Factors like stress, allergies, insomnia contribute to those unwanted kilos, finds study.
"It's not just calories in and out - if it were that simple everyone in the world would be the way that they want to be," the Daily Telegraph quoted Dr Pamela Waritan as saying.
The co-director of the Metabolic and Nutritional Program at the University of South Florida College of Medicine, Dr Smith said in her new book that for every extra 0.45kg someone carries on their frame, they put 1.36 million kg of stress on their joints each year.
"Sometimes it's confusing to people if they have an allergic response. You can have two kinds of allergic response, one is what we call an IgE, which can be shrimp, and is pretty immediate, for example, resulting in shortness of breath or a rash.
"The other which is termed as an IgG response, is an allergy which can usually happen a couple of days later. People may gain weight, get a stomach ache or a headache but they don't equate it to the food (they have eaten)."
Symptoms include abdominal pain, backache, dark circles under the eyes, diarrhoea, dizziness, hives, muscle aches and pains, a persistent cough and memory changes.
Dr Smith said lack of sleep and stress were also contributing to weight gain.
"When people stay stressed they put weight on around the middle. And I do think modern life affects it for many different reasons. Insomnia and sleep deprivation, people that don't get 6 1/2 hours a night of good solid sleep also suffer."
Her book 'A Guide to Solving Your Weight-Loss Puzzle: Why You Can't Lose Weight' also reveals being toxic is another key factor.
Symptoms of toxicity include abdominal bloating, belching, cramping, gas, heartburn and weight gain, depression, itching, muscle aches and pains and skin rashes.