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Episode 11 - Obesity: a bird’s eye view

by Break Nutrition | Podcast

Short summary: In episode 11 Gabor selects 2 papers on the physiology of migrating birds that fatten up for their voyage and we discuss what this can tell us about human obesity. The first paper is from 2002 by Bairlein and is called “How to get fat: nutritional mechanisms of seasonal fat accumulation in migratory songbirds The second is called “Adipose energy stores, physical work, and the metabolic syndrome: lessons from hummingbirds and is by Hargrove et al. from 2005.

How to get fat: nutritional mechanisms of seasonal fat accumulation in migratory songbirds (Bairlein 2002)

  • Seasonal fruit eating is an important aspect of how migratory birds prepare for their migratory flight
  • In a couple of weeks the body mass of birds can double; 70-80% is from fat in most birds, the rest is muscle mass
  • Carbs fuel the takeoff of birds but not their endurance flying)
  • “Seasonal migratory fattening is due to endogenously programmed circannual shifts of a body mass set point”
  • Some 20-40% more calories fully account for some bird’s extra body mass, but not in others
  • “In garden warblers, for example, the daily body mass gain is associated with a 60% increase in assimilated energy intake, of which 40% is due to an increase in the amount of food eaten (gross food intake), while 20% can be ascribed to an in- creased assimilation efficiency”
  • seasonally higher plasma cholesterol in some birds may reflect increased bile acid need to use fats
    • another adaptation, for e.g., is the dry intestinal mass increasing 25% tjat coincides with “significant increase in energy utilization efficiency” è in other birds, intestinal mass decreases actually
  • The authors categorize birds into 2 groups: “granivorous species with no significant variation, and insectivorous species with pronounced temporal increase in food assimilation efficiency”
  • “seasonal frugivory is primarily the result of changes in food preferences
  • Amazingly, “birds are able to detect specific nutrient deficiencies and respond quickly and appropriately by effectively regulating the selection and intake of specific nutrients
  • The authors say how the “sugar content does not appear to be of any significant importance for migratory fattening. High-sugar diets did not promote fat accumulation nor did very low-sugar diets hamper fattening in garden warblers”
  • The “[Randle cycle] may explain the birds’ preference for lipid rather than carbohydrate diets after the metabolism and the enzyme machinery has adopted to the high lipid oxidation capacity”

Adipose energy stores, physical work, and the metabolic syndrome: lessons from hummingbirds (Hargrove et al. 2005

  • fat tissue has a pathological connotation = wrong!
  • 1-2g of fat allows a human to climb 50ft (15m) whilst a Ruby-throat bird flies across the Gulf of Mexico for about 500-600 miles using that same amount of fat!
  • Nectars contain as much as 38% (~1 M) sugars (mostly sucrose)”
  • When humming birds overnight fast they have an RQ 0.7 and when they feed it is0
  • The top end beats per minute of some birds is 1260 & they breathe about 250 times per minute. They have one of the highest metabolic rates per body weight
  • Flying hummingbirds use ~ x10 more O2 per grame of muscle tissue than best human athletes do
  • When birds have an RQ > 1 this is consistent with (1) De Novo Lipogenesis (2) carbohydrate oxidation
  • In some hummingbirds the “fasted glucose is about 17 mM (300 mg/dl), and it increases to about 42 mM (740 mg/dl) after feeding”
  • For these bird, “the flight is possible only because the energy yield per gram of fat is 10 fold higher than for hydrated glycogen
  • In some birds, their “typical flight across the Gulf of Mexico will require about 75% of the birds’ energy stores”
  • One possible reason for their lowered HbA1c is that the “lifespan of red blood cells in birds can be 21 days or less vs. about 120 days for humans, so there may be less opportunity for glycation”
  • There’s some speculation as to “Whether or not birds avoid obesity and diabetes by dint of their rates of living…”


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