Diane Abbott says it took her brother to tell her off before she realised she needed to address her diabetes.
Ms Abbott had to temporarily step down as shadow home secretary just days before the general election after a series of uncomfortable interviews.
She told BBC Breakfast she had not been managing her type 2 diabetes.
Now back on the Labour frontbench, she said: “Stress wasn’t an issue, my blood sugar level was. I am back to fighting fitness.”
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During the election campaign, Ms Abbott faced criticism for her performance after she stumbled several times during interviews and appeared unable to give detailed answers.
In one interview with LBC Radio, she mistakenly said plans to boost police numbers by 10,000 would cost £300,000. It led to a barrage of criticism from the Conservatives who said she could not “add up”.
She also pulled out of an interview with BBC’s Woman’s Hour with just a few minutes’ notice.
Just 48 hours before polling day, Labour said Ms Abbott was taking a period of sick leave and would be replaced “indefinitely” by Lyn Brown.
After the election Ms Abbott revealed in an interview with the the Guardian that she had been diagnosed with Type 2 diabetes two years earlier and her condition was “out of control” during the campaign.
The Hackney North and Stoke Newington MP, who increased her majority in the 8 June election, said she struggled after facing six or seven interviews in a row without eating enough food – vital to managing her blood sugar levels.
Type 2 diabetes is a long-term condition that causes a person’s blood sugar level to become too high.
It can be controlled by taking medication to reduce glucose to normal levels and also by making lifestyle and dietary changes.
‘Stress wasn’t an issue’
Now back in the shadow cabinet, Ms Abbott told BBC Breakfast, in her first interview since returning to work, her Type 2 diabetes had been “an issue at certain points” during the election campaign.
But she said: “I’m feeling a lot better,” adding: “It took my brother to ring me up and tell me off and tell me about the importance of eating properly and glucose tablets.”
Urging others to get themselves tested for type 2 diabetes, she said she was now taking “all my brother’s good advice”.
Questioned as to whether her illness was stress related and if so would she be capable of being home secretary, she said: “Stress wasn’t an issue, my blood sugar levels was. I am back to fighting fitness.”
Asked if she was subjected to racism and misogyny in the campaign, she said: “I think politicians complaining about the media is like sailors complaining about the weather.”
She said resigning because of her ill health was “never a consideration”. “Everyone who has type 2 diabetes knows it is a perfectly manageable condition,” she added.