Some science behind the scenes

Metabolic process

Once you have chewed and swallowed your food it heads to the stomach where it is digested.

Carbohydrates,  proteins, fats and nucleic acids are digested in the stomach by the acid in the stomach and, more importantly, enzymes such as pepsin.  The muscular walls of the stomach churn around and break the food down mixing it with gastric juices until it becomes a creamy paste called acid chyme. Nerves and hormones control the secretion of gastric juices so that they are only produced when food is about to be eaten or is in the gut.

There is a type of opening called the pyloric sphincter that guards the opening to the small intestine.  Once the food has become acid chyme this sphincter opens and the chyme flows into the duodenum a little at a time [the duodenum is the first bit of the small intestine].  Hydrogencarbonate ions in the duodenum,  secreted by the pancreas,  neutralise the acid in the chyme.

The intestine is where all the absorption of nutrients takes place.

In the intestine there are folds and twists which provide a huge surface area capable of absorbing all the nutrients.  There are finger like projections in the wall called villi adding to the surface area.  Each villus is supplied with a good blood supply and it is the blood that carries the extracted sugars, amino acids, minerals and other water soluble substances such as vitamins to the liver for further processing.  There are things called ‘lacteals’ which transport any fat soluble substances.  So it is here that differences between people can result – efficient intestine, inefficient intestine.

Once all the nutrients have been absorbed in the small intestine, the intestine contents, which are now in the form of faeces, pass slowly along the large intestine.  The main function of the large intestine is to reabsorb water into the blood stream, making the faeces less bulky.  This is a vital function, a lot of water enters the gut every day and if this were not reabsorbed we would become severely dehydrated.

So it is the duodenum that is the site of intense chemical activity and the place where nutrients get extracted.  How is each type of food stuff processed?

Carbohydrates

Whenever we eat carbohydrates such as bread, sweet foods, fruit [bananas, apples, pears, orange etc], fruit smoothies, pizza and pasta, french fries and hamburger buns, breakfast cereals or porridge, potato crisps, ice cream, sweet fizzy drinks, chocolate, snack bars, rice, doughnuts, jelly, maple syrup, honey, puddings such as sago or rice pudding or treacle tart,  fruit yoghurts, muffins, bagels, tortillas, pancakes, biscuits, dried fruit [raisins, prunes, apricots etc] tomato ketchup, wine and beer, and so on, our digestive system responds by converting it to glucose.

In the intestines, pancreatic amylase converts starch into maltose; maltase converts maltose into glucose; sucrase converts sucrose into fructose and glucose; and lactase converts lactose into glucose and galactose.   So what we can see is that carbohydrates tend to get converted to glucose – instant energy.

In effect, the principal source of glucose is carbohydrates.

The brain requires a constant supply of glucose and is very sensitive to deficiencies, so we can’t do without glucose – however, the amount of carbohydrate we eat is often far far in excess of what we actually need to keep our brain going!   

We cannot do without carbohydrate.  Carbohydrates have a role in combining with proteins and lipids to produce glycoproteins and glycolipids. 

Glycoproteins are important integral cell membrane proteins, where they play a role in cell–cell interactions.  Glycoproteins also help the immune system – antibodies are glycoproteins; are used in blood clotting and help in sperm egg interactions [so if you really cut down the carbs you could reduce your fertility and bleed more easily].

TSH [Thyroid stimulating hormone] is a glycoprotein. Glycolipids provide energy, but also serve as markers for cellular recognition.

However, there are carbohydrates in many vegetables such as sweet corn, carrots, peas, root vegetables and so on, so eat your vegetables and you will be getting plenty

Alcoholic beverages

It is fairly obvious I think that sugary fizzy drinks count as carbohydrate but what about alcoholic beverages – drinks which may have very little sugar or fructose content because the original sugars have been converted by fermentation or distillation into ethanol, commonly known as alcohol?

In the liver, the enzyme alcohol dehydrogenase oxidizes ethanol into acetaldehyde, which is then further oxidized into acetic acid by acetaldehyde dehydrogenase. Acetic acid is esterified with coenzyme A to produce acetyl CoA. Acetyl CoA carries the acetyl moiety into the citric acid cycle, which produces energy by oxidizing the acetyl moiety into carbon dioxide. Acetyl CoA is an intermediate that is common in the metabolism of sugars and fats; it is the product of glycolysis, the breakdown of glucose. 

So if we sum up this cycle, the breakdown of alcohol does not produce glucose, but it does produce energy via Acetyl CoA.  In principle therefore as long as we don’t overdue the amount there is no effect on the glucose receptors.

But what we tend to forget is that wine and beer in particular as well as fortified wines also contain sugars and fructose from the fruit and other ingredients such as malt that have not been converted into alcohol because the fermentation process is never ‘complete’.  This means that again although alcohol/ethanol does not contribute to, for example,  diabetes because it has very little impact on the glucose receptors, wines beers and spirits certainly cannot be assumed to be ‘calorie free’! as there are carbohydrates in all these drinks that will get converted to glucose.  Beer as you can see is very high in carbohydrates, so a beer drinker is at greater risk of getting diabetes than a wine drinker.

The data in the chart below were taken from the USDA website.

ITEM

RED
WINE
  5 OZ
147.5g

WHITE
WINE
  5 OZ
147.5g

REGULAR
Bud BEER
12 OZ
356.4g

LIGHT
Bud BEER
12 OZ
354g

DISTILLED
80 PROOF
1.5 OZ
42g

DISTILLED
86 PROOF
1.5 OZ
42g

Calories

106

100

146

99

97

105

Carbohydrates

2.51g

1.18g

13.20

4.60g

0

0.00g

Water

130.54g

132.16g

328.96g

337.01g

27.97g

26.84g

Alcohol

13.70g

13.70g

12.80g

11.30g

14.00g

15.10g

Fats and Proteins

If you eat proteins and fats such as meat and poultry, fish and other seafood [oysters, clams, roe, scallops etc], cheese, eggs, milk, olive oil and other pure vegetable oils, butter, plain yoghurt, as well as various leafy vegetables, most root vegetables, tomatoes and nuts, these help to build cells but may also supply energy.  We don’t actually need carbohydrates for energy we can get energy from fats and protein.

Animal Source

Amount
(mg / 100g)

 

 

 

 

 

It is worth adding that all our vitamin needs and all our mineral needs can also be supplied via proteins and fats. 

 

This is even true of vitamin C – once thought to be only present in fruits and vegetables. 

 

The overwhelming majority of species of animals (but not humans) and plants synthesise their own vitamin C.

 

Therefore, some animal products can be used as sources of dietary vitamin C.  The chart to the left shows the sources.

 

We tend to forget that tea and coffee can provide us with vitamin K and manganese.

 

Calf liver (raw)

36

Beef liver (raw)

31

Oysters (raw)

30

Cod roe (fried)

26

Pork liver (raw)

23

Lamb brain (boiled)

17

Chicken liver (fried)

13

Lamb liver (fried)

12

Calf adrenals (raw)

11

Lamb heart (roast)

11

Lamb tongue (stewed)

6

Human milk (fresh)

4

Goat milk (fresh)

2

Camel milk (fresh)

5

Cow milk (fresh)

2

The main problem with this group of food products is that the energy they produce is very ‘slow release’, as such carbohydrates are a useful addition if you need quick energy bursts – instant energy.  If you sit behind a desk all day and the TV at night, however, you probably don’t really need the carbohydrates we saw in the first list at all

So how do we get energy from proteins and fats?

Proteins - Proteins are a source of amino acids that are used to synthesise new proteins for growth and repair.  Twenty amino acids are needed by human beings, 11 of which can be synthesised by the body.  The remaining 9 amino acids must be obtained from our food and drink and are called essential amino acids.  The body cannot store excess amino acids, so we need a regular supply of protein within our diet.  Animal products such as meat, eggs, milk and cheese are good sources of protein because they contain all the essential amino acids.  Most plant foods are incomplete, but if you eat a variety of plants all the amino acids can be obtained. 

In the intestine, proteases are enzymes that digest proteins, protein generally gets converted to essential amino acids – cell building products, but dietary protein can also be used as metabolic fuel.  During prolonged exercise, for example, protein can supply about 5 to 10% of energy requirements but not via the direct ‘glucose route’, instead the liver converts amino acids to glucose.

Fats   - Bile produced by the liver cells transforms large lumps of fat into smaller usable droplets - triglycerides.  Triglycerides are the main constituents of vegetable oil (typically more unsaturated) and animal fats (typically more saturated). 

Lipase then converts these ‘triglycerides’  into fatty acids and glycerol.  When the body uses stored fat as a source of energy, glycerol and fatty acids are also released into the bloodstream. 

  • The glycerol component can be converted into glucose by the liver and, thus, provide energy for cellular metabolism.  So fats are the second source of ‘energy’ but are really slow release energy.
  • Fatty acids [and hence fats]seem to have acquired a bad name in the medical literature.  But this is incorrect, we need fatty acids.  Fats are a major source of material for cell membranes and some essential fatty acids are used in controlling  blood pressure and in the body’s immune response.

Cholesterol, for example, is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals.  It is an essential structural component of mammalian cell membranes and is required to establish proper membrane permeability and fluidity. In addition, cholesterol is an important component for the manufacture of bile acids, steroid hormones, and vitamin D, so without it, we get sick and die.

Fat pound for pound, produces far more energy than carbohydrates, which is why – when you see the calories of fatty foods, the calories are so high.  Pure fat can generate more than twice as much energy as carbohydrates [each gram of fat provides about 37.6kj per gram, carbohydrates about 16.7 kj per gram].

So everyone needs fats in their diet, but you don’t need much.  In order to use fats as a source of energy, for example, you do not need much at all  – hence the advice not to overdo the fat intake.

Observations

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