Energy expediture and its role in weight gain and lose

We use our net energy for maintenance (average daily expediture is 2000 kcal for female, 2500 kcal for male), for physical activity (exercising) and for growth. Our daily energy expediture can be higher or lower, depending on body size, illness or physical activity. The maintenance part is essential for our survival (things such as heart beat, brain function, respiration).

Now i will introduce some new terms. Basal maintenance in fasting state is called Basal Metabolic Rate (BMR). To this we will add the energy cost of digestion and absorption (dietary induced thermogenesis - approx. 10% of the caloric value of the meal). BMR covers 60-70% of the daily energy expediture and it is different between individuals, depending of the lean body mass (body mass remaining after we subtract the fat). Tall people with lot of muscle will have a much higher BMR than short fat people.

Physical activity is measured by PAL (Physical Activity Level) and is defined by the daily amount of physical activity of a person. This vary from 1.4 for sedentary people to 2.0 for athletes, with 1.6-1.7 being the average Joe. Can be measured though the oxygen consumption (indirect calorimetry).

As for growth, we will think at the net energy required to make extra tissue. It is close to zero for weight stable adults, relevant for children and pregnant women.

We store energy as fat, a dense and easy to use form of energy. The average amount of fat stored by a normal individual is 90.000 kcal (enough to support us for 30 days of fasting, but this can last for months if you are obese). Average amount of proteins stored is 36000 kcal, but you can use only 12000 kcal in case of emergency, before you start to compromise some of the normal body functions. Average amount of carbohydrates stored is 1400 kcal in the muscle and 350 kcal in the liver as glycogen, and it is used to maintain the blood glucose level in between meals.

When the energy intake is exceeding the energy expediture, the excess is stored as fat and we will experience weight gain. After a certain margin (let's say 600 kcal), the body weight tends to level off after a while, even if initially will go up, as the energy expediture also increase with the weight gain, until a new equilibrium is reached at a higher body weight. Why, you will ask? Because the BMR will increase as the body weight increase, even if the BMR is mainly determined by the lean body mass. The energy cost of the physical activity will be higher with greater body-weight (you need more energy to use your body as your overall body mass increase.

The opposite happens if we lose weight. Weight loss is rapid at first, but progressively slows down as the energy deficit decrease. Losing weight, the energy expediture decrease as well as BMR and the energy required for physical activity decrease, until a new equilibrium point is reached. This is the reason why every diet will work well at first, but lose efficiency over time.

Next post will be about weight management and obesity.

Energy value of nutrients

Today we will talk about the energy value, Atwater factors, metabolizable energy and kilocalories.

Our energy value come 30-70% from carbs, 15-50% from fats and 10-25% from proteins. The Adwater factors are:

-gross energy - all the energy present in the food products (not all is available)
-digestible energy - amount of energy absorbed, dependent of the digestibility of the food (carbs 98%, fats 95%, proteins 92%)
-metabolizable energy - what is left after accounting the energy in feces and urine.

So, the gross energy is made by the digestible energy and the one not available. The digestible energy is made by the metabolizable energy and the energy eliminated through feces and urine. The energy eliminated is mostly made from proteins (30% of it as urea). The calories we see on the food package are in fact equal with the metabolizable energy.

In terms of kilo-calories (kcal), we have the carbohydrates and proteins with 4 kcal, fats with 9 kcal and alcohol with 7 kcal. In terms of dietary fiber, the insoluble one is subtracted from the carbs content, and the soluble ones have 4 kcal per gram. According to the EU rules, all the fiber is considered to have 2 kcal per gram.

The Atwater factor is an average, but in real life the digestibility can be different. For example, from the thermic effect of food point of view (dietary induced thermogenesis), the transient increase in energy expediture following meal consumption, caused by digestion, absorption and processing of the nutrients, it is higher for proteins than for carbohydrates and fats. Also, proteins, fats and carbs have different effects on hunger and satiety.

In conclusion, the traditional view that all energy is equal regardless of the source (a calorie is a calorie) is overly simplistic and often wrong in real life.

Next post: energy expediture.

Feedback regulations from adipose tissue (fat)

The adipose tissue plays a central role in the long-term regulation of the food intake.

When we gain body fat, the fat cells increase the leptin release, affecting the satiety cells in the hypothalamus, and we decrease our food intake, decreasing the energy storage.

When we lose body fat, the leptin release is decreasing, the hunger cells in  the hypothalamus are stimulated, increasing the food intake, increasing the energy storage.

The leptin reduction during weight loss is one of the reason of being difficult to maintain a diet indefinitely.

In the next post i will write about the energy value of the nutrients.

Dietary fats - Part two (Triglycerides digestion)

Many foods contains substantial amounts of dietary fats. Cooking oils (Sunflower, peanut, olive) got 100% fats as triglycerides. Butter and mayo - got more than 80% fat, bacon and sausages - around 40% fats, as opposed to fruits and veg - little to no fat.

We can have cholesterol only in foods of animal origin (highest amount in egg yolk and shrimps). Saturated fatty acids can be found in lard, butter, coconut oil, and unsaturated fatty acids in vegetable oils. Soy bean oil - has linolenic acids (C18:3), coconut oil - has medium chain fatty acids (8-12 chains, fish oil - has fatty acids with 20-22 chain length. Flax-seed oil is a good source of essential fatty acids (50% linolenic acids).

Fat digestion - we would talk especially about triglycerides digestion. The breakdown of a triglycerides molecule is done under the influence of an enzyme called lipase. It is broken in 2 fatty acids and one monoglyceride attached to a fatty acids. We got lingual lipase, in the mouth, with a role in fat taste detection more than digestion. The process starts in the stomach under the gastric lipase, and it is continued in the intestines, with the pancreatic lipase doing most of the job. When dietary fat enters in the intestines, a hormone called cholecystokinin (CCK) is released, to activate the gall bladder contractions, and the secretion of bile acids. Bile acids are made from cholesterol and are stored in the gall bladder with cholesterol and phospholipids, and has the role of an emulsifier, changing fat in little droplets, for pancreatic lipase easy access. The main bile acid is the cholic acid, can be attached easily to amino-acids like taurine and glycine (making taurocholate and glycocholate - conjugated acids). The broken down fatty acids and the monoglycerides resulted from the digestion of triglycerides form a special structure called mycelle and the bile acids play an important role in it. Then they are taken up by the enterocyte (intestinal cell) and reconverted into triglycerides via a process called re-esterification. Then they are packaged into special particles called chylomicrons - which carry the dietary fats through the body, using the lymphatic circulation (small vessels called lacteals) to reach the subclavian vein.

Fats - intro

The fats, also known by the scientific (fancy) name of lipids, are digested and released in the blood stream, as lipoproteins (ULDL, HDL, LDL), and then stored as fat tissue. An important role is played by the essential fatty acids (linoleic acid and linolenic acid), but about this we will write a bit later.

There are different tipes of fat, and the first classification is by saturation grade (saturated (triglycerides, phospholipids, sterols) and unsaturated. Out of dietary lipids, the triglycerides make 95% of our daily fat. A triglyceride is made from 3 fatty acids linked via a glycerol molecule.
Another classification is made according to the state of the lipids at room temperature - if solid  is called fat, if liquid is called oil.

Fatty acids - are made from a chain of hydrogenated carbon atoms connected to a carboxil group. The chain length could be anywhere between 3 to 24 carbon atoms. The degree of saturation is related to the number of double bonds - unsaturated fatty acids got at least on double bond (mono-unsaturated f.a. has one, poli-unsaturated f.a. at least two double bonds). Point of unsaturation - where the double bonds are located in the fatty acid molecule - n (omega) and delta designation (shows all double bonds. The most abundant fatty acids in our food are oleic and palmitic acids (2/3 of our daily intake).

According to the point of unsaturation, the most common acids in our diet are C16:0 - palmitic acid, C18:0 stearic acid, C18:2 linoleic acid, C18:3 linolenic acic and C18:1 oleic acid.
The lenght of the chain is related to the melting point, shorther the chain, lower the melting point. The more unsaturated, the lower the melting point also. Liquids such as vegetable oils (made from sunflower, corn, olive, peanut, canola or soy) are mostly unsaturated, while solid fats like butter are mostly saturated fatty acids.

Triglycerides are mostly saturated, more stable and less prone to oxidation (one of the important oxidation reaction is the hydrogenation - unsaturated fatty acids become saturated). Hydrogenation is a common process in food manufacturing (or used to be) but it is also transforming the Cis fatty acids in Trans fatty acids, with negative influence on health. The Trans fatty acids are made during industrial processing and in the stomach of ruminants such as cows.

Phospholipids contains a diglyceride, a phosphate and a simple organic molecule such as choline. The got a polar and an apolar side (amphipathic properties). Are major components of cell membranes (you can find them in plant and animal food). They are added during food preparation as emulsifiers, to help the oil and the water to create a stable emulsion (prevents oil and water to separate). They line up tail-to-tail in the membrane of the cell to form a bylayer, as a barrier for entry of many molecules into the cell.

Sterols are the third lipid component in the diet. The main one is known as cholesterol, found in foods of animal origin. The plants have sterols and stanols, most of them being eliminated via stools. Added to the food to reduce the absorption of cholesterol, they can potentially lead to a 10% reduction of the cholesterol in the blood stream .