Reports suggest there are health benefits associated with making a permanent change to our eating patterns, including weight loss and a reduction in chronic inflammation associated with coronary heart disease (CHD)1,2,3.
Consuming food within a set timeframe, say between the hours of 10:00 and 18:00 or even 12:00 and 18:00 each day, is called ‘time-restricted feeding’ (TRF); consuming less calories on 2 non-consecutive days of the week and eating normally for the other 5 days – as in Michael Moseley’s 5:2 plan linked here – is popularly known as ‘intermittent fasting’ (IF); whilst fasting every other day, is termed ‘alternate day fasting’ (ADF).
The format is relatively simple. With IF, calories are restricted to around 500 or 600 per day on the two ‘fast’ days. For TRF and ADF, non-caloric beverages only are consumed during fasting periods. Outside these times there are generally no dietary restrictions, providing food intake contains macro and micronutrients in the required amounts, so no over consumption or binge eating.
But is this just another fad or does scientific evidence support these claims?
Let’s first review the physiological states which describe energy availability in relation to the last meal if we’re not fasting but eating ‘normally’, in tune with our circadian rhythm. Note that each of these states will merge with the next. There is no ‘on/off switch’ where one state immediately flips to another:
- The fed state – lasting for up to four hours after eating, dependent on the type and amount of food intake
- The post-absorptive state – starting about three to four hours after the last meal, dependent on the type and amount of food consumed and physical activity during that time, when the contents of the last meal have been digested and absorbed by the body
- The early fasting state – starting several hours after a meal and beyond the post-absorptive state
The fed, post-absorptive and early fasting states would typically be experienced at different times during the day, where say breakfast is consumed early in the morning, lunch at midday, dinner in the evening, followed by a fast until the next morning. Each state brings about a gradual change in the physiological response from the body, as the food eaten is utilised. Energy metabolism is precisely controlled by insulin and other hormones, whose concentrations are continually regulated in relation to the availability of nutrients. This is summarised below4, where a flux between the storage and release of nutrients is demonstrated:
Post breakfast, lunch, evening meal and snacking – the fed state
Approximately 15 to 30 minutes after a meal of carbohydrates, proteins and fats, the products of digestion – glucose, amino acids and fats respectively – begin making their way into the blood stream. Insulin, a hormone produced by the pancreas, is released in response to a rise in blood glucose. Insulin brings about a lowering of the concentration of glucose in the blood by facilitating its movement into muscle cells and the liver where it is used for energy/stored as glycogen, or into fat cells (adipose tissue) to be stored as fat. Amino acids are used to create proteins. Fatty acids are stored in fat cells.
As there is a plentiful supply of nutrients, the body does not need to turn to its reserves for nourishment, meaning energy from glycogen and fat stores is not required at this point.
Between meals – the post-absorptive state
The level of glucose in the blood starts to decrease and with it that of insulin, since it is no longer needed in large quantities to shunt excess glucose from the bloodstream. This state may not be fully realised however, if re-feeding occurs within three or four hours of the previous meal in the form of snacks or the next meal. If re-feeding is delayed glycogen stores in the liver will start to be converted back to glucose and released into the bloodstream to maintain blood glucose concentrations and glycogen in muscle will be converted to glucose and used as a source of energy by the muscle.
Before breakfast – the early fasting state
After several hours without food, blood concentrations of glucose and the hormone insulin are usually at their lowest. However, the brain must have a continual supply of glucose. To ensure blood glucose concentrations remain stable, glycogen stored in the liver during the fed state is converted to glucose and released into the bloodstream along with glucose that is newly synthesised by the liver. The latter is known as gluconeogenesis and uses lactate, a by-product of glucose breakdown or alanine, an amino acid from protein breakdown.
Let’s now consider what happens if food doesn’t arrive within about 12 hours of the last meal:
The fasted state
Dependent on the size and composition of the last meal, stores of energy in the body decline and metabolic responses adapt. In response to low levels of insulin in the blood, fatty acids start to be released from the fat cells in adipose tissue and become a source of energy for organs such as the brain, skeletal muscle and even adipose tissue itself in the form of ketone bodies.
Ketogenesis is the name given to the production of ketone bodies in the liver from the breakdown of fatty acids. Ketones include β-hydroxybutyrate (BHB), acetoacetate (AcAc) and to a lesser extent acetone derived from AcAc. The levels of ketone bodies begin to rise within 8 to 12 hours of the last meal and continue to do so if re-feeding does not occur within 48 hours5. Whilst high concentrations of ketones in the blood can be dangerous to health, as in diabetic ketoacidosis, this report2 cites evidence that ketones in lower concentrations are physiologically beneficial, influencing how proteins and other molecules affect health and aging. Low concentrations of BHB, AcAc and acetone can be achieved through fasting, adopting a ketogenic diet or physical exertion.
As well as weight loss6, studies report other beneficial effects of fasting including an increase in metabolic rate7, improved insulin sensitivity8 (a reduction in the amount of insulin needed to move excess glucose from the blood into liver, muscle and fat cells), a reduction in atherogenic small dense low-density lipoprotein (sdLDL) cholesterol6 (raised sdLDL is an indicator of CHD risk) and a reduction in markers of inflammation3 which are often seen in relation to chronic disease.
Summary
For healthy individuals the evidence would suggest that TRF/IF/ADF could be an effective tool for weight loss and to improve metabolic health. It’s also easier to count hours than it is to read labels and count calories. However, before doing a little jig, thinking the perfect panacea for all dietary ails has arrived, check through these cautionary points:
- As noted in an earlier post (That ‘D’ Word), as far as weight loss regimes are concerned one size does not necessarily fit all, so whilst TRF/IF/ADF may give results for some they won’t necessarily work for/suit everyone
- Though TRF/IF/ADF do appear to bring health benefits, the specific mechanisms for this are not fully understood3
- Research on TRF/IF/ADF has in the main been conducted on overweight or obese respondents so there is minimal data covering benefits for non-overweight or non-obese individuals6. However, this study7 did discover positive health benefits without weight loss.
- It’s unclear if the benefits of fasting are due to anything other than the consumption of fewer calories
- The adverse effects of weight gain are insidious and may take years to develop, whilst in contrast work done to show the effects of TRF/IF/ADF are of relatively short intervention; evidence – both positive and negative – from longer term studies will, no doubt, evolve over time
Tips
At the very least, eat only at mealtimes, avoid snacking between meals, keep active and schedule the last meal of the day for early in the evening, leaving time for food to digest before bed.
And, in case I haven’t already mentioned it, NO SNACKING!
References
- JOHNSTONE, A. Fasting for weight loss: an effective strategy or latest dieting trend?. Int J Obes 2015, 39, 727–733. Available from: https://doi.org/10.1038/ijo.2014.214
- NASSER, S., VIALICHKA, V., BIESIEKIERSKA, M., BALCERCZYK, A., PIROLA, L. Effects of ketogenic diet and ketone bodies on the cardiovascular system: Concentration matters. World J Diabetes 2020; 11(12), 584-595. Available from: DOI: 10.4239/wjd.v11.i12.584
- de CABO, R. and MATTSON, M. P. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med 2019, 381: 2541-2551. Available from: DOI: 10.1056/NEJMra1905136
- FRAYN, Keith N. Metabolic Regulation: A Human Perspective. 3rd ed. Chichester: Wiley-Blackwell, 2010, pp 206-211
- BROWNING, J. D., BAXTER, J., SATAPATI, S., BURGESS, S. C. The effect of short-term fasting on liver and skeletal muscle lipid, glucose, and energy metabolism in healthy women and men. Journal of Lipid Research 2012, 53(3), 577-586. Available from: https://doi.org/10.1194/jlr.P020867
- BHUTANI, S., KLEMPEL, M.C., KROEGER, C.M., TREPANOWSKI, J.F., VARADY, K.A. Alternate day fasting and endurance exercise combine to reduce body weight and favourably alter plasma lipids in obese humans. Obesity 2013, 21, 1370–1379. Available from: https://doi.org/10.1002/oby.20353
- ZAUNER, C., SCHNEEWEISS, B., KRANZ, A., MADL, C., RATHEISER, K., KRAMER, L., ROTH, E., SCHNEIDER, B., LENZ, K., Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. The American Journal of Clinical Nutrition 2000, 71(6), 1511–1515. Available from: https://doi.org/10.1093/ajcn/71.6.1511
- SUTTON, E. F., BEYL, R., EARLY, K. S., CEFALU, W. T., RAVUSSIN, E., PETERSON, C. M. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism 2018, 27(6), 1212-1221. Available from: https://doi.org/10.1016/j.cmet.2018.04.010
