(Preface: This post can be simplified as detailing of digestive breakdown, absorption into the blood stream, consumption by way of metabolism and the inevitable ‘storage’ of the surplus. Remembering this ‘model’ or simplification will help.)

How food sustains life, an overview: An average office-going Indian adult needs about 2000 calories per day to enable him to do everything, maintain body temperature and keep his internal organs functioning. He needs about 60 grams of proteins to repair damaged tissue and build new ones which accounts for about 250 calories. He gets these as follows:

The food intake is digested in his GI tract and the residue – the undigested digestibles and dietary fiber – are ejected as feces.

The simplified digestion products – glucose (and other simple sugars), fatty acids, glycerol and amino acids and micronutrients – are absorbed into the blood stream across the GI tract wall.

These nutrients spread out in the body riding in blood and are consumed in metabolism. The products of energy metabolism, usually just carbon dioxide and water, are breathed out. The difference in the energy content of input to metabolism and the exiting gaseous products is the energy that he uses i.e. material has vanished to release energy.

Body has an elaborate mechanism to store nutrients (glucose and fatty acids, glycerol) for a scenario of ‘starvation’ and such storage has to be large enough to provide the aforesaid calorie requirement for a few days. Obviously, such storage has to be in the form of an energy-dense nutrient in a large enough quantity  spread out in the body. Fat is such a nutrient (along with a limited quantity of carbohydrates).

Such stored materials break down into metabolizable simple nutrients to again become usable nutrients in the blood (muscles use their storage directly) which undergo metabolism to release energy and sustain life during starvation i.e. continuation of life (however subdued) in absence of partial or full input of nutrition.

Thus digestion, absorption, circulation, metabolism, storage (when possible), retrieval (when required) constitute the gross life sustaining scheme. (Ref. Posts 2: HEALTH, HAPPINESS, LIFE AND FOOD (PART I) : WHAT ARE THEY? and 3: HEALTH, HAPPINESS, LIFE AND FOOD (PART II) : DEFINING ‘FOOD’ AND USING IT SMARTLY and the previous post: OBESITY AND FOOD – ACCUMULATION OF THE UNUSED: PART I (UNDERSTANDING OBESITY).)

(We have already noted that the physical states of food, water and oxygen – all external to the body – is a fascinating scheme of nature.) 

When the life-sustaining role of food goes awry: Obviously, controlled food intake will send limited nutrients into bloodstream, efficient metabolism will use them smartly to release adequate energy and heat and muscle repair and building and hence there will be little storage of any kind. Of course, the nutrient intake must be consistent with physical activity level. Thus,

1. Food intake controls are the first step that controls obesity. We will soon realize that such automatic controls do exist; only, they can be flawed.
2. Metabolism must have controls that use circulating (and muscles’ in-house) nutrients efficiently failing which unwarranted diversion to storage can happen. (Ref. Post 15: OXYGEN, FOOD AND LIFE : PART I (HOW OXYGEN MEDIATES IN LIFE AND FOOD PROCESSING).
3. Whenever required, the withdrawal of nutrients from storage and their breakdown into usable forms must happen at a rate that matches the rate of energy requirement. If the released availability and its combustion are too slow vis-a-vis activity level, fatigue (inducing stoppage of requirement i.e. exercise) may set in.
4. Sustained deposits to storage is the direct result of sustained surplus of input over requirement. (Meet Ramesh in post 11: INDIAN CONSUMER AND PROCESSED FOODS – DIVERSITY MEETS DIVERSITY!). A sedentary lifestyle, love for sweets, icecream, chocolates, butter/ghee, biscuits and cookies, even tea and coffee along with watching TV, reading, video-games and table-work, promote obesity.
5. Sustained withdrawals from storage with limited and smart dietary inputs (i.e. serious aerobic exercise and dieting) leads to slimming. The unwelcome nature of both the factors is the commonest cause of personal failure of slimming and commercial success of all shades of pills, diet regimens and ‘easy’ exercise programs.
6. Smart food habits, regular aerobic exercise, weight-watching, positive temperament, periodic pathological analysis and preemptive physician counseling and on-going corrections lead to a long, balanced, slim, energetic life. (Meet Geeta in post 11: INDIAN CONSUMER AND PROCESSED FOODS – DIVERSITY MEETS DIVERSITY!).

This sets up the sequence of digestion, absorption (into blood stream) and metabolism as the nitty gritty of obesity/slimming.

Essential physiology of food digestion: Given the focus of our blog, we cannot go into minutiae of digestion of food in the gastro-intestinal tract. Visit posts 2 and 3: Health, Happiness, Life and Food: Parts I and II for a glimpse. Let’s encapsulate it as a gradual breakdown of carbohydrates, proteins and oils/fats into simple constituents that can be and are absorbed into blood stream along with some micronutrients and the passage of the residue into the colon where the soluble and insoluble carbohydrates do some magical life-easing things before getting out! (Ref. post 2: HEALTH, HAPPINESS, LIFE AND FOOD (PART I) : WHAT ARE THEY?)

(Special notes: 1.The slurry that the cecum (the first part of large intestine or colon) receives from the small intestine (thru the ileo-cecal valve) for eventual ejection, has hardly received any attention. It is an anomaly because happiness itself resides in the colon; it is loaded with possibilities. The colon proceedings are complex and developing insights in them is what finance experts would call a potential high return investment.

2. We have already noted how oils/fats and water are the only liquids in our food and how their ‘mutual exclusivity’ is a masterly natural scheme. This ‘exclusivity’ stems from the fact that that while water is a small, simple, strongly electrically charged molecule, oils are large, nominally charged molecules. This distinguishes oils even in their digestion in the small intestine, where bile salts coming in with the bile juice (produced by the liver and stored by the gall bladder embedded in the liver) break oil into small droplets i.e. emulsify it. Only then the pancreatic lipase can act on this temperory water-oil mixture and digest it into fatty acids and glycerol. Not surprisingly, such ‘speciality’ of oil/fat persists in the way it circulates in the blood, is oxidized to release energy and is stored as concentrated and large chemical energy. We will bypass most of this complex ‘speciality’ as non-essential for our purpose.)

In a nutshell: Ref. posts 1 and 2: Health, Happiness, Life and Food: Parts I and II. Now, an overview of digestion:

1. Starch, the most abundant carbohydrate in the diet, is digested in the mouth and the small intestine with amylase enzymes into smaller molecules and ultimately into glucose.

Cellulose (along with other insoluble fiber) remains undigested except to a very limited extent in the colon – the large intestine, by the microbiome i.e. ‘external’ enzymes.

Sugar (table sugar, sucrose) is digested in the small intestine into glucose and fructose.

Lactose (milk sugar) is digested in the small intestine into glucose and galactose – some digestion happens in the colonic microbiome. (Fructose and galactose are very close cousins of glucose and are converted into glucose – body’s favorite energy source – in the liver and hence we will focus on glucose as the ‘simple sugar’ that gives energy.)

2. Proteins digest stage-wise into simpler (smaller) peptides and eventually into amino acids. For simplicity that serves our purpose without compromising on the rigor, we will treat amino acids as the digestion product that enters the blood.
3. Fats digest into mono-glycerides, fatty acids and glycerol, much like peptides and amino acids from proteins. (Coconut oil is special; most of its glycerides reach the liver directly where they are broken down and released for combustion for energy. But we will bypass this nitty gritty as inconsequential for our purposes but note the faster availability of a lot of energy in an emergency.)

Absorption of nutrients into blood, distribution and storage: Most of the absorption of water and nutrients occurs in the small intestine, especially its last portion (ileum) whose inner wall is designed with arrangement to increase the surface area to facilitate absorption of a large amount of water, digested nutrients, minerals and vitamins.  Absorption per se’ is quite complex but, for our purpose, we can treat glucose, fats (recombined in the intestinal wall from fatty acids and glycerol) and amino acids as the ready-for-use nutrients carried by blood all over the body.

With blood as an efficient ‘carrier’ and heart as the untiring mobilizer, they are metabolized in tissues and organs to release energy, maintain body temperature, build and repair tissue and get stored for future use whenever required.

An overview of the physiology of energy release and tissue building:

1. Glucose is the primary energy source that is quickly and efficiently oxidized to release energy to do work and produce carbon dioxide and water. (We will see how glucose evolved into this ‘favorite’ energy source, bypassing fructose and galactose – its close cousins in our diet – in the post on Diabetes.) Brain is particularly partial to glucose as energy source; yawning at the end of a strenuous brain exercise is a sign of brain crying out for a glucose shot, sometimes, also a whoosh of oxygen. It is a good idea to pop a large toffee in the middle of a 3 hour exam and go to the window and breathe deeply while working on that research paper on the computer. ‘Glucose saline’ is the commonest intravenous, life-sustaining drip to bed-ridden patients with impaired dietary intakes.
2. Blood glucose level changes continue in a narrow range in a 24 hour cycle. Refer the next post for details. The size of peaks, their timings and duration, for a given meal are loaded dietary and metabolic stories.
3. The surplus glucose (in the event of carbohydrate intake exceeding requirement) is stored in the liver and muscles as ‘glycogen’ – starch-like glucose polymer that is insoluble in water. But this storage is limited and rarely exceeds equivalent of 2000 calories – the daily requirement of an average adult. In absence of dietary replenishment, it would be exhausted in a day. Hence, whatever is still left,  is stored in adipose tissue as fat, so you can get fat thru rice and even tea/coffee. This also proves the utility of fat as a concentrated energy source ‘for the rainy day’. Such adipose tissue is found all over the body, especially abdominal region, buttocks, back and even muscles. Obviously, a fat/oil and carbohydrate rich diet (sweets, rice, butter/ghee, fried stuff) in a large quantity and without aerobic activity is invitation to obesity.
4. The exhaustion of glucose is sequential. The circulating blood glucose is the first to be gobbled up when aerobic activity starts, lowering its level quickly in absence of input. Soon liver starts releasing glucose into blood (glucogenesis) from its glycogen storage which quickly reaches the consuming muscles. Simultaneously, and if the activity is vigorous enough (sustained brisk walk, cycling, resistance/weight lifting), muscles use up their own glycogen storage similarly. If dietary support is still pending, adipose fat enters the fray.
5. Fat release from adipose tissue and its combustion are slow and energy-inefficient processes. Hence, reluctance to continue exercise sets in if a quick dietary glucose shot continues to be denied – classical ‘exhaustion’. However, if the exercise is continued, the fat begins to burn; obviously, this is the beginning of slimming. It is easy to see why it is difficult. The much-touted ‘keto diet’ is rich in fat, low in carbohydrates and moderate in proteins; it is a questionable fad if slimming is the objective. It is so called because fat metabolism generates ‘keto bodies’ in the blood which work like glucose.

Next Post:

Obesity and food – accumulation of the unused: Part III

Inherent limitations of metabolism and poor obesity control

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