison, a mouse with a normal amount of adipose tissue is also shown (right).]]
In animals, adipose tissue (fatty tissue) is the body's means of storing metabolic energy over extended periods of time. Adipocytes (fat cells) store fat derived from the diet and from liver metabolism. Under energy stress these cells may degrade their stored fat to supply fatty acids and also glycerol to the circulation. These metabolic activities are regulated by several hormones (e.g., insulin, glucagon and epinephrine). Adipose tissue also secretes the hormone leptin. the glycerol component of triglycerides can be converted into glucose, via gluconeogenesis by conversion into dihydroxyacetone phosphate and then into glyceraldehyde 3-phosphate, for brain fuel when it is broken down. Fat cells may also be broken down for that reason if the brain's needs ever outweigh the body's.
Triglycerides cannot pass through cell membranes freely. Special enzymes on the walls of blood vessels called lipoprotein lipases must break down triglycerides into free fatty acids and glycerol. Fatty acids can then be taken up by cells via fatty acid transport proteins (FATPs).
Triglycerides, as major components of very-low-density lipoprotein (VLDL) and chylomicrons, play an important role in metabolism as energy sources and transporters of dietary fat. They contain more than twice as much energy (approximately 9kcal/g or 38kJ/g) as carbohydrates (approximately 4kcal/g or 17kJ/g).
Nutritional and health aspects
The most common type of fat, in human diet and most living beings, is a triglyceride, an ester of the triple alcohol glycerol and three fatty acids. The molecule of a triglyceride can be described as resulting from a condensation reaction (specifically, esterification) between each of glycerol's –OH groups and the HO– part of the carboxyl group of each fatty acid, forming an ester bridge with elimination of a water molecule .
Other less common types of fats include diglycerides and monoglycerides, where the esterification is limited to two or just one of glycerol's –OH groups. Other alcohols, such as cetyl alcohol (predominant in spermaceti), may replace glycerol. In the phospholipids, one of the fatty acids is replaced by phosphoric acid or a monoester thereof.
The benefits and risks of various amounts and types of dietary fats have been the object of much study, and are still highly controversial topics.
- Another review found limited evidence for a positive relationship between consuming animal fat and incidence of colorectal cancer. Tallow (beef fat) is about 50% monounsaturated fat, Other sources include hazelnut, avocado oil, macadamia nut oil, grapeseed oil, groundnut oil (peanut oil), sesame oil, corn oil, popcorn, whole grain wheat, cereal, oatmeal, almond oil, hemp oil, and tea-oil camellia. typically 25% of total fat.
and stroke.
Guidelines
<div class="noprint">thumb|upright=1.8|[[Reference ranges for blood tests, showing usual ranges for triglycerides (increasing with age) in orange at right]]</div>
The National Cholesterol Education Program has set guidelines for triglyceride levels:
{| class="wikitable"
|-
! colspan=2 | Level
! rowspan=2 | Interpretation
|-
! (mg/dL)
! (mmol/L)
|-
| < 150
| < 1.70
| Normal range – low risk
|-
| 150–199
| 1.70–2.25
| Slightly above normal
|-
| 200–499
| 2.26–5.65
| Some risk
|-
| 500 or higher
| > 5.65
| Very high – high risk
|}
These levels are tested after fasting for 8 to 12 hours. Triglyceride levels remain temporarily higher for a period after eating.
The AHA recommends an optimal triglyceride level of 100mg/dL (1.1mmol/L) or lower to improve heart health.
Reducing triglyceride levels
See also
- Diet and heart disease
- Fatty acid synthesis
- Food composition data
- Western pattern diet
References
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