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Feast or Fast Before Exercise to Boost Fat-Burning?
New findings help answer a key question: Is it better to eat before or after exercise?

05/11/2017 By Michelle Lee with Craig Weatherby

Should you eat before or after exercise?

There are as many answers to this question as there are personal trainers and fitness magazines.

Likewise, ads and articles push claims for alleged “fat-burning” foods — most of which lack persuasive evidence of that effect.

Fish and fish oil may promote fat-burning, thanks to the metabolic effects of their omega-3 fats. For more on that, see Fish Oil Fuels Fat-Burning in Mice and Exercise + Omega-3s = Perfect Weight Loss Pair.

Now, the results of a new study from Britain provide guidance on the timing of meals in relation to exercise.

Before we examine the new study, let's review the types of fat in your body and the different ways in which they affect fat-burning.

Types of body fat and their behaviors
Your body has three basic types of fat cells:

  • Brown cells burn fat, in part to regulate body temperature.
  • Beige cells — which were recently discovered — behave much like brown cells.
  • White cells store calories, and — especially in overweight people — release hormones and other messenger chemicals that promote hunger, inflammation, and weight gain.

Brown and beige fat-burning cells are abundant in babies, but their numbers drop as we grow into adulthood.

Previous research has shown that “activating” brown fat boosts fat-burning in the body, thereby helping prevent weight gain and obesity.

Unless we adjust our diets and activity levels to boost the numbers of brown and beige cells, fat accumulates in our white cells, which leads to weight gain.

New study sheds light on the “before or after” debate
A randomized clinical study from the UK’s University of Bath tested whether — in order to promote fat-burning — it’s better to fast or feast before your workout (Chen YC et al. 2017).

To answer that question, the authors tested the effects of eating or fasting before exercise on gene expression in adipose (fat) tissue.

The British team asked 10 overweight men to walk for an hour at a moderate pace — first before eating a high-carb, 650-calorie breakfast, and then after eating that same meal.

Each test was conducted separately to ensure clear results, and blood and fat tissue samples were gathered from the participants before and after each workout.

The results showed that two genes known to influence fat-burning displayed very different levels of “expression”, depending on whether the men fasted or ate before their walk.

The men who fasted before exercise enjoyed increased expression of two genes, indicating that body fat was used to fuel the workout, which would lead to increased fat-burning.

But the opposite was true when the men ate before exercising — they displayed declines in the expression of these two fat-burning genes.

In other words, eating before exercise caused the men’s bodies to rely on their recently consumed calories, rather than stored fat, to fuel their workout.

Study co-author Dylan Thompson, Ph.D., wrote that after eating, adipose tissue “is busy responding to the meal and a bout of exercise at this time will not stimulate the same [beneficial] changes in adipose tissue.”

As he said, “This means that exercise in a fasted [empty stomach] state might provoke more favorable changes in adipose tissue, and this could be beneficial for health in the long term.”

Although it requires confirmation, this study strongly suggests that we should fast before exercise, and wait a while before breaking our pre-workout fast.

Workouts “train” your body fat
Five years ago, a joint British-French team reviewed relevant evidence from their own studies and several others (Thompson D et al. 2012).

They wanted to see whether and how adipose (fat) tissue behaves during exercise and physical activity.

And the results of their review led to some important, exercise-motivating insights.

As the UK team's trial just found, fasting before exercise promotes fat-burning.

Previously, the British-French team found evidence that fat tissue responds to exercise in other ways — ones that add health benefits.

First, activation of fat tissue by exercise raises blood flow within fatty tissue, prompting it to release helpful chemical messengers for several hours afterwards.

The result is a strong, lasting message to your muscles that they should continue to burn fat as fuel.

Second, the authors of the 2012 British-French review concluded that exercise raises your baseline metabolism and prompts body fat to burn fat.

Professor Dylan Thompson — who co-authored the 2012 British-French review as well as the recent British trial — put it this way: “Just looking at the weighing scales when you increase your physical activity will only tell you part of the story — your adipose tissue may be doing some quite amazing things underneath, even without getting any smaller.”

And the results of the new study suggest that exercise before eating helps “train” your body to burn fat.


Sources

  • Chen YC, Travers RL, Walhin JP, Gonzalez JT, Koumanov F, Betts JA, Thompson D. Feeding influences adipose tissue responses to exercise in overweight men. Am J Physiol Endocrinol Metab. 2017 Mar 14:ajpendo.00006.2017. doi: 10.1152/ajpendo.00006.2017. [Epub ahead of print]
  • Dekker MJ, Graham TE, Ooi TC, Robinson LE. Exercise prior to fat ingestion lowers fasting and postprandial VLDL and decreases adipose tissue IL-6 and GIP receptor mRNA in hypertriacylglycerolemic men. J Nutr Biochem. 2010 Oct;21(10):983-90. doi: 10.1016/j.jnutbio.2009.08.004. Epub 2009 Dec 1.
  • Højbjerre L, Rosenzweig M, Dela F, Bruun JM, Stallknecht B. Acute exercise increases adipose tissue interstitial adiponectin concentration in healthy overweight and lean subjects. Eur J Endocrinol. 2007 Nov;157(5):613-23.
  • Lee BC, Lee J. Cellular and molecular players in adipose tissue inflammation in the development of obesity-induced insulin resistance. Biochim Biophys Acta. 2014 Mar;1842(3):446-62. doi: 10.1016/j.bbadis.2013.05.017. Epub 2013 May 22. Review.
  • Makki K, Froguel P, Wolowczuk I. Adipose tissue in obesity-related inflammation and insulin resistance: cells, cytokines, and chemokines. ISRN Inflamm. 2013 Dec 22;2013:139239. doi: 10.1155/2013/139239. eCollection 2013 Dec 22. Review.
  • Sjögren P, Sierra-Johnson J, Kallings LV, Cederholm T, Kolak M, Halldin M, Brismar K, de Faire U, Hellénius ML, Fisher RM. Functional changes in adipose tissue in a randomised controlled trial of physical activity. Lipids Health Dis. 2012 Jun 21;11:80. doi: 10.1186/1476-511X-11-80.
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  • University of Bath. How training gets your fat fit. February 28, 2012. Accessed at http://www.bath.ac.uk/news/2012/02/28/adipose-tissue/
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  • Walhin JP, Richardson JD, Betts JA, Thompson D. Exercise counteracts the effects of short-term overfeeding and reduced physical activity independent of energy imbalance in healthy young men. J Physiol. 2013 Dec 15;591(24):6231-43. doi: 10.1113/jphysiol.2013.262709. Epub 2013 Oct 28.