Nutrition 4 [Controlling energy balance]

Homoeostatic mechanisms contributes to energy balance

• Energy stores

• Obesity

• Ob + db genes

• Obesity and evolution

Overview of food processing in mammalian digestive tract

Energy balance

ATP

produces from oxidation of carbohydrates, proteins and fats in cellular respiration.

Energy stores

• Preference of use = carbs + fats then proteins

• Fats 2x energy released per gram cf. others

• Excess molecules are converted to storage molecules:

  • 1st site → glycogen

  • 2nd site → adipose (fat) cells

glycogen

a polymer glucose, stored in liver and muscle. Tight regulation of glycogen levels

Adipose cells

Store energy as fat when glycogen stores full

Energy released diagram

Obesity

Overnourishment (consuming too many calories) causes obesity, the excessive accumulation of fat

Obesity problems that causes health problems

• Type 2 diabetes

• Cardiovascular disease

• Colon cancer

• Breast cancer

Fat cells from the abdomen of a human

• Strands of connective tissue, shown in yellow, hold the fat storing adipose cells, shown in red in place.

• Body weight is regulated by homeostatic mechanism

• Feedback circuits control storage and metabolism of fat

• Hormones regulate ‘‘Satiety centre’’ in the brain

Appetite-regulating hormones

Secreted by various organs, hormones reach brain via bloodstream..

Green → appetite stimulant

Red → appetite suppressant

Ghrelin

• Secreted by stomach wall

• Triggers hunger at meal times

• (increases in dieters, hence difficult to stay on diet!)

Insulin

• Produced by pancreas after rise in blood sugar

• As well as other functions also suppresses appetite

Leptin

• Produced by adipose tissue and suppresses appetite

• Decrease in body fat causes levels to fall, hence appetite increases.

PYY

• Secreted by small intestine after meals

• Appetite suppressant that counters effect of ghrelin

ob + db genes in appetite regulation

• Mutations in chronically obese mice investigated

• Mutation in ob gene leads to mouse eating voraciously = obese

• ob gene required to produce satiety factor

• db gene required to respond to that factor

  • ob gene actually encodes for a hor

Obesity + Evolution

• Hunter gatherer ancestors → seeds and plants, with occasional meat from hunting ‘‘feast or famine’’.

• Promotes individual with high capacity for storing = more likely to survive lean times

Offspring of petrel

• Parents on long food trips

• Food has high fat content but little protein

• Chicks have to ‘‘overfeed’’ to get enough protein

• Can survive while parents are away

• Eventually need to fast for several days to lose weight to be able to fly

ob genes

• ob gene required to produce satiety factor

• the gene encodes for a hormone, leptin

db genes

• db gene required to respond to that factor

• the gene encodes for leptin receptor

Mammalian hibernation

• Physiological and morphological changes to survival seasonal low temperatures and food scarcity

• Prolonged bouts of ‘‘torpor’’ save energy, body temp falls (to 0-10*C)

• Reduced metabolic rate (-5% of active levels)

13-lined ground squirrel

• consumes no food for 5-6 months

• Rely on fat stores for energy

• Changes in gastrointestinal tract particularly noticeable.

Ground squirrel jejunum

• Stained with antibody that detects brush border enzyme sucrase-isomaltase (marked by arrows)

• (A) active animal (B) hibernating / torpor animal

Hibernation reduces pancreatic amylase levels

• Torpid → no food for 6 weeks

• Amylase activity down 50%

• Amylase protein expression down 40%

• Remaining levels facilitate digestion upon awakening in spring

GI tract during hibernation, what happens?

• During hibernation GI tract is reduced in size / weight, with reduction in number of cells = not needed for digestion

• Preservation of intestinal gene expression during hibernation so still totally functional - no lag time in recovery upon awakening

Urea Nitrogen Salvaging & Hibernation

Indian and Burmese python

Burmese python digestion

• Burmese python swallowing a rat, killed by constriction

• 24 hours after consuming a rat meal (>50% of snake’s body mass). Further distention of body due to the build up of gases within ingested rat).

Python digestion

• Pythons wait for suitable prey to wander near; may wait for weeks on end

• Equipped to ingest large animals if give the chance (up to 70% of their own weight)

• Within 24 hours of feeding, double the mass of mid-gut through growth of new epithelia.

• 20-fold increase in transport proteins (e.g. glucose and amino acid transporters)

• Metabolic rate increases 40-fold

Python post feeding

• Small intestine of similar-size pythons fasted, or at 2 and 10 days postfeeding (DPF). By 2 DPF, intestine in diameter due to hypertrophy of the epithelial cells; response reversed by 10 DPF

Model for lengthening of microvilli

• begins after onset of feeding

• involves structural proteins actin and villin

• 5-fold increase in villi and apical surface area during digestion

Positron emission tomography (PET) images of fasted and fed (1 DPF) Burmese python

• injected with 2-[18F]fluoro-2-deoxyglucose

• bright areas = regions experiencing high rates of glucose metabolism

  • Difference between images is actually greater, intensity of the fasted image had to be increased 1000-fold in order to view